THE
TROUVELOT
ASTRONOMICAL DRAWINGS
MANUAL

BY

E. L. TROUVELOT,

FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE
AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL
SOCIETY OF GREAT BRITAIN; IN CHARGE OF A
GOVERNMENT EXPEDITION TO OBSERVE THE
TOTAL SOLAR ECLIPSE OF 1878.

NEW YORK

CHARLES SCRIBNER'S SONS

1882

[INTRODUCTION]

During a study of the heavens, which has now been continued for more than fifteen years, I have made a large number of observations pertaining to physical astronomy, together with many original drawings representing the most interesting celestial objects and phenomena.

With a view to making these observations more generally useful, I was led, some years ago, to prepare, from this collection of drawings, a series of astronomical pictures, which were intended to represent the celestial phenomena as they appear to a trained eye and to an experienced draughtsman through the great modern telescopes, provided with the most delicate instrumental appliances. Over two years were spent in the preparation of this series, which consisted of a number of large drawings executed in pastel. In 1876, these drawings were displayed at the United States Centennial Exhibition at Philadelphia, forming a part of the Massachusetts exhibit, in the Department of Education and Science.

The drawings forming the present series comprise only a part of those exhibited at Philadelphia; but, although fewer in number, they are quite sufficient to illustrate the principal classes of celestial objects and phenomena.

While my aim in this work has been to combine scrupulous fidelity and accuracy in the details, I have also endeavored to preserve the natural elegance and the delicate outlines peculiar to the objects depicted; but in this, only a little more than a suggestion is possible, since no human skill can reproduce upon paper the majestic beauty and radiance of the celestial objects.

The plates were prepared under my supervision, from the original pastel drawings, and great care has been taken to make the reproduction exact.

The instruments employed in the observations, and in the delineation of the heavenly bodies represented in the series, have varied in aperture from 6 to 26 inches, according to circumstances, and to the nature of the object to be studied. The great Washington refractor, kindly placed at my disposal by the late Admiral C. H. Davis, has contributed to this work, as has also the 26 inch telescope of the University of Virginia, while in the hands of its celebrated constructors, Alvan Clark & Sons. The spectroscope used was made by Alvan Clark & Sons. Attached to it is an excellent diffraction grating, by Mr. L. M. Rutherfurd, to whose kindness I am indebted for it.

Those unacquainted with the use of optical instruments generally suppose that all astronomical drawings are obtained by the photographic process, and are, therefore, comparatively easy to procure; but this is not true. Although photography renders valuable assistance to the astronomer in the case of the Sun and Moon, as proved by the fine photographs of these objects taken by M. Janssen and Mr. Rutherfurd; yet, for other subjects, its products are in general so blurred and indistinct that no details of any great value can be secured. A well-trained eye alone is capable of seizing the delicate details of structure and of configuration of the heavenly bodies, which are liable to be affected, and even rendered invisible, by the slightest changes in our atmosphere.

The method employed to secure correctness in the proportions of the original drawings is simple, but well adapted to the purpose in view. It consists in placing a fine reticule, cut on glass, at the common focus of the objective and the eye-piece, so that in viewing an object, its telescopic image, appearing projected on the reticule, can be drawn very accurately on a sheet of paper ruled with corresponding squares. For a series of such reticules I am indebted to the kindness of Professor William A. Rogers, of the Harvard College Observatory.

The drawings representing telescopic views are inverted, as they appear in a refracting telescope—the South being upward, the North downward, the East on the right, and the West on the left. The Comet, the Milky-Way, the Eclipse of the Moon, the Aurora Borealis, the Zodiacal Light and the Meteors are represented as seen directly in the sky with the naked eye. The Comet was, however, drawn with the aid of the telescope, without which the delicate structure shown in the drawing would not have been visible.

The plate representing the November Meteors, or so-called "Leonids," may be called an ideal view, since the shooting stars delineated, were not observed at the same moment of time, but during the same night. Over three thousand Meteors were observed between midnight and five o'clock in the morning of the day on which this shower occurred; a dozen being sometimes in sight at the same instant. The paths of the Meteors, whether curved, wavy, or crooked, and also their delicate colors, are in all cases depicted as they were actually observed.

In the Manual, I have endeavored to present a general outline of what is known, or supposed, on the different subjects and phenomena illustrated in the series. The statements made are derived either from the best authorities on physical astronomy, or from my original observations, which are, for the most part, yet unpublished.

The figures in the Manual relating to distance, size, volume, mass, etc., are not intended to be strictly exact, being only round numbers, which can, therefore, be more easily remembered.

It gives me pleasure to acknowledge that the experience acquired in making the astronomical drawings published in Volume VIII. of the Annals of the Harvard College Observatory, while I was connected with that institution, has been of considerable assistance to me in preparing this work; although no drawings made while I was so connected have been used for this series.

E. L. TROUVELOT.

Cambridge, March, 1882.

[INTRODUCTION]
[LIST OF PLATES]

[LIST OF PLATES][1]

PLATE

[I. GROUP OF SUN-SPOTS AND VEILED SPOTS.]
Observed June 17, 1875, at 7 h. 30m. A. M.
[II. SOLAR PROTUBERANCES.]
Observed May 5, 1873, at 9h. 40m. A. M.
[III. TOTAL ECLIPSE OF THE SUN.]
Observed July 29, 1878, at Creston, Wyoming Territory.
[IV. AURORA BOREALIS.]
As observed March 1, 1872, at 9h. 25m. P. M.
[V. THE ZODIACAL LIGHT.]
Observed February 20, 1876.
[VI. MARE HUMORUM.]
From a study made in 1875.
[VII. PARTIAL ECLIPSE OF THE MOON.]
Observed October 24, 1874.
[VIII. THE PLANET MARS.]
Observed September 3, 1877, at 11h. 55m. P. M.
[IX. THE PLANET JUPITER.]
Observed November 1, 1880, at 9h. 30m. P. M.
[X. THE PLANET SATURN.]
Observed November 30, 1874, at 5th. 50m. P. M.
[XI. THE GREAT COMET OF 1881.]
Observed on the night of June 25-26, at 1h. 30m. A. M.
[XII. THE NOVEMBER METEORS.]
As observed between midnight and 3 o'clock A. M., on the night
of November 13-14, 1868.
[XIII. PART OF THE MILKY-WAY.]
From a study made during the years 1874, 1875 and 1876.
[XIV. STAR-CLUSTER IN HERCULES.]
From a study made in June, 1877.
[XV. THE GREAT NEBULA IN ORION.]
From a study made in the years 1873-76.

[1]For Key to the Plates, see Appendix.

Reproduced from the Original Drawings, by Armstrong & Company,
Riverside Press, Cambridge, Mass.

[GENERAL REMARKS ON THE SUN]

The Sun, the centre of the system which bears its name, is a self-luminous sphere, constantly radiating heat and light.

Its apparent diameter, as seen at its mean from the Earth, subtends an angle of 32', or a little over half a degree. A dime, placed about six feet from the eye, would appear of the same proportions, and cover the Sun's disk, if projected upon it.

That the diameter of the Sun does not appear larger, is due to the great distance which separates us from that body. Its distance from the Earth is no less than 92,000,000 miles. To bridge this immense gap, would require 11,623 globes like the Earth, placed side by side, like beads on a string.

The Sun is an enormous sphere whose diameter is over 108 times the diameter of our globe, or very nearly 860,000 miles. Its radius is nearly double the distance from the Earth to the Moon. If we suppose, for a moment, the Sun to be hollow, and our globe to be placed at the centre of this immense spherical shell, not only could our satellite revolve around us at its mean distance of 238,800 miles, as now, but another satellite, placed 190,000 miles farther than the Moon, could freely revolve likewise, without ever coming in contact with the solar envelope.

The circumference of this immense sphere measures 2,800,000 miles. While a steamer, going at the rate of 300 miles a day, would circumnavigate the Earth in 83 days, it would take, at the same rate, nearly 25 years to travel around the Sun.

The surface of the Sun is nearly 12,000 times the surface of the Earth, and its volume is equal to 1,300,000 globes like our own. If all the known planets and satellites were united in a single mass, 600 such compound masses would be needed to equal the volume of our luminary.

Although the density of the Sun is only one-quarter that of the Earth, yet the bulk of this body is so enormous that, to counterpoise it, no less than 314,760 globes like our Earth would be required.

The Sun uniformly revolves around its axis in about 25½ days. Its equator is inclined 7° 15' to the plane of the ecliptic, the axis of rotation forming, therefore, an angle of 82° 45' with the same plane. As the Earth revolves about the Sun in the same direction as that of the Sun's rotation, the apparent time of this rotation, as seen by a terrestrial spectator, is prolonged from 25½ days to about 27 days and 7 hours.

The rotation of the Sun on its axis, like that of the Earth and the other planets, is direct, or accomplished from West to East. To an observer on the Earth, looking directly at the Sun, the rotation of this body is from left to right, or from East to West.

The general appearance of the Sun is that of an intensely luminous disk, whose limb, or border, is sharply defined on the heavens. When its telescopic image is projected on a screen, or fixed on paper by photography, it is noticed that its disk is not uniformly bright throughout, but is notably more luminous in its central parts. This phenomenon is not accidental, but permanent, and is due in reality to a very rare but extensive atmosphere which surrounds the Sun, and absorbs the light which that body radiates, proportionally to its thickness, which, of course, increases towards the limb, to an observer on the Earth.

THE ENVELOPING LAYERS OF THE SUN

The luminous surface of the Sun, or that part visible at all times, and which forms its disk, is called the Photosphere, from the property it is supposed to possess of generating light. The photosphere does not extend to a great depth below the luminous surface, but forms a comparatively thin shell, 3,000 or 4,000 miles thick, which is distinct from the interior parts, above which it seems to be kept in suspense by internal forces. From the observations of some astronomers it would appear that the diameter of the photosphere is subject to slight variations, and, therefore, that the solar diameter is not a constant quantity. From the nature of this envelope, such a result does not seem at all impossible, but rather probable.

Immediately above the photosphere lies a comparatively thin stratum, less than a thousand miles in thickness, called the Reversing Layer. This stratum is composed of metallic vapors, which, by absorbing the light of particular refrangibilities emanating from the photosphere below, produces the dark Fraunhofer lines of the solar spectrum.

Above the reversing layer, and resting immediately upon it, is a shallow, semi-transparent gaseous layer, which has been called the Chromosphere, from the fine tints which it exhibits during total eclipses of the Sun, in contrast with the colorless white light radiated by the photosphere below. Although visible to a certain extent on the disk, the chromosphere is totally invisible on the limb, except with the spectroscope, and during eclipses, on account of the nature of its light, which is mainly monochromatic, and too feeble, compared with that emitted by the photosphere, to be seen.

The chromospheric layer, which has a thickness of from 3,000 to 4,000 miles, is uneven, and is usually upheaved in certain regions, its matter being transported to considerable elevations above its general surface, apparently by some internal forces. The portions of the chromosphere thus lifted up, form curious and complicated figures, which are known under the names of Solar Protuberances, or Solar Flames.

Above the chromosphere, and rising to an immense but unknown height, is the solar atmosphere proper, which is only visible during total eclipses of the Sun, and which then surrounds the dark body of the Moon with the beautiful rays and glorious nimbus, called the Corona.

These four envelopes: the photosphere, the reversing layer, the chromosphere, and the corona, constitute the outer portions of our luminary.

Below the photosphere little can be seen, although it is known, as will appear below, that at certain depths cloud-like forms exist, and freely float in an interior atmosphere of invisible gases. Beyond this all is mystery, and belongs to the domain of hypothesis.

STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE

The apparent uniformity of the solar surface disappears when it is examined with a telescope of sufficient aperture and magnifying powers. Seen under good atmospheric conditions, the greater part of the solar surface appears mottled with an infinite number of small, bright granules, irregularly distributed, and separated from each other by a gray-tinted background.

These objects are known under different names. The terms granules and granulations answer very well for the purpose, as they do not imply anything positive as to their form and true nature. They have also been called Luculœ, Rice Grains, Willow Leaves, etc., by different observers.

Although having different shapes, the granulations partake more or less of the circular or slightly elongated form. Their diameter, which varies considerably, has been estimated at from 0".5 to 3", or from 224 to 1,344 miles. The granulations which attain the largest size appear, under good atmospheric conditions, to be composed of several granules, closely united and forming an irregular mass, from which short appendages protrude in various directions.

The number of granulations on the surface of the Sun varies considerably under the action of unknown causes. Sometimes they are small and very numerous, while at other times they are larger, less numerous, and more widely separated. Other things being equal, the granulations are better seen in the central regions of the Sun than they are near the limb.

Usually the granulations are very unstable; their relative position, form, and size undergoing continual changes. Sometimes they are seen to congregate or to disperse in an instant, as if acting under the influence of attractive and repulsive forces; assembling in groups or files, and oftentimes forming capricious figures which are very remarkable, but usually of short duration. In an area of great solar disturbances, the granulations are often stretched to great distances, and form into parallel lines, either straight, wavy, or curved, and they have then some resemblance to the flowing of viscous liquids.

The granulations are usually terminated either by rounded or sharply pointed summits, but they do not all rise to the same height, as can be ascertained with the spectroscope when they are seen sidewise on the limb. In the regions where they are most abundant, they usually attain greater elevations, and when observed on the limb with the spectroscope, they appear as slender acute flames.

The granulations terminated by sharply-pointed crests, although observed in all latitudes, seem to be characteristic of certain regions. A daily study of the chromosphere, extending over a period of ten years, has shown me that the polar regions are rarely ever free from these objects, which are less frequent in other parts of the Sun. In the polar regions they are sometimes so abundant that they completely form the solar limb. These forms of granulation are comparatively rare in the equatorial zones, and when seen there, they never have the permanency which they exhibit in the polar regions. When observed in the equatorial regions, they usually appear in small groups, in the vicinity of sun spots, or they are at least enclosed in areas of disturbances where such spots are in process of formation. In these regions they often attain greater elevations than those seen in high latitudes.

As we are certain that in the equatorial zones these slender flames (i. e., granulations) are a sure sign of local disturbance, it may be reasonably supposed that the same kind of energy producing them nearly always prevails in the polar regions, although it is there much weaker, and never reaches beyond certain narrow limits.

Studied with the spectroscope, the granulations are found to be composed in the main of incandescent hydrogen gas, and of an unknown substance provisionally called "helium." Among the most brilliant of them are found traces of incandescent metallic vapors, belonging to various substances found on our globe.

The chromosphere is not fixed, but varies considerably in thickness in its different parts, from day to day. Its thickness is usually greater in the polar regions, where it sometimes exceeds 6,700 miles. In the equatorial regions the chromosphere very rarely attains this height, and when it does, the rising is local and occupies only a small area. In these regions it is sometimes so shallow that its depth is only a few seconds, and is then quite difficult to measure. These numbers give, of course, the extreme limits of the variations of the chromosphere; but, nearly always, it is more shallow in the equatorial regions; and, as far as my observations go, the difference in thickness between the polar and equatorial zones is greater in years of calm than it is in years of great solar activity. But ten years of observation are not sufficient to warrant any definite conclusions on this subject.

There is undoubtedly some relation between the greater thickness of the chromosphere in the polar regions, and the abundance and permanence of the sharply-pointed granulations observed in the same regions. This becomes more evident when we know that the appearance of similarly-pointed flames in the equatorial zones is always accompanied with a local thickening of the chromosphere. The thickening in the polar regions may be only apparent, and not due to a greater accumulation of chromospheric gases there; but may be caused by some kind of repulsive action or polarity, which lifts up and extends the summit of the granulations in a manner similar to the well-known mode of electric repulsion and polarity.

As it seems very probable that the heat and light emanating from the Sun are mainly generated at the base of the granulations, in the filamentary elements composing the chromosphere and photosphere, it would follow that, as the size and number of these objects constantly vary, the amount of heat and light emitted by the Sun should also vary in the same proportion.

The granulations of the solar surface are represented on Plate I., and form the general background to the group of Sun-spots forming the picture.

THE FACULÆ

Although the solar surface is mainly covered with the luminous granulations and the grayish background above described, it is very rare that its appearance is so simple and uniform as already represented. For the most part, on the contrary, it is diversified by larger, brighter, and more complicated forms, which are especially visible towards the border of the Sun. Owing to their extraordinary brilliancy, these objects have been called Faculœ (torches).

Although the faculæ are very seldom seen well beyond 50 heliocentric degrees from the limb, yet they exist, and are as numerous in the central parts of the disk as they are towards the border; since they form a part of the solar surface, and participate in its movement of rotation. Their appearance near the limb has been attributed to the effect of absorption produced by the solar atmosphere on the light from the photosphere; but this explanation seems inadequate, and does not solve the problem. The well-known fact that the solar protuberances—which are in a great measure identical with the faculæ—are much brighter at the base than they are at the summit, perhaps gives a clue to the explanation of the phenomenon; especially since we know that, in general, the summit of the protuberances is considerably broader than their base. When these objects are observed in the vicinity of the limb, they present their brightest parts to the observer, since, in this position, they are seen more or less sidewise; and, therefore, they appear bright and distinct. But as the faculæ recede from the limb, their sides, being seen under a constantly decreasing angle, appear more and more foreshortened; and, therefore, these objects grow less bright and less distinct, until they finally become invisible, when their bases are covered over by the broad, dusky summit generally terminating the protuberances.

The faculæ appear as bright and luminous masses or streaks on the granular surface of the Sun, but they differ considerably in form and size. Two types at least are distinguishable. In their simplest form they appear either as isolated white spots, or as groups of such spots covering large surfaces, and somewhat resembling large flakes of snow. In their most characteristic types they appear as intensely luminous, heavy masses, from which, in most cases, issue intricate ramifications, sometimes extending to great distances. Generally, the ramifications issuing from the masses of faculæ have their largest branches directed in the main towards the eastern limb of the Sun. Some of these branches have gigantic proportions. Occasionally they extend over 60° and even 80° of the solar surface, and, therefore, attain a length of from 450,000 to 600,000 miles.

Although the faculæ may be said to be seen everywhere on the surface of the Sun, there is a vast difference in different regions, with regard to their size, number, and brilliancy. They are largest, most abundant, and brightest on two intermediate zones parallel to the solar equator, and extending 35° or 40° to the north and to the south of this line. The breadth of these zones varies considerably with the activity of the solar forces. When they are most active, the faculæ spread on either side, but especially towards the equator, where they sometimes nearly meet those of the other zone. In years of little solar activity the belts formed by the faculæ are very narrow—the elements composing them being very few and small, although they never entirely disappear.

The faculæ are very unstable, and are constantly changing: those of the small types sometimes form and vanish in a few minutes. When an area of disturbance of the solar surface is observed for some time, all seems in confusion; the movements of the granulations become unusually violent; they congregate in all sorts of ways, and thus frequently form temporary faculæ. Action of this kind is, for the most part, peculiar to the polar regions of the Sun.

The larger faculæ have undoubtedly another origin, as they seem to be mainly formed by the ejection of incandescent gases and metallic vapors from the interior of the photosphere. In their process of development some of the heavy masses of faculæ are swollen up to great heights, being torn in all sorts of ways, showing large rents and fissures through which the sight can penetrate.

Very few faculæ are represented in Plate I.; several streaks are shown at the upper left-hand corner, some appearing as whitish ramifications among the granulations representing the general solar surface.

[SUN-SPOTS AND VEILED SPOTS]
PLATE I

Besides the brilliant faculæ already described, much more conspicuous markings, though of a totally different character, are very frequently observed on the Sun. On account of their darkish appearance, which is in strong contrast with the white envelope of our luminary, these markings were called Maculæ, or Sun-spots, by their earlier observers.

The Sun-spots are not equally distributed on the solar surface; but like the faculæ, to which they are closely related, they occupy two zones—one on each side of the equator. These zones are comprised between 10° and 35° of north latitude, and 10° and 35° of south latitude. Between these two zones is a belt 20° in width, where the Sun-spots are rarely seen.

Above the latitudes 35° north and south, the Sun-spots are rare, and it is only occasionally, and during years of great solar activity, that they appear in these regions; in only a few cases have spots of considerable size been seen there. A few observers, however, have seen spots as far as 40° and 50 from the equator; and La Hire even observed one in 70° of north latitude; but these cases are exceedingly rare. It is not uncommon, however, to see very small spots, or groups of such spots, within 8° or 10° from the poles.

The activity of the Sun is subject to considerable fluctuation, and accordingly the Sun-spots vary in size and number in different years. During some years they are large, complicated, and very numerous; while in others they are small and scarce, and are sometimes totally absent for weeks and months together. The fluctuations in the frequency of Sun-spots are supposed to be periodical in their character, although their periods do not always appear to recur at exactly regular intervals. Sometimes the period is found to be only nine years, while at other times it extends to twelve years. The period generally adopted now is 11⅒ years, nearly; but further investigations are needed to understand the true nature of the phenomenon.

The number of Sun-spots does not symmetrically augment and diminish, but the increase is more rapid than the diminution.

The period of increase is only about four years, while that of decrease is over seven years; each period of Sun-spot maximum being nearer the preceding period of Sun-spot minimum than it is to that next following.

The cause of these fluctuations in the solar energy is at present wholly unknown. Some astronomers, however, have attributed it to the influence of the planets Venus and Jupiter, the period of revolution of the latter planet being not much longer than the Sun-spot period; but this supposition lacks confirmation from direct observations, which, so far, do not seem to be in favor of the hypothesis. At the present time the solar activity is on the increase, and the Sun-spots will probably reach their maximum in 1883. The last minimum occurred in 1879, when only sixteen small groups of spots were observed during the whole year.

Sun-spots vary in size and appearance; but, unless they are very small, in which case they appear as simple black dots, they generally consist of two distinct and well-characterized parts, nearly always present. There is first, a central part, much darker than the other, and sharply divided from it, called the "Umbra;" second, a broad, irregular radiated fringe of lighter shade, completely surrounding the first, and called the "Penumbra."

Reduced to its simplest expression, a Sun-spot is a funnel-shaped opening through the chromosphere and the photosphere. The inner end of the funnel, or opening, gives the form to the umbra, while its sloping sides form the penumbra.

The umbra of Sun-spots, whose outlines approximately follow the irregularities of the penumbral fringe, has a diameter which generally exceeds the width of the penumbral ring. Sometimes it appears uniformly black throughout; but it is only so by contrast, as is proved when either Mercury or Venus passes near a spot during a transit over the Sun's disk. The umbra then appears grayish, when compared with the jet-black disk of the planet.

The umbra of spots is rarely so simple as just described; but it is frequently occupied, either partly or wholly, by grayish and rosy forms, somewhat resembling loosely-entangled muscular fibres. These forms have been called the Gray and Rosy Veils. Frequently these veils appear as if perforated by roundish black holes, improperly called Nuclei, which permit the sight to penetrate deeper into the interior. To all appearance the gray and rosy veils are of the same nature as the chromosphere and the faculæ, and are therefore mainly composed of hydrogen gas.

Whatever can be known about the interior of the Sun, must be learned from the observations of these openings, which are comparatively small. But whatever this interior may be, we certainly know that it is not homogeneous. Apparently, the Sun is a gigantic bubble, limited by a very thin shell. Below this shell exists a large open space filled with invisible gases, in which, through the openings constituting the Sun-spots, the gray and rosy veils described above are occasionally seen floating.

The fringe forming the penumbra of spots is much more complicated than the umbra. In its simpler form, it is composed of a multitude of bright, independent filaments of different forms and sizes, partly projecting one above the other, on the sloping wall of the penumbra, from which they seem to proceed. Seen from the Earth, these filaments have somewhat the appearance of thatched straw, converging towards the centre of the umbra. It is very rare, however, that the convergence of the penumbral filaments is regular, and great confusion sometimes arises from the entanglement of these filaments. Some of these elements appear straight, others are curved or loop-shaped; while still others, much larger and brighter than the rest, give a final touch to this chaos of filaments, from which results the general thatched and radiating appearance of the penumbra.

The extremities of the penumbral filaments, especially of those forming the border of the umbra, are usually club-shaped and appear very brilliant, as if these elements had been superheated by some forces escaping through the opening of the spots.

Besides these characteristics, the Sun-spots have others, which, although not always present, properly belong to them. Comparatively few spots are so simple as the form just described. Very frequently a spot is accompanied by brilliant faculæ, covering part of its umbra and penumbra, and appearing to form a part of the spot itself.

When seen projected over Sun-spots, the faculæ appear intensely bright, and from these peculiarities they have been called Luminous Bridges. They are, in fact, bridges, but in most cases they are at considerable heights above the spots, kept there by invisible forces. When such spots with luminous bridges approach the Sun's limb, it is easy to see, by the rapid apparent displacement which they undergo, that they are above the general level.

When the spots are closing up, the inverse effect is sometimes observed. On several occasions, I have seen huge masses of faculæ advance slowly over the penumbra of a spot and fall into the depths of the umbra, resembling gigantic cataracts. I have seen narrow branches of faculæ, which, after having fallen to great depths in the umbra, floated across it and disappeared under the photosphere on the opposite side. I have also seen luminous bridges, resembling cables, tightly stretched across the spots, slackening slowly, as if loosened at one end, and gently curving into the umbra, where they formed immense loops, large enough to receive our globe.

It is to be remarked that, in descending under the photospheric shell, the bright faculæ and the luminous bridges gradually lose their brilliancy. At first they appear grayish, but in descending farther they assume more and more the pink color peculiar to the rosy veils. The pinkish color acquired by the faculæ when they reach a certain depth under the photosphere, is precisely the color of the chromosphere and of the solar protuberances, as seen during total eclipses of the Sun—a fact which furnishes another proof that the faculæ are of the same nature as the protuberances.

I record here an observation which, at first sight, may appear paradoxical; but which seems, however, to be of considerable importance, as it shows unmistakably that the solar light is mainly, if not entirely, generated on its surface, or at least very near to it. On May 26, 1878, I observed a large group of Sun-spots at a little distance from the east limb of the Sun. The spot nearest to the limb was partly covered over on its eastern and western sides by bright and massive faculæ which concealed about two-thirds of the whole spot, only a narrow opening, running from north to south, being left across the middle of the spot. Owing to the rotundity of the Sun, the penumbra of this spot, although partly covered by the faculæ, could, however, be seen on its eastern side, since the sight of the observer could there penetrate sidewise under the faculæ. Upon that part of the penumbra appeared a strong shadow, representing perfectly the outline of the facular mass situated above it. The phenomenon was so apparent that no error of observation was possible, and a good drawing of it was secured. If this faculæ had been as bright beneath as it was above, it is evident that no shadow could have been produced; hence the light of these faculæ must have been mainly generated on or very near their exterior surfaces. This, with the well-proved fact that the bright faculæ lose their light in falling into the interior of the Sun, seems to suggest the idea that the bright light emitted by the faculæ, and very probably all the solar light, can be generated only on its surface; the presence of the coronal atmosphere being perhaps necessary to produce it. Several times before this observation, I had suspected that some faculæ were casting a shadow, but as this seemed so improbable, my attention was not awakened until the phenomenon became so prominent that it could not escape notice.

With due attention, some glimpses of the phenomenon can frequently be observed through the openings of some of the faculæ projecting over the penumbra of Sun-spots. It is very seldom that the structure of the penumbra is seen through such openings, which usually appear as dark as the umbra of the large spots, although they do not penetrate through the photosphere like the latter. It is only when the rents in the faculæ are numerous and quite large, that the penumbral structure is recognized through them. Since these superficial rents in the faculæ do not extend through the photosphere, and appear black, it seems evident that the penumbra seen through them cannot be as bright as it is when no faculæ are projected upon it, and therefore that the faculæ intercept light from the exterior surface, which would otherwise reach the penumbra.

While the matter forming the faculæ sometimes falls into the interior of the Sun, the same kind of matter is frequently ejected in enormous quantities, and with great force, from the interior, through the visible and invisible openings of the photosphere, and form the protuberances described in the following section of this manual (Solar protuberances) It is not only the incandescent hydrogen gas or the metallic vapors which are thus ejected, but also cooler hydrogen gas, which sometimes appears as dark clouds on the solar surface. On December 12, 1875, I observed such a cloud of hydrogen issuing from the corner of a small Sun-spot. It traveled several thousand miles on the solar surface, in a north-easterly direction, before it became invisible.

Solar spots are formed in various ways; but, for the most part, the apparition of a spot is announced beforehand, by a great commotion of the solar surface at the place of its appearance, and by the formation of large and bright masses of faculæ, which are usually swollen into enormous bubbles by the pressure of the internal gases. These bubbles become visible in the spectroscope while they are traversing the solar limb, as they are then presented to us sidewise. Under the action of the increasing pressure, the base of the faculæ is considerably stretched, and, its weakest side finally giving way, the facular mass is torn in many places from the solar surface, and is perforated by holes of different sizes and forms. The holes thus made along the border of the faculæ appear as small black spots, separated more or less by the remaining portion of the lacerated faculæ, and they enlarge more and more at the expense of the intervening portions, which thus become very narrow. This perforated side of the faculæ, offering less resistance, is gradually lifted up, as would be the cover of a box, for example, while its opposite side remains attached to the surface. The facular matter separating the small black holes is greatly stretched during this action, and forms long columns and filaments. These appear as luminous bridges upon the large and perfectly-formed spot, which is then seen under the lifted facular masses. The spots thus made visible are soon freed from the facular masses, which are gradually shifted towards the opposite side.

In such cases the spots are undoubtedly formed under the faculæ before they can be seen. This becomes evident when such spots, not yet cleared from the faculæ covering them, are observed near the east limb; since in this position the observer can see through the side-openings of the faculæ, and sometimes recognize the spots under their cover.

It frequently occurs that the spots thus formed under the faculæ continue to be partly covered by the facular clouds, the forces at work in them being apparently too feeble to shift them aside. In such cases these spots are visible when they are in the vicinity of the limb, where they are seen sidewise; but when observed in the east, in being carried forward by the solar rotation towards the centre of the disk, they gradually diminish in size, and finally become invisible. The disappearance of these spots, however, is only apparent, being due to the fact that, as they advance towards the centre of the disk, our lateral view of them is gradually lost, and they are finally hidden from sight by the overhanging faculæ which then serve as a screen between the observer and the spot. This class of spots may be called Lateral Spots, from the fact that they can only be seen laterally, and near the Sun's limb.

Solar spots are also formed in various other ways. Some, like those represented in Plate I., appearing without being announced by any apparent disturbance of the surface, or by the formation of any faculæ, form and develop in a very short time. Others, appearing at first as very small spots having an umbra and a penumbra, slowly and gradually develop into very large spots. This mode of formation, which would seem to be the most natural, is, however, quite rare. Spots of this class have a duration and permanence not observed in those of any other type. These spots of slow and regular development are never accompanied by faculæ or luminous bridges, nor have they any gray or rosy veils in their interior; a fact which may, perhaps, account for their permanent character.

Another class of spots, which is also rare, appear as long and narrow crevasses showing the penumbral structure of the ordinary spots; but these rarely have any umbra. These long, and sometimes exceedingly narrow fissures of the solar envelopes, with their radiated penumbral structure, strongly suggest the idea that the photosphere is composed of a multitude of filamentary elements having the granulations for summits. Such a crevasse is represented on Plate I., and unites the two spots which form the group.

The duration of Sun-spots varies greatly. Some last only for a few hours; while others continue for weeks and even months at a time, but not without undergoing changes.

The modes of disappearance of Sun-spots are as various as those of their apparition. The spots rarely close up by a gradual diminution or contraction of their umbra and penumbra. This mode of disappearance belongs exclusively to the spots deprived of faculæ and veils. One of the most common modes of the disappearance of a spot is its invasion by large facular masses, which slowly advance upon its penumbra and umbra and finally cover it entirely. It is a process precisely the reverse of that in which spots are formed by the shifting aside of the faculæ, as above described. In other types, the spots close up by the gradual enlargement of the luminous bridges traversing them, which are slowly transformed into branches of the photosphere, all of the characteristics of which they have acquired. In many cases, the spots covered over by the faculæ continue to exist for some time, hidden under these masses, as is often proved, either by the appearance of small spots on the facular mass left at the place they occupied, or even by the reappearance of the same spot.

Apart from the general movement of rotation of the solar surface, some of the spots seem to be endowed with a proper motion of their own, which becomes greater the nearer the spots are to the solar equator. According to the observations of Mr. Carrington, the period of rotation of the Sun, as deduced from the observations of the solar spots during a period of seven years, is 25 days at the equator; while at 50° of heliocentric latitude it is 27 days. But the period of rotation, as derived from the observations of spots occupying the same latitude, is far from being constant, as it varies at different times, with the frequency of the spots and with the solar activity, so that at present the law of these variations is not well known. From the character of the solar envelope, it seems very natural that the rotation should differ in the different zones and at different times, since this envelope is not rigid, but very movable, and governed by forces which are themselves very variable.

Although it is a general law that the spots near the equator have a more rapid motion than those situated in higher latitudes, yet, in many cases, the proper motion of the spots is more apparent than real. For the most part, the changes of form and the rapid displacements observed in some spots are only apparent, and due to the fact that the large masses of faculæ which are kept in suspense above them are very unstable, and change position with the slightest change in the forces holding them in suspension. Since in these cases we view the spots through the openings of the faculæ situated above them, the slightest motion of these objects produces an apparent motion in the spots, although they have remained motionless. Accordingly, it has been remarked that of all the spots, those which have the greater proper motion are precisely those which have the most faculæ and luminous bridges; while the other spots in the same regions, but not attended by similar phenomena, are comparatively steady in their movement. These last spots are undoubtedly better adapted than any others to exhibit the rotation of the Sun; but it is probable that this period of rotation will never be known with accuracy, simply because the solar surface is unstable, and does not rotate uniformly.

The Sun-spots have a remarkable tendency to form into groups of various sizes, but whatever may be the number of spots thus assembled, the group is nearly always composed of two principal spots, to which the others are only accessories. The tendency of the Sun-spots to assemble in pairs is general, and is observed in all latitudes, even among the minute temporary groups formed in the polar regions. Whenever several are situated quite close together, those belonging to the same group can be easily recognized by this character. Whatever may be the position of the axis of the two principal spots of a group when it is first formed, this axis has a decided tendency to place itself parallel to the solar equator, no matter to what latitude the group belongs; and if it is disturbed from this position, it soon returns to it when the disturbance has ceased.

It is also remarkable that the spots observed at the same time remain in nearly the same parallel of latitude for a greater or less period of time; but they keep changing their position from year to year, their latitude decreasing with the activity of the solar forces.

Among the Sun-spots, those associated with faculæ form the groups which attain the largest proportions. When such groups acquire an apparent diameter of 1' or more, they are plainly visible to the naked eye, since for a spot to be visible to the naked eye on the Sun, it need only subtend an angle of 50". I have sometimes seen such groups through a smoky atmosphere, when the solar light was so much reduced that the disk could be observed directly and without injury to the sight.

The largest spot which ever came under my observation was seen during the period from the 13th to the 19th of November, 1870. This spot, which was on the northern hemisphere of the Sun, was conspicuous among the smaller spots constituting the group to which it belonged, and followed them on the east. On November 16th, when it attained its largest size, the diameter of its penumbra occupied fully one-fifth of the diameter of the Sun; its real diameter being, therefore, not less than 172,000 miles, or nearly 22 times the diameter of the Earth. As the umbra of this spot occupied a little more than one-third of its whole diameter, seven globes like our own, placed side by side on a straight line, could easily have passed through this immense gap. To fill the area of this opening, about 45 such globes would have been needed. This spot was, of course, very easily seen with the naked eye, its diameter being almost eight times that required for a spot to be visible without a telescope.

Ancient historians often speak of obscurations of the Sun, and it has been supposed by some astronomers that this phenomenon might have been due in some cases to the apparition of large spots. A few spots on the surface of the Sun, like that just described, would sensibly reduce its light.

Besides the ordinary Sun-spots already described, others are at times observed on the surface of the Sun, which show some of the same characteristics, but never attain so large proportions. They always appear as if seen through a fog, or veil, between the granulations of the solar surface. On account of their vagueness and ill-defined contours, I have proposed for these objects the term, "Veiled Spots." Veiled spots have a shorter duration than the ordinary spots, the smaller types sometimes forming and vanishing in a few minutes. Some of the larger veiled spots, however, remain visible for several days in succession, and show the characteristics of other spots in regard to the arrangement of their parts.

The veiled spots have no umbra or penumbra, although they are usually accompanied by faculæ resembling those seen near the ordinary spots. They are frequently seen in the polar regions, but are there always of small size and of short duration. The veiled spots are larger, and more apt to arrange themselves into groups, in the regions occupied by the ordinary spots, and it is not rare to observe such spots transform themselves into ordinary spots, and vice versa. The veiled spots, therefore, seem to be ordinary spots filled up, or covered over by the granulations and semi-transparent gases composing the chromospheric layer. That it is so, becomes more evident, from the fact that large Sun-spots in process of diminution are sometimes gradually covered with faint and scattered granules which descend in long, narrow filaments, and become less and less distinguishable as they attain greater depth. This phenomenon, associated with the fact that the luminous bridges seen over the Sun-spots which are closing up are sometimes transformed into branches which show the characteristic structure of the photosphere, goes far to prove that the solar envelopes are mainly composed of an innumerable quantity of radial filaments of varying height.

PLATE I.—GROUP OF SUN-SPOTS AND VEILED SPOTS.

Observed on June 17th 1875 at 7 h. 30 m. A.M.

The group of Sun-spots represented in Plate I., was observed and drawn on June 17th, 1875, at 7h. 30m. A. M. The first traces of this group were seen on June 15th, at noon, and consisted of three small black dots disseminated among the granulations. At that time, no disturbance of the surface was noticeable, and no faculæ were seen in the vicinity of these spots. On June 16th, at 8 o'clock A. M., the three small spots had become considerably enlarged, and, as usual, the group consisted of two principal spots. Between these two spots all was in motion: the granulations, stretched into long, wavy, parallel lines, had somewhat the appearance of a liquid in rapid motion. At 1 o'clock, P. M., on the same day, the group had considerably enlarged; the faculæ, the granulations, and the penumbral filaments being interwoven in an indescribable manner. On the morning of the 17th, these spots had assumed the complicated form and development represented in the drawing; while at the same time two conspicuous veiled spots were seen on the left hand, at some distance above the group.

Some luminous bridges are visible upon the left hand spot, traversing the penumbra and umbra of this spot in various directions. The umbra of one of the spots is occupied, and partly filled with gray and rosy veils, similar to those above described, and the granulations of the solar surface form a background to the group of spots.

This group of spots was not so remarkable for its size as for its complicated structure. The diameter of the group from east to west was only 2½ minutes of arc, or about 67,000 miles. The upper part of the umbra of the spot situated on the right hand side of the group was nearly 7,000 miles in diameter, or less by 1,000 miles than the diameter of the Earth. Some of the long filaments composing that part of the penumbra, situated on the left hand side of the same spot, were 17,000 miles in length. One of these fiery elements would be sufficient to encircle two-thirds of the circumference of the Earth.

[SOLAR PROTUBERANCES]
PLATE II

The chromosphere forming the outlying envelope of the Sun, is subject, as has been shown above, to great disturbances in certain regions, causing considerable upheavals of its surface and violent outbursts of its gases. From these upheavals and outbursts of the chromosphere result certain curious and very interesting forms, which are known under the name of "Solar Protuberances," "Prominences," or "Flames."

These singular forms, which could, until recently, be observed only during the short duration of the total eclipses of the Sun, can now be seen on every clear day with the spectroscope, thanks to Messrs. Janssen and Lockyer, to whose researches solar physics is so much indebted.

PLATE II.—SOLAR PROTUBERANCES.

Observed on May 5, 1873 at 9h, 40m. A.M.

The solar protuberances, the Sun-spots, and the faculæ to which they are closely related, are confined within the same general regions of the Sun, although the protuberances attain higher heliocentric latitudes.

There is certainly a very close relation between the faculæ and the solar protuberances, since when a group of the faculæ traverses the Sun's limb, protuberances are always seen at the same place. It seems very probable that the faculæ and the protuberances are in the main identical. The faculæ may be the brighter portion of the protuberances, consisting of gases which are still undergoing a high temperature and pressure; while the gases which have been relieved from this pressure and have lost a considerable amount of their heat, may form that part of the protuberances which is only visible on the Sun's limb.

A daily study of the solar protuberances, continued for ten years, has shown me that these objects are distributed on two zones which are equidistant from the solar equator, and parallel with it. The zone arrangement of the protuberances is more easily recognized during the years of minimum solar activity, as in these years the zones are very narrow and widely separated. During these years the belt of protuberances is situated between 40° and 45° of latitude, north and south. In years of great solar activity the zones spread considerably on either side of these limits, especially towards the equator, which they nearly reach, only a narrow belt, usually free from protuberances, remaining between them. Towards the poles the zones do not spread so much, and there the space free from protuberances is considerably greater than it is at the equator.

During years of maximum solar activity, the protuberances, like the Sun-spots and the faculæ, are very numerous, very large, and very complicated—sometimes occupying a great part of the whole solar limb. As many as twenty distinct flames are sometimes observed at one time. In years of minimum solar activity, on the contrary, the prominences are very few in number, and they are of small size; but, as far as my observations go, they are never totally absent.

In general, the solar flames undergo rapid changes, especially those which are situated in the vicinity of Sun-spots, although they occasionally remain unchanged in appearance and form for several hours at a time. The protuberances situated in higher latitudes are less liable to great and sudden changes, often retaining the same form for several days. The changes observed in the protuberances of the equatorial regions are due in part to the comparatively great changes in their position with respect to the spectator, which are occasioned by the rotation of the Sun. This rotation, of course, has a greater angular velocity on the equator than in higher latitudes. In most cases, however, the changes of the equatorial protuberances are too great and too sudden to be thus explained. They are, in fact, due to the greater solar activity developed in the equatorial zones, and wherever spots are most numerous.

The solar protuberances appear under various shapes, and are often so complicated in appearance that they defy description. Some resemble huge clumsy masses having a few perforations on their sides; while others form a succession of arches supported by pillars of different styles. Others form vertical or inclined columns, often surmounted by cloud-like masses, or by various appendages, which sometimes droop gracefully, resembling gigantic palm leaves. Some resemble flames driven by the wind; others, which are composed of a multitude of long and narrow filaments, appear as immense fiery bundles, from which sometimes issue long and delicate columns surmounted by torch-like objects of the most fantastic pattern. Some others resemble trees, or animal forms, in a very striking manner; while still others, apparently detached from the solar limb, float above it, forming graceful streamers or clouds of various shapes. Some of the protuberances are very massive, while others are so thin and transparent as to form a mere veil, through which more distant flames can easily be seen.

Notwithstanding this variety of form, two principal classes of solar protuberances may be recognized: the cloud-like or quiescent, and the eruptive or metallic protuberances.

The first class, which is the most common, comprises all the cloud-like protuberances resting upon the chromosphere or floating about it. The protuberances of this type often obtain enormous horizontal proportions, and it is not rare to see some among them occupying 20° and 30° of the solar limb. The height attained by protuberances of this class does not correspond in general to their longitudinal extent; although some of their branches attain considerable elevations. These prominences very seldom have the brilliancy displayed by the other type, and are sometimes so faint as to be seen with difficulty. Although it is generally stated by observers that some of the protuberances belonging to this class are detached from the solar surface, and kept in suspension above the surface, like the clouds in our atmosphere, yet it seems to me very doubtful whether protuberances are ever disconnected from the chromosphere, since, in an experience of ten years, I have never been able to satisfy myself that such a thing has occurred. Many of them have appeared to me at first sight to be detached from the surface, but with a little patience and attention I was always able to detect faint traces of filamentary elements connecting them with the chromosphere. Quite often I have seen bright protuberances gradually lose their light and become invisible, while soon after they had regained it, and were as clearly visible as before. Observations of this kind seem to show that while the prominences are for the most part luminous, there are also a few which are non-luminous and invisible to the eye. These dark and invisible forms are most generally found in the vicinity of Sun-spots in great activity. When observing such regions with the spectroscope, it is not rare to encounter them in the form of large dark spots projecting on the solar spectrum near the hydrogen lines. On July 28th, 1872, I observed with the spectroscope a dark spot of this kind issuing from the vicinity of a large Sun-spot, and extending over one-fifth of the diameter of the Sun. This object had been independently observed in France a little earlier by M. Chacornac with the telescope, in which it appeared as a bluish streak.

The second class of solar protuberances, comprising the eruptive type, is the most interesting, inasmuch as it conveys to us a conception of the magnitude and violence of the solar forces. The protuberances of this class, which are always intensely bright, appear for the most part in the immediate vicinity of Sun-spots or faculæ. These protuberances, which seem to be due to the outburst of the chromosphere, and to the violent ejection of incandescent gases and metallic vapors from the interior of the Sun, sometimes attain gigantic proportions and enormous heights.

While the spectrum of the protuberances of the cloudy type is simple, and usually composed of four hydrogen lines and the yellow line D₃, that of the eruptive class is very complicated, and, besides the hydrogen lines and D₃, it often exhibits the bright lines of sodium, magnesium, barium, titanium, and iron, and occasionally, also, a number of other bright lines.

The phenomena of a solar outburst are grand and imposing. Suddenly immense and acute tongues and jets of flames of a dazzling brilliancy rise up from the solar limb and extend in various directions. Some of these fiery jets appear perfectly rigid, and remain apparently motionless in the midst of the greatest disorder. Immense straps and columns form and rise in an instant, bending and waving in all sorts of ways and assuming innumerable shapes. Sometimes powerful jets resembling molten metal spring up from the Sun, describing graceful parabolas, while in their descent they form numerous fiery drops which acquire a dazzling brilliancy when they approach the surface.

The upward motion of the protuberances in process of formation is sometimes very rapid. Some protuberances have been observed to ascend in the solar atmosphere at the rate of from 120 to 497 miles a second. Great as this velocity may appear, it is nevertheless insignificant when compared with that sometimes attained by protuberances moving in the line of sight instead of directly upwards. Movements of this kind are indicated by the displacement of the bright or dark lines in the spectrum. A remarkable instance of this kind occurred on the 26th of June, 1874. On that day I observed a displacement of the hydrogen C line corresponding to a velocity of motion of 1,600 miles per second. The mass of hydrogen gas in motion producing such a displacement was, according to theory, moving towards the Earth at this incredible rate, when it instantly vanished from sight as if it had been annihilated, and was seen no more.

Until recently the protuberances had not been observed to rise more than 200,000 miles above the solar surface; but, on October 7th, 1880, a flame, which had an elevation of 80,000 miles when I observed it at 8h. 55m. A. M., had attained the enormous altitude of 350,000 miles when it was observed at noon by Professor C. A. Young. If we had such a protuberance on the Earth, its summit would be at a height sufficient not merely to reach, but to extend 100,000 miles beyond the Moon.

Although the solar protuberances represented in Plate II. have not the enormous proportions attained by some of these objects, yet they are as characteristic as any of the largest ones, and afford a good illustration of the purely eruptive type of protuberances. The height of the largest column in the group equals 4' 43", or a little over 126,000 miles. A large group of Sun-spots was in the vicinity of these protuberances when they were observed and delineated.

[TOTAL ECLIPSE OF THE SUN]
PLATE III

A solar eclipse is due to the passage of the Moon directly between the observer and the Sun. Such an eclipse can only occur at New Moon, since it is only at that time that our satellite passes between us and the Sun. The Moon's orbit does not lie precisely in the same plane as the orbit of the Earth, but is inclined about five degrees to it, otherwise an eclipse of the Sun would occur at every New Moon, and an eclipse of the Moon at every Full Moon.

Since the Moon's orbit is inclined to that of the Earth, it must necessarily intersect this orbit at two opposite points. These points are called the nodes of the Moon's orbit. When our satellite passes through either of the nodes when the Moon is new, it appears interposed to some extent between the Sun and the Earth, and so produces a solar eclipse; while if it passes a node when the Moon is full, it is more or less obscured by the Earth's shadow, which then produces an eclipse of the Moon. But, on the other hand, when the New Moon and the Full Moon do not coincide with the passage of our satellite through the nodes of its orbit, no eclipse can occur, since the Moon is not then on a line with the Sun and the Earth, but above or below that line.

Owing to the ellipticity of the Moon's orbit, the distance of our satellite from the Earth varies considerably during each of its revolutions around us, and its apparent diameter is necessarily subject to corresponding changes. Sometimes it is greater, sometimes it is less, than the apparent diameter of the Sun. If it is greater at the time of a solar eclipse, the eclipse will be total to a terrestrial observer stationed nearly on the line of the centres of the Sun and Moon, while it will be only partial to another observer stationed further from this line. But the Moon's distance from the Earth may be so great and its apparent diameter consequently so small that even those observers nearest the central line of the eclipse see the border of the Sun all round the black disk of the Moon; the eclipse is then annular. Even during the progress of one and the same eclipse the distance of the Moon from the parts of the Earth towards which its shadow is directed may vary so much that, while the eclipse is total to some observers, others equally near the central line, but stationed at a different place, will see it as annular.

The shadow cast by the Moon on the Earth during total eclipses, travels along upon the surface of the Earth, in consequence of the daily movement of rotation of our globe combined with the movements of the Earth and Moon in their orbits. The track of the Moon's shadow over the Earth's surface has a general eastward course, so that the more westerly observers see it earlier than those east of them. An eclipse may continue total at one place for nearly eight minutes, but in ordinary cases the total phase is much shorter.

The nodes of the Moon's orbit do not invariably occupy the same position, but move nearly uniformly, their position with regard to the Sun, Earth, and Moon being at any time approximately what it formerly was at a series of times separated by equal intervals from each other. Each interval comprises 223 lunations, or 18 years, 11 days, and 7 or 8 hours. The eclipses which occur within this interval are almost exactly repeated during the next similar interval. This period, called the "Saros," was well known to the ancients, who were enabled by its means to predict eclipses with some certainty.

PLATE III.—TOTAL ECLIPSE OF THE SUN.

Observed July 29, 1878, at Creston, Wyoming Territory

A total eclipse of the Sun is a most beautiful and imposing phenomenon. At the predicted time the perfectly round disk of the Sun becomes slightly indented at its western limb by the yet invisible Moon. This phenomenon is known as the "first contact."

The slight indentation observed gradually increases with the advance of the Moon from west to east, the irregularities of the surface of our satellite being plainly visible on the border of the dark segment advancing on the Sun's disk. With the advance of the Moon on the Sun, the light gradually diminishes on the Earth. Every object puts on a dull and gloomy appearance, as when night is approaching; while the bright sky, losing its light, changes its pure azure for a livid grayish color.

Two or three minutes before totality begins, the solar crescent, reduced to minute proportions, gives comparatively so little light that faint traces of the Sun's atmosphere appear on the western side behind the dark body of the Moon, whose limb then becomes visible outside of the Sun. I observed this phenomenon at Creston during the eclipse of 1878. From 15 to 20 seconds before totality, the narrow arc of the Sun's disk not yet obscured by the Moon seems to break and separate towards the extremities of its cusps, which, thus divided, form independent points of light, which are called "Baily's beads." A moment after, the whole solar crescent breaks into numerous beads of light, separated by dark intervals, and, suddenly, they all vanish with the last ray of Sunlight, and totality has begun with the "second contact." This phenomenon of Baily's beads is undoubtedly caused by the irregularities of the Moon's border, which, on reaching the solar limb, divide the thin solar crescent into as many beads of light and dark intervals as there are peaks and ravines seen sidewise on that part of the Moon's limb.

With the disappearance of the last ray of light, the planets and the stars of the first and second magnitude seem to light up and become visible in the sky. The darkness, which had been gradually creeping in with the progress of the eclipse, is then at its maximum. Although subject to great variations in different eclipses, the darkness is never so great as might be expected from the complete obscuration of our luminary, as the part of our atmosphere which is still exposed to the direct rays of the Sun, reflects to us some of that light, which thus diminishes the darkness resulting from the disappearance of the Sun. Usually the darkness is sufficient to prevent the reading of common print, and to deceive animals, causing them to act as if night was really approaching. During totality the temperature decreases, while the humidity of the atmosphere augments.

Simultaneously with the disappearance of Baily's beads, a pale, soft, silvery light bursts forth from behind the Moon, as if the Sun, in disappearing, had been vaporized and expanded in all directions into soft phosphorescent rays and streamers. This pale light is emitted by gases constituting the solar atmosphere surrounding the bright nucleus now obscured by the dark body of our satellite. This solar atmosphere is called Corona, from its distant resemblance to the aureola, or glory, represented by ancient painters around the heads of saints.

With the bursting forth of the corona, a very thin arc of bright white light is seen along the Moon's limb, where the solar crescent has just disappeared. This thin arc of light is the reversing layer, which, when observed with the spectroscope at that moment, exhibits bright lines answering to the dark lines of the ordinary solar spectrum. Immediately above this reversing layer, and concentric with it, appears the pink-colored chromospheric layer, with its curiously shaped flames and protuberances. During totality, the chromosphere and protuberances are seen without the aid of the spectroscope, and appear of their natural color, which, although somewhat varying in their different parts, is, on the whole, pinkish, and similar to that of peach-blossoms; yet it is mixed here and there with delicate prismatic hues, among which the pink and straw colors predominate.

The color of the corona seems to vary in every eclipse, but as its tints are very delicate, it may depend, in a great measure, upon the vision of the observer; although there seems to be no doubt that there are real variations. At Creston, in 1878, it appeared to both Professor W. Harkness and myself of a decided pale greenish hue.

The corona appears under different forms, and has never been observed twice alike. Its dimensions are also subject to considerable variations. Sometimes it appears regular and very little extended, its distribution around the Sun being almost uniform; although in general it spreads a little more in the direction of the ecliptic, or of the solar equator. At other times it appears much larger and more complicated, and forms various wings and appendages, which in some cases, as in 1878, extend to immense distances; while delicate rays radiate in straight or curved lines from the spaces left in the polar regions between the wings. The corona has sometimes appeared as if divided by immense dark gaps, apparently free from luminous matter, and strongly resembling the dark rifts seen in the tails of comets. This was observed in Spain and Sicily during the total eclipse of the Sun in 1870. Different structures, forming wisps and streamers of great length, and interlaced in various ways, are sometimes present in the corona, while faint but more complicated forms, distantly resembling enormous solar protuberances with bright nuclei, have also been observed.

As the Moon continues its eastward progress, it gradually covers the chromosphere and the solar protuberances on the eastern side of the Sun; while, at the same time, the protuberances and the chromosphere on the opposite limb gradually appear from under the retreating Moon. Then, the thin arc of the reversing layer is visible for an instant, and is instantly followed by the appearance of a point of dazzling white light, succeeded immediately by the apparition of Daily's beads on each side, and totality is over, with this third contact. The corona continues to be visible on the eastern side of the Sun for several minutes longer, and then rapidly vanishes.

The thin solar crescent increases in breadth as the Moon advances; while, at the same time, the darkness and gloom spread over nature gradually disappear, and terrestrial objects begin to resume their natural appearance. Finally the limb of the Moon separates from that of the Sun at the instant of "fourth contact," and the eclipse is over.

The phenomena exhibited by the corona in different eclipses are very complex, and, so far, they have not been sufficiently studied to enable us to understand the true nature of the solar atmosphere. From the spectral analysis of the corona, and the phenomena of polarization, it has been learned, at least, that while the matter composing the upper part of the solar atmosphere is chiefly composed of an unknown substance, producing the green line 1474, its lower part is mainly composed of hydrogen gas at different temperatures, a part of which is self-luminous, while the other part only reflects the solar light. But the proportion of the gaseous particles emitting light, to those simply reflecting it, is subject to considerable variations in different eclipses. At present it would seem that in years of great solar disturbances, the particles emitting light are found in greater quantity in the corona than those reflecting it; but further observations will be required to confirm these views.

It is very difficult to understand how the corona, which in certain eclipses extends only one diameter of the Sun, should, in other cases, as in 1878, extend to the enormous distance of twelve times the same diameter. Changes of such magnitude in the solar atmosphere, if due to the operation of forces with which we are acquainted, cannot yet be accounted for by what is known of such forces. Their causes are still as mysterious as those concerned in the production of the monstrous tails displayed by some comets on their approach to the Sun.

Plate 3, representing the total eclipse of the Sun of July 29th, 1878, was drawn from my observations made at Creston, Wyoming Territory, for the Naval Observatory. The eclipse is represented as seen in a refracting telescope, having an aperture of 6⅓ inches, and as it appeared a few seconds before totality was over, and when the chromosphere was visible on the western limb of the Sun. The two long wings seen on the east and west side of the Sun, appeared considerably larger in the sky than they are represented in the picture.

[THE AURORA BOREALIS]
PLATE IV

The name of Polar Auroras is given to certain very remarkable luminous meteoric phenomena which appear at intervals above the northern or the southern horizons of both hemispheres of the Earth. When the phenomenon is produced in our northern sky, it is called "Aurora Borealis," or "Northern Lights;" and when it appears in the southern sky, it is called "Aurora Australis," or southern aurora.

Marked differences appear in the various auroras observed from our northern latitudes. While some simply consist in a pale, faint luminosity, hardly distinguishable from twilight, others present the most gorgeous and remarkable effects of brightness and colors.

A great aurora is usually indicated in the evening soon after twilight, by a peculiar grayish appearance of the northern sky just above the horizon. The grayish vapors giving that appearance, continuing to form there, soon assume a dark and gloomy aspect, while they gradually take the form of a segment of a circle resting on the horizon. At the same time that this dark segment is forming, a soft pearly light, which seems to issue from its border, spreads up in the sky, where it gradually vanishes, being the brightest at its base. This arc of light, gradually increasing in extent as well as in brightness, reaches sometimes as far as the polar star. On some rare occasions, one or two, and even three, concentric arches of bright light form one above the other over the dark segment, where they appear as brilliant concentric rainbows. While the aurora continues to develop and spread out its immense arc, the border of the dark segment loses its regularity and appears indented at several places by patches of light, which soon develop into long, narrow, diverging rays and streamers of great beauty. For the most part the auroral light is either whitish or of a pale, greenish tint; but in some cases it exhibits the most beautiful colors, among which the red and green predominate. In these cases the rays and streamers, which are usually of different colors, produce the most magnificent effects by their continual changes and transformations.

The brightness and extent of the auroral rays are likewise subject to continual changes. An instant suffices for their development and disappearance, which may be succeeded by the sudden appearance of others elsewhere, as though the original streamers had been swiftly transported to a new place while invisible. It frequently happens that all the streamers seem to move sidewise, from west to east, along the arch, continuing meanwhile to exhibit their various changes of form and color. For a time, these appearances of motion continue to increase, a succession of streamers alternately shooting forth and again fading, when a sudden lull occurs, during which all motion seems to have ceased. The stillness then prevailing is soon succeeded by slight pulsations of light, which seem to originate on the border-of the dark segment, and are propagated upwards along the streamers, which have now become more numerous and active. Slow at first, these pulsations quicken by degrees, and after a few minutes the whole northern sky seems to be in rapid vibration. The lively upward and downward movement of these streamers entitles them to the name of "merry dancers" given them in northern countries where they are frequent.

Long waves of light, quickly succeeded by others, are propagated in an instant from the horizon to the zenith; these, in their rapid passage, cause bends and curves in the streamers, which then, losing their original straightness, wave and undulate in graceful folds, resembling those of a pennant in a gentle breeze. Although the coruscations add to the grandeur of the spectacle, they tend to destroy the diverging streamers, which, being disconnected from the dark segment, or torn in various ways, are, as it were, bodily carried up towards the zenith.

In this new phase the aurora is transformed into a glorious crown of light, called the "Corona." From this corona diverge in all directions long streamers of different colors and forms, gracefully undulating in numerous folds, like so many banners of light. Some of the largest of these streamers appear like fringes composed of short transverse rays of different intensity and colors, producing the most fantastic effects, when traversed by the pulsations and coruscations which generally run across these rays during the great auroral displays.

The aurora has now attained its full development and beauty. It may continue in this form for half an hour, but usually the celestial fires begin to fade at the end of fifteen or twenty minutes, reviving from time to time, but gradually dying out. The northern sky usually appears covered by gray and luminous streaks and patches after a great aurora, these being occasionally rekindled, but more often they gradually disappear, and the sky resumes its usual appearance.

The number of auroras which develop a corona near the zenith is comparatively small in our latitudes; but many of them, although not exhibited on so grand a scale, are nevertheless very interesting. On some very rare occasions the auroral display has been confined almost exclusively to the dark segment, which appeared then as if pierced along its border by many square openings, like windows, through which appeared the bright auroral light.

PLATE IV.—AURORA BOREALIS.

As observed March 1, 1872, at 9h. 25m. P.M.

Among the many auroras which I have had occasion to observe, none are more interesting, excepting the type first described, than those which form an immense arch of light spanning the heavens from East to West. This form of aurora, which is quite rare, I last observed on September 12th, 1881. All the northern sky was covered with light vapors, when a small auroral patch appeared in the East at about 20° above the horizon. This patch of light, gradually increasing westward, soon reached the zenith, and continued its onward progress until it arrived at about 20° above the western horizon, where it stopped. The aurora then appeared as a narrow, wavy band of light, crossed by numerous parallel rays of different intensity and color. These rays seemed to have a rapid motion from West to East along the delicately-fringed streamer, which, on the whole, moved southward, while its extremities remained undisturbed. Aside from the apparent displacement of the fringes, a singular vibrating motion was observed in the auroral band, which was traversed by pulsations and long waves of light. The phenomena lasted for about twenty minutes, after which the arch was broken in many places, and it slowly vanished.

The aurora usually appears in the early part of the evening, and attains its full development between ten and eleven o'clock. Although the auroral light may have apparently ceased, yet the phenomenon is not at an end, as very often a solitary ray is visible from time to time; and even towards morning these rays sometimes become quite numerous. On some occasions the phenomenon even continues through the following day, and is manifested by the radial direction of the cirrus-clouds in the heights of our atmosphere. In 1872 I, myself, observed an aurora which apparently continued for two or three consecutive days and nights. In August, 1859, the northern lights remained visible in the United States for a whole week.

The height attained by these meteors is considerable, and it is now admitted that they are produced in the rarefied air of the upper regions of our atmosphere. From the researches of Professor Elias Loomis on the great auroras observed in August and September, 1859, it was ascertained that the inferior part of the auroral rays had an altitude of 46 miles, while that of their summits was 428 miles. These rays had, therefore, a length of 382 miles. From the observation of thirty auroral displays, it has been found that the mean height attained by the summit of these streamers above the Earth's surface was 450 miles.

But if the auroral streamers are generally manifested at great heights in our atmosphere, it would appear from the observations of persons living in the regions where the auroras are most frequent, as also from those who have been stationed in high northern and southern latitudes, that the phenomenon sometimes descends very low. Both Sabine and Parry saw the auroral rays projected on a distant mountain; Ross saw them almost at sea-level projected on the polar ice; while Wrangel, Franklin, and others observed similar phenomena. Dr. Hjaltalin, who has lived in latitude 64° 46' north, and has made a particular study of the aurora, on one occasion saw the aurora much below the summit of a hill 1,600 feet high, which was not very far off.

The same aurora is sometimes observed on the same night at places very far distant from one another. The great aurora borealis of August 28th, 1859, for instance, was seen over a space occupying 150° in longitude—from California to the Ural Mountains in Russia. It even appears now very probable that the phenomenon is universal on our globe, and that the northern lights observed in our hemisphere are simultaneous with the aurora australis of the southern hemisphere. The aurora of September 2d, 1859, was observed all through North and South America, the Sandwich Islands, Australia, and Africa; the streamers and pulsations of light of the north pole responding to the rays and coruscations of the south pole. Of thirty-four auroras observed at Hobart Town, in Tasmania, twenty-nine corresponded with aurora borealis observed in our hemisphere.

The auroral phenomena, although sometimes visible within the tropics, are, however, quite rare in these regions. For the most part they are confined within certain zones situated in high latitudes north and south. The zone where they are most frequent in our hemisphere forms an ellipse, which has the north pole at one of its foci; while the other is situated somewhere in North America, in the vicinity of the magnetic pole. The central line of the zone upon which the auroras seem to be most frequent passes from the northern coast of Alaska through Hudson's Bay and Labrador to Iceland, and then follows the northern coast of Europe and Asia. The number of auroras diminishes as the observer recedes from this zone, and it is only in exceptional cases that they are seen near the equator. Near the pole the phenomenon is less frequent than it is in the region described. In North America we occupy a favorable position for the observation of auroras, as we are nearer the magnetic poles than are the Europeans and Asiatics, and we consequently have a greater number of auroras in corresponding latitudes.

The position of the dark auroral segment varies with the place occupied by the observer, and its centre always corresponds with the magnetic meridian. In our Eastern States the auroral segment appears a little to the west of the north point; but as the observer proceeds westward it gradually approaches this point, and is due north when seen from the vicinity of Lake Winnipeg. At Point Barrow, in the extreme north-west of the United States, the aurora is observed in the east. In Melville Islands, Parry saw it in the south; while in Greenland it is directly in the west.

It is stated that auroras are more numerous about the equinoxes than they are at any other seasons; and also, when the earth is in perigee, than when it is in apogee. An examination which I have made of a catalogue by Professor Loomis, comprising 4,137 auroras observed in the temperate zone of our hemisphere from 1776 to 1873, sustains this statement. During this period, one hundred more auroras were recorded during each of the months comprising the equinoxes, than during any other months of the year; while eighty more auroras were observed when the earth was in perigee, than when it was in apogee. But to establish the truth of this assertion on a solid basis, more observations in both hemispheres will be required.

The aurora is not simply a terrestrial phenomenon, but is associated in some mysterious way with the conditions of the Sun's surface. It is a well-known fact that terrestrial magnetism is influenced directly by the Sun, which creates the diurnal oscillations of the magnetic needle. Between sunrise and two o'clock, the north pole of the needle moves towards the west in our northern hemisphere, and in the afternoon and evening it moves the other way. These daily oscillations of the needle are not uniform in extent; they have a period of regular increase and decrease. At a given place the daily oscillations of the magnetic needle increase and decrease with regularity during a period which is equal to 10⅓ years. As this period closely coincides with the Sun-spot period, the connection between the variation of the needle and these solar disturbances has been recognized.

Auroral phenomena generally accompany the extraordinary perturbations in the oscillations of the magnetic needle, which are commonly called "magnetic storms," and the greater the auroral displays, the greater are the magnetic perturbations. Not only is the needle subject to unusual displacements during an aurora, but its movements seem to be simultaneous with the pulsations and waving motions of the delicate auroral streamers in the sky. When the aurora sends forth a coruscation, or a streamer in the sky, the magnetic needle responds to it by a vibration. The inference that the auroral phenomena are connected with terrestrial magnetism is further supported by the fact that the centre of the corona is always situated exactly in the direction of that point in the heavens to which the dipping needle is directed.

It has been found that the aurora is a periodical phenomenon, and that its period corresponds very closely with those of the magnetic needle and Sun-spots. The years which have the most Sun-spots and magnetic disturbances have also the most auroras. There is an almost perfect similarity between the courses of the three sets of phenomena, from which it is concluded that the aurora is connected in some mysterious way with the action of the Sun, as well as with the magnetic condition of the earth.

A very curious observation, which has been supposed to have some connection with this subject, was made on Sept. 1st, 1859, by Mr. Carrington and Mr. Hodgson, in England. While these observers, who were situated many miles from one another, were both engaged at the same time in observing the same Sun-spot, they suddenly saw two luminous spots of dazzling brilliancy bursting into sight from the edge of the Sun-spot. These objects moved eastward for about five minutes, after which they disappeared, having then traveled nearly 34,000 miles. Simultaneously with these appearances, a magnetic disturbance was registered at Kew by the self-registering magnetic instruments. The very night that followed these observations, great magnetic perturbations, accompanied by brilliant auroral displays, were observed in Europe. A connection between the terrestrial magnetism and the auroral phenomena is further proved by the fact that, before the appearance of an aurora, the magnetic intensity of our globe considerably increases, but diminishes as soon as the first flashes show themselves.

The auroral phenomena are also connected in some way with electricity, and generate serious disturbances in the electric currents traversing our telegraphic lines, which are thus often rendered useless for the transmission of messages during great auroral displays. It sometimes happens, however, during such displays, that the telegraphic lines can be operated for a long distance, without the assistance of a battery; the aurora, or at least its cause, furnishing the necessary electric current for the working of the line. During auroras, the telephonic lines are also greatly affected, and all kinds of noises and crepitations are heard in the instruments.

Two observations of mine, which may have a bearing on the subject, present some interest, as they seem to indicate the action of the aurora on some of the clouds of our atmosphere. On January 6th, 1872, after I had been observing a brilliant aurora for over one hour, an isolated black cumulus cloud appeared at a little distance from the western extremity of the dark auroral segment. This cloud, probably driven by the wind, rapidly advanced eastward, and was soon followed by a succession of similar clouds, all starting from the same point. All these black clouds apparently followed the same path, which was not a straight line, but parallel to and concentric with the border of the dark auroral segment. When the first cloud arrived in the vicinity of the magnetic meridian passing through the middle of the auroral arc, it very rapidly dissolved, and on reaching this meridian became invisible. The same phenomenon was observed with the succession of black clouds following, each rapidly dissolving as it approached the magnetic meridian. This phenomenon of black clouds vanishing like phantoms in crossing the magnetic meridian, was observed for nearly an hour. On June 17th, 1879, I observed a similar phenomenon during a fine auroral display. About midway between the horizon and the polar star, but a little to the west of the magnetic meridian, there was a large black cumulo-stratus cloud which very slowly advanced eastward. As it progressed in that direction, its eastern extremity was dissolved in traversing the magnetic meridian; while, at the same time, several short and quite bright auroral rays issued from its western extremity, which in its turn dissolved rapidly, as if burned or melted away in the production of the auroral flame.

It seems to be a well observed fact, that during auroras, a strong sulphurous odor prevails in high northern latitudes. According to Dr. Hjaltalin, during these phenomena, "the ozone of the atmosphere increases considerably, and men and animals exposed out of doors emit a sulphurous odor when entering a heated room." The Esquimaux and other inhabitants of the northern regions assert that great auroras are sometimes accompanied by crepitations and crackling noises of various sorts. Although these assertions have been denied by several travelers who have visited the regions of these phenomena, they are confirmed by many competent observers. Dr. Hjaltalin, who has heard these noises about six times in a hundred observations, says that they are especially audible when the weather is clear and calm; but that when the atmosphere is agitated they are not heard. He compares them to the peculiar sound produced by a silk cloth when torn asunder, or to the crepitations of the electric machine when its motion is accelerated. "When the auroral light is much agitated and the streamers show great movements, it is then that these noises are heard at different places in the atmosphere."

The spectrum of the auroral light, although it varies with almost every aurora, always shows a bright green line on a faint continuous spectrum. In addition to this green line I have frequently observed four broad diffused bands of greater refrangibility in the spectra of some auroras. In two cases, when the auroras appeared red towards the west, the spectrum showed a bright red line, in addition to the green line and the broad bands described. These facts evidently show that the light of the aurora is due to the presence of luminous vapors in our atmosphere; and it may reasonably be supposed that these vapors are rendered luminous by the passage of electric discharges through them.

[THE ZODIACAL LIGHT]
PLATE V

In our northern latitudes may be seen, on every clear winter and spring evening, a column of faint, whitish, nebulous light, rising obliquely above the western horizon. A similar phenomenon may also be observed in the east, before day-break, on any clear summer or autumn night. To this pale, glimmering luminosity the name of "Zodiacal Light" has been given, from the fact that it lies in the zodiac along the ecliptic.

In common with all the celestial bodies, the zodiacal light participates in the diurnal motion of the sky, and rises and sets with the constellations in which it appears. Aside from this apparent motion, it is endowed with a motion of its own, accomplished from west to east, in a period of a year. In its motion among the stars, the zodiacal light always keeps pace with the Sun, and appears as if forming two faint luminous wings, resting on opposite sides of this body. In reality it extends on each side of the Sun, its axis lying very nearly in the plane of the ecliptic.

In our latitudes the phenomena can be observed most advantageously towards the equinoxes, in March and September, when twilight is of short duration. As we proceed southward it becomes more prominent, and gradually increases in size and brightness. It is within the tropical regions that the zodiacal light acquires all its splendor: there it is visible all the year round, and always appears very nearly perpendicular to the horizon, while at the same time its proportions and brilliancy are greatly increased.

PLATE V.—THE ZODIACAL LIGHT.

Observed February 20, 1876

The zodiacal light appears under the form of a spear-head, or of a narrow cone of light whose base apparently rests on the horizon, while its summit rises among the zodiacal constellations. In general appearance it somewhat resembles the tail of a large comet whose head is below the horizon. The most favorable time to observe this phenomenon in the evening, is immediately after the last trace of twilight has disappeared; and in the morning, one or two hours before twilight appears. When observed with attention, it is seen that the light of the zodiacal cone is not uniform, but gradually increases in brightness inwardly, especially towards its base, where it sometimes surpasses in brilliancy the brightest parts of the Milky-Way. In general, its outlines are vague and very difficult to make out, so gradually do they blend with the sky. On some favorable occasions, the luminous cone appears to be composed of several distinct concentric conical layers, having different degrees of brightness, the inner cone being the most brilliant of all. There is a remarkable distinction between the evening and morning zodiacal light. In our climate, the morning light is pale, and never so bright nor so extended as the evening light.

In general, the zodiacal light is whitish and colorless, but in some cases it acquires a warm yellowish or reddish tint. These changes of color may be accidental and due to atmospheric conditions, and not to actual change in the color of the object. Although the zodiacal light is quite bright, and produces the impression of having considerable depth, yet its transparency is great, since all the stars, except the faint ones, can be seen through its substance.

The zodiacal light is subject to considerable variations in brightness, and also varies in extent, the apex of its cone varying in distance from the Sun's place, from 40 to 90 degrees. These variations cannot be attributed to atmospheric causes alone, some of them being due to real changes in the zodiacal light itself, whose light and dimensions increase or decrease under the action of causes at present unknown. From the discussion of a series of observations on the zodiacal light made at Paris and Geneva, it appears certain that its light varies from year to year, and sometimes even from day to day, independently of atmospheric causes. Some of my own observations agree with these results, and one of them, at least, seems to indicate changes even more rapid. On December 18th, 1875, I observed the zodiacal light in a clear sky free from any vapors, at six o'clock in the evening. At that time, the point of its cone was a little to the north of the ecliptic, at a distance of about 90 degrees from the Sun's place. Ten minutes later, its summit had sunk down 35 degrees, the cone then being reduced to nearly one-half of its original dimensions. Ten minutes later, it had risen 25 degrees, and was then 80 degrees from the Sun's place, where it remained all the evening. On March 22d, 1878, the sky was very clear and the zodiacal light was bright when I observed it, at eight o'clock. At that moment the apex of the cone of light was a little to the south of the Pleiades, but this cone presented an unusual appearance never noticed by me before, its northern border appearing much brighter and sharper than usual, while at the same time its axis of greatest brightness appeared to be much nearer to this northern border than it was to the southern. After a few minutes of observation it became evident that the northern border was extending itself, as stars which were at some distance from it became gradually involved in its light. At the same time that this border spread northward, it seemed to diffuse itself, and after a time the cone presented its usual appearance, having its southern border brighter and better defined than the other. It would have been impossible to attribute this sudden change to an atmospheric cause, since only one of the borders of the cone participated in it, and since some very faint stars near this northern border were not affected in the least while the phenomenon occurred. Besides these observations, Cassini, Mairan, Humboldt, and many other competent observers have seen pulsations, coruscations and bickerings in the light of the cone, which they thought could not be attributed to atmospheric causes. It has also been observed that at certain periods the zodiacal light has shone with unusual intensity for months together.

When this phenomenon is observed from the tropical regions, it is found that its axis of symmetry always corresponds with its axis of greatest brightness, and that both lie in the plane of the ecliptic, which divides its cone into two equal parts. But when the zodiacal light is observed in our latitude, the axis of symmetry does not correspond with the axis of greatest brightness, and both axes are a little to the north of this plane, the axis of symmetry being the farther removed. Furthermore, as already stated, the southern border of the cone always appears better defined and brighter than the corresponding northern margin. It is very probable, if not absolutely certain, that these phenomena are exactly reversed when the zodiacal light is observed from corresponding latitudes in the southern hemisphere, and that there, its axes, both of symmetry and of greatest brightness, appear south of the ecliptic, while the northern margin is the brightest. This seems to be established by the valuable observations of Rev. George Jones, made on board the U. S. steam frigate Mississippi, in California, Japan, and the Southern Ocean. "When I was north of the ecliptic," says this observer, "the greatest part of the light of the cone appeared to the north of this line; when I was to the south of the ecliptic, it appeared to be south of it; while when my position was on the ecliptic, or in its vicinity, the zodiacal cone was equally divided by this line."

Besides the zodiacal light observed in the East and West, some observers have recognized an exceedingly faint, luminous, gauzy band, about 10 or 12 degrees wide, stretching along the ecliptic from the summit of the western to that of the eastern zodiacal cone. This faint narrow belt has been called the Zodiacal Band. It has been recognized by Mr. H. C. Lewis, who has made a study of this phenomenon, that the zodiacal band has its southern margin a little brighter and a little sharper than the northern border. This observation is in accordance with similar phenomena observed in the zodiacal light, and may have considerable importance.

In 1854, Brorsen recognized a faint, roundish, luminous spot in a point of the heavens exactly opposite to the place occupied by the Sun, which he has called "Gegenschein," or counter-glow. This luminous spot has sometimes a small nucleus, which is a little brighter than the rest. Night after night this very faint object shifts its position among the constellations, keeping always at 180 degrees from the Sun. The position of the counter-glow, like that of the zodiacal light and zodiacal band, is not precisely on the plane of the ecliptic, but a little to the north of this line. It is very probable that near the equator the phenomenon would appear different and there would correspond with this plane.

There seems to be some confusion among observers in regard to the spectrum of the zodiacal light. Some have seen a bright green line in its spectrum, corresponding to that of the aurora borealis; while others could only see a faint grayish continuous spectrum, which differs, however, from that of a faint solar light, by the fact that it presents a well-defined bright zone, gradually blending on each side with the fainter light of the continuous spectrum. I have, myself, frequently observed the faint continuous spectrum of the zodiacal light, and on one occasion recognized the green line of the aurora; but it might have been produced by the aurora itself, as yet invisible to the eye, and not by the zodiacal light, since, later in the same evening, there was a brilliant auroral display. If it were demonstrated that this green line exists in the spectrum of the zodiacal light, the fact would have importance, as tending to show that the aurora and the zodiacal light have a common origin.

Rev. Geo. Jones describes a very curious phenomenon which he observed several times a little before the moon rose above the horizon. The phenomenon consisted in a short, oblique, luminous cone rising from the Moon's place in the direction of the ecliptic. This phenomenon he has called the Moon Zodiacal Light. In 1874, I had an opportunity to observe a similar phenomenon when the Moon was quite high in the sky. By taking the precaution to screen the Moon's disk by the interposition of some buildings between it and my eye, I saw two long and narrow cones of light parallel to the ecliptic issuing from opposite sides of our satellite. The phenomenon could not possibly be attributed to vapors in our atmosphere, since the sky was very clear at the moment of the observation. Later on, these appendages disappeared with the formation of vapors near the Moon, but they reappeared an hour later, when the sky had cleared off, and continued visible for twenty minutes longer, and then disappeared in a clear sky.

Although the zodiacal light has been studied for over two centuries, no wholly satisfactory explanation of the phenomenon has yet been given. Now, as in Cassini's time, it is generally considered by astronomers to be due to a kind of lens-shaped ring surrounding the Sun, and extending a little beyond the Earth's orbit. This ring is supposed to lie in the plane of the ecliptic, and to be composed of a multitude of independent meteoric particles circulating in closed parallel orbits around the Sun. But many difficulties lie in the way of this theory. It seems as incompetent to explain the slow and rapid changes in the light of this object as it is to explain the contractions and extensions of its cone. It fails, moreover, to explain the flickering motions, the coruscations observed in its light, or the displacement of its cone and of its axes of brightness and symmetry by a mere change in the position of the observer. Rev. Geo. Jones, unable to explain by this theory the phenomena which came under his observation, has proposed another, which supposes the zodiacal light to be produced by a luminous ring surrounding the Earth, this ring not extending as far as the orbit of the Moon. But this theory also fails in many important points, so that at present no satisfactory explanation of the phenomenon can be given.

As the phenomenon is connected in some way with the Sun, and as we have many reasons to believe this body to be always more or less electrified, it might be supposed that the Sun, acting by induction on our globe, develops feeble electric currents in the rarefied gases of the superior regions of our atmosphere, and there forms a kind of luminous ridge moving with the Sun in a direction contrary to the diurnal motion, and so producing the zodiacal light. On this hypothesis, the counter-glow would be the result of a smaller cone of light generated by the solar induction on the opposite point of the Earth.

Plate 5, which sufficiently explains itself, represents the zodiacal light as it appeared in the West on the evening of February 20th, 1876. All the stars are placed in their proper position, and their relative brightness is approximately shown by corresponding variations in size—the usual and almost the only available means of representation. Of course, it must be remembered that a star does not, in fact, show any disk even in the largest telescopes, where it appears as a mere point of light, having more or less brilliancy. The cone of light rises obliquely along the ecliptic, and the point forming its summit is found in the vicinity of the well-known group of stars, called the Pleiades, in the constellation of Taurus, or the Bull.

[THE MOON]
PLATE VI

In its endless journey through space, our globe is not solitary, like some of the planets, but is attended by the Moon, our nearest celestial neighbor. Although the Moon does not attain to the dignity of a planet, and remains a secondary body in the solar system, yet, owing to its proximity to our globe, and to the great influence it exerts upon it by its powerful attraction, it is to us one of the most important celestial bodies.

While the Moon accompanies the Earth around the Sun, it also revolves around the Earth at a mean distance of 238,800 miles. For a celestial distance this is only a trifling one; the Earth in advancing on its orbit travels over such a distance in less than four hours. A cannon ball would reach our satellite in nine days; and a telegraphic dispatch would be transmitted there in 1½ seconds of time, if a wire could be stretched between us and the Moon.

Owing to the ellipticity of the Moon's orbit, its distance from the Earth varies considerably, our satellite being sometimes 38,000 miles nearer to us than it is at other times. These changes in the distance of the Moon occasion corresponding changes from 29' to 33' in its apparent diameter. The real diameter of the Moon is 2,160 miles, or a little over one-quarter the diameter of our globe; our satellite being 49 times smaller than the Earth.

The mean density of the materials composing the Moon is only ⁶⁄₁₀ that of the materials composing the Earth, and the force of gravitation at the surface of our satellite is six times less than it is at the surface of our globe. If a person weighing 150 lbs. on our Earth could be transported to the Moon, his weight there would be only 25 lbs.

The Moon revolves around the Earth in about 27⅓ days, with a mean velocity of one mile per second, the revolution constituting its sidereal period. If the Earth were motionless, the lunar month would be equal to the sidereal period; but owing to its motion in space, the Sun appears to move with the Moon, though more slowly, so that after having accomplished one complete revolution, our satellite has yet to advance 2¼ days before reaching the same apparent position in regard to the Earth and the Sun that it had at first. The interval of time comprised between two successive New Moons, which is a little over 29½ days, constitutes the synodical period of the Moon, or the lunar month.

The Moon is not a self-luminous body, but, like the Earth and the planets, it reflects the light which it receives from the Sun, and so appears luminous. That such is the case is sufficiently demonstrated by the phases exhibited by our satellite in the course of the lunar month. Every one is familiar with these phases, which are a consequence of the motion of the Moon around the Earth. When our satellite is situated between us and the Sun, it is New Moon; since we cannot see its illuminated side, which is then turned away from us towards the Sun. When, on the contrary, it reaches that point of its orbit which, in regard to us, is opposite to the Sun's place, it is Full Moon; since from the Earth we can only see the fully illuminated side of our satellite. Again, when the Moon arrives at either of the two opposite points of its orbit, the direction of which from the Earth is at right angles with that of the Sun, it is either the First or the Last Quarter; since in these positions we can only see one-half of its illuminated disk.

The curve described by the Moon around the Earth lies approximately in a plane, this plane being inclined about 5° to the ecliptic. Since our satellite, in its motion around us and the Sun, closely follows the ecliptic, which is inclined 23½° to the equator, it results that when this plane is respectively high or low in the sky, the moon is also high or low when crossing the meridian of the observer. In winter that part of the ecliptic occupied by the Sun is below the equator, and, consequently, the New Moons occurring in that season are low in the sky, since at New Moon our satellite must be on the same side of the ecliptic with the Sun. But the Full Moons in the same season are necessarily high in the sky, since a Full Moon can only occur when our satellite is on the opposite side of the ecliptic from the Sun, in which position it is, of course, as many degrees above the equator as the Sun is below. The Full Moon which happens nearest to the autumnal equinox is commonly called the Harvest Moon, from the fact that, after full, its delays in rising on successive evenings are very brief and therefore favorable for the harvest work in the evening. The same phenomenon occurs in every other lunar month, but not sufficiently near the time of Full Moon to be noticeable. When, in spring, a day or two after New Moon, our satellite begins to show its thin crescent, its position on the ecliptic is north as well as east of that occupied by the Sun; hence, its horns are nearly upright in direction, and give it a crude resemblance to a tipping bowl, from which many people who are unaware of its cause, and that this happens every year, draw conclusions as to the amount of rain to be expected.

One of the most remarkable features of the Moon's motions is that our satellite rotates on its axis in exactly the same period of time occupied by its revolution around the Earth, from which it results that the Moon always presents to us the same face. To explain this peculiarity, astronomers have supposed that the figure of our satellite is not perfectly spherical, but elongated, so that the attraction of the Earth, acting more powerfully upon its nearest portions, always keeps them turned toward us, as if the Moon were united to our globe by a string. It is not exactly true, however, that the Moon always presents its same side to us, although its period of rotation exactly equals that of its revolution; since in consequence of the inclination of its axis of rotation to its orbit, combined with the irregularities of its orbital motion about us, apparent oscillations in latitude and in longitude, called librations, are created, from which it results that nearly ⁶⁄₁₀ of the Moon's surface is visible from the Earth at one time or another.

The Moon is a familiar object, and every one is aware that our satellite, especially when it is fully illuminated, presents a variety of bright and dark markings, which, from their distant resemblance to a human face, are popularly known as "the man in the moon." A day or two after New Moon, when the thin crescent of our satellite is visible above the western horizon after sunset, the dark portion of its disk is plainly visible, and appears of a pale, ashy gray color, although not directly illuminated by the Sun. This phenomenon is due to the Earth-shine, or to that portion of solar light which the illuminated surface of our globe reflects to the dark side of the Moon, exactly in the same manner that the Moon-shine, on our Earth, is due to the solar light reflected to our globe by the illuminated Moon.

Seen with a telescope of moderate power, or even with a good opera-glass, the Moon presents a peculiar mottled appearance, and has a strong resemblance to a globe made of plaster of Paris, on the surface of which numerous roundish, saucer-shaped cavities of various sizes are scattered at random. This mottled structure is better seen along the boundary line called the terminator, which divides the illuminated from the dark side of the Moon. The line of the terminator always appears jagged, and it is very easy to recognize that this irregularity is due to the uneven and rugged structure of the surface of our satellite.

A glance at the Moon through a larger telescope shows that the bright spots recognized with the naked eye belong to very uneven and mountainous regions of our satellite, while the dark ones belong to comparatively smooth, low surfaces, comparable to those forming the great steppes and plains of the Earth. When examined with sufficient magnifying power, the white, rugged districts of the Moon appear covered over by numerous elevated craggy plateaus, mountain-chains, and deep ravines; by steep cliffs and ridges; by peaks of great height and cavities of great depth. This rugged formation, which is undoubtedly of volcanic origin, gives our satellite a desolate and barren appearance. The rugged tract occupies more than one-half of the visible surface of the Moon, forming several distinct masses, the principal of which occupy the south and south-western part of the disk. That this formation is elevated above the general level is proved by the fact that the mountains, peaks, and other objects which compose it, all cast a shadow opposite to the Sun; and further, that the length of these shadows diminishes with the elevation of the Sun above the lunar horizon.

Since Galileo's time the surface of the Moon has been studied by a host of astronomers, and accurate maps of its topographical configuration have been made, and names given to all features of any prominence. It may even be said that in its general features, the visible surface of our satellite is now better known to us than is the surface of our own Earth.

One of the most striking and common features of the mountainous districts of the Moon, is the circular, ring-like disposition of their elevated parts, which form numerous crater-like objects of different sizes and depths. Many thousands of crater-like objects are visible on the Moon through a good telescope, and, considering how numerous the small ones are, there is, perhaps, no great exaggeration in fixing their number at 50,000, as has been done by some astronomers. These volcanic regions of the Moon cannot be compared to anything we know, and far surpass in extent those of our globe. The number and size of the craters of our most important volcanic regions in Europe, in Asia, in North and South America, in Java, in Sumatra, and Borneo, are insignificant when compared with those of the Moon. The largest known craters on the Earth give only a faint idea of the magnitude of some of the lunar craters. The great crater Haleakala, in the Sandwich Islands, probably the largest of the terrestrial volcanoes, has a circumference of thirty miles, or a diameter of a little less than ten miles. Some of the great lunar craters, called walled plains, such as Hipparchus, Ptolemæus, etc., have a diameter more than ten times larger than that of Haleakala, that of the first being 115 miles and that of the last 100 miles. These are, of course, among the largest of the craters of the Moon, although there are on our satellite a great number of craters above ten miles in diameter.

The crater-forms of the Moon have evidently appeared at different periods of time, since small craters are frequently found on the walls of larger ones; and, indeed, still smaller craters are not rarely seen on the walls of these last. The walls of the lunar craters are usually quite elevated above the surrounding surface, some of them attaining considerable elevations, especially at some points, which form peaks of great height. Newton, the loftiest of all, rises at one point to the height of 23,000 feet, while many others range from ten to twenty thousand feet in height. Several craters have their floor above the general surface—Plato, for instance. Wargentin has its floor nearly on a level with the summit of its walls, showing that at some period of its history liquid lavas, ejected from within, have filled it to the brim and then solidified. The floors of some of the craters are smooth and flat, but in general they are occupied by peaks and abrupt mountainous masses, which usually form the centre. Many of their outside walls are partly or wholly covered by numerous ravines and gullies, winding down their steep declivities, branching out and sometimes extending to great distances from their base. It would seem that these great volcanic mouths have at some time poured out torrents of lavas, which, in their descent, carved their passage by the deep gullies now visible. Sometimes, also, the crater slopes are strewn with debris, giving them a peculiar volcanic appearance.

Notwithstanding their many points of similarity with the volcanoes of the Earth, the lunar craters differ from them in many particulars, showing that volcanic forces acting on different globes may produce widely different results. For example, the floors of terrestrial craters are usually situated at considerable elevations above the general surface, while those of the lunar craters are generally much depressed, the height of their walls being only about one-half the depth of their cavities. Again, while on the Earth the mass of the volcanic cones far exceeds the capacity of their openings, on the Moon it is not rare to see the capacity of the crater cavities exceeding the mass of the surrounding walls. On the Earth, the volcanic cones and mouths are comparatively regular and smooth, and are generally due to the accumulation of the ashes and the debris of all kinds which are ejected from the volcanic mouths. On the Moon, very few craters show this character, and for the most part their walls have a very different structure, being irregular, very rugged, and composed of a succession of concentric ridges, rising at many points to great elevations, and forming peaks of stupendous height. Again, many of the larger terrestrial craters have their interior occupied by a central cone, or several such cones, having a volcanic mouth on their summits; on the Moon such central cones are very rare. Although many of the large lunar craters have their interior occupied by central masses which have been often compared to the central cones of our great volcanoes, yet these objects have a very different character and origin. For the most part, they are mountainous masses of different forms—having very rarely any craters on them—and seem to have resulted from the crowding and lifting up of the crater floor by the phenomena of subsidence, of which these craters show abundant signs. Besides, the terrestrial craters are characterized by large and important lava streams, while on the Moon the traces of such phenomena are quite rare, and when they are shown, they generally differ from those of the Earth by their numerous and complicated ramifications, and also by the fact that many of these lava streamlets take their origin at a considerable distance from the crater slopes, and are grooved and depressed as if the burning liquids which are supposed to have produced them had subsequently disappeared, by evaporation or otherwise, leaving the furrow empty.

The dark spots of the Moon, when viewed through a telescope, exhibit a totally different character, and show that they belong to a different formation from that of the brighter portions. These darker tracts do not seem to have had a direct volcanic origin like the latter, but rather appear to have resulted from the solidification of semi-fluid materials, which have overflowed vast areas at different times. The surface of this system is comparatively smooth and uniform, only some small craters and low ridges being seen upon it. The level and dark appearance of these areas led the ancient astronomers to the belief that they were produced by a liquid strongly absorbing the rays of light, and were seas like our seas. Accordingly, these dark surfaces were called Maria, or Seas, a name which it is convenient to retain, although it is well known to have originated in an error. The so-called seas of the Moon are evidently large flat surfaces similar to the deserts, steppes, pampas, and prairies of the Earth in general appearance. The great plains of the Moon are at a lower level than that of the other formation, and that which first attracts the observer's attention is the fact that they are surrounded almost on all sides by an irregular line of abrupt cliffs and mountain chains, showing phenomena of dislocation. This character of dislocation, which is general, and is visible everywhere upon the contours of the plains, seems to indicate that phenomena of subsidence, either slow or rapid, have occurred on the Moon; while, at the same time, the sunken surfaces were overflowed by a semi-fluid liquid, which solidified afterwards. The evidences of subsidence and overflowing become unmistakable when we observe that, along the borders of the gray plains, numerous craters are more or less embedded in the gray formation, only parts of the summit of their walls remaining visible, to attest that once large craters existed there. The farther from the border of the plain the vestiges of these craters are observed, the deeper they are embedded in the gray formation. That phenomena of subsidence have occurred on a grand scale on the Moon, is further indicated by the fact that the singular systems of fractures called clefts and rifts generally follow closely the outside border of the gray plains, often forming parallel lines of dislocation and fractures. In the interior regions of the gray formation, these fractures are comparatively rare.

The gray, lava-like formation is obviously of later origin than the mountainous system to which belong the embedded craters above described. Its comparatively recent origin might also be inferred from the smallness of its craters and its low ridges. The few large craters observed on this formation evidently belong to the earlier system.

The color of this system of gray plains is far from being uniform. In general appearance it is of a bluish gray, but when observed attentively, large areas appear tinted with a dusky olive-green, while others are slightly tinged with yellow. Some patches appear brownish, and even purplish. A remarkable example of the first case is seen on the surface, which encloses within a large parallelogram the two conspicuous craters, Aristarchus and Herodotus. This surface evidently belongs to a different system from that of the Oceanus Procellarum surrounding it, as, besides its color, which totally differs from that of the gray formation, its surface shows the rugged structure of the volcanic formation.

When the Moon is full, some very curious white, luminous streaks are seen radiating from different centres, which, for the most part, are important craters, occupied by interior mountains. The great crater Tycho is the centre of the most imposing of the systems of white streaks. Some of the diverging rays of this great centre extend to a distance equal to one-quarter of the Moon's circumference, or about 1,700 miles. The true nature of these luminous streaks is unknown, but it seems certain that they have their origin in the crater from which they diverge. They do not form any relief on the surface, and are seen going up over the mountains and steep walls of the crater, as well as down the ravines and on the floors of craters.

The Moon seems to be deprived of an atmosphere; or, if it has any, it must be so excessively rare that its density is less than of the density of the Earth's atmosphere, since delicate tests afforded by the occultation of stars have failed to reveal its presence. Although no atmosphere of any consequence exists on the Moon, yet phenomena which I have observed seem to indicate the occasional presence there of vapors of some sort. On several occasions, I have seen a purplish light over some parts of the Moon, which prevented well-known objects being as distinctly seen as they were at other times, causing them to appear as if seen through a fog. One of the most striking of these observations was made on January 4th, 1873, on the crater Kant and its vicinity, which then appeared as if seen through luminous purplish vapors. On one occasion, the great crater Godin, which was entirely involved in the shadow of its western wall, appeared illuminated in its interior by a faint purplish light, which enabled me to recognize the structure of this interior. The phenomenon could not be attributed in this case to reflection, since the Sun, then just rising on the western wall of the crater, had not yet grazed the eastern wall, which was invisible. It is not impossible that a very rare atmosphere composed of such vapors exists in the lower parts of the Moon.

If the Moon has no air, and no liquids of any sort, it seems impossible that its surface can maintain any form of life, either vegetable or animal, analogous to those on the Earth. In fact, nothing indicating life has been detected on the Moon—our satellite looking like a barren, lifeless desert. If life is to be found there at all, it must be of a very elementary nature. Aside from the want of air and water to sustain it, the climatic conditions of our satellite are very unfavorable for the development of life. The nights and days of the Moon are each equal to nearly fifteen of our days and nights. For fifteen consecutive terrestrial days the Sun's light is absent from one hemisphere of the Moon; while for the same number of days the Sun pours down on the other hemisphere its light and heat, the effects of which are not in any way mitigated by an atmosphere. During the long lunar nights the temperature must at least fall to that of our polar regions, while during its long days it must be far above that of our tropical zone. It has been calculated that during the lunar nights the temperature descends to 23° below zero, while during the days it rises to 468°, or 256° above the boiling point.

It has been a question among astronomers whether changes are still taking place at the surface of the Moon. Aside from the fact that change, not constancy, is the law of nature, it does not seem doubtful that changes occur on the Moon, especially in view of the powerful influences of contraction and dilatation to which its materials are submitted by its severe alternations of temperature. From the distance at which we view our satellite, we cannot expect, of course, to be able to see changes, unless they are produced on a large scale. Theoretically speaking, the largest telescopes ever constructed ought to show us the Moon as it would appear to the naked eye from a distance of 40 miles; but in practice it is very different. The difficulty is in the fact that, while we magnify the surface of a telescopic image, we are unable to increase its light; so that, practically, in magnifying an object, we weaken its light proportionally to the magnifying power employed. The light of the Moon, especially near the terminator, where we almost always make our observations, is not sufficiently bright to bear a very high magnifying power, and only moderate ones can be applied to its study. What we gain by enlarging an object, we more than lose by the weakening of its light. Besides, a high magnifying power, by increasing the disturbances generally present in our atmosphere, renders the telescopic image unsteady and very indistinct. On the whole, the largest telescopes now in existence do not show us our satellite better than if we could see it with the naked eye from a distance of 300 miles or more. At such a distance only considerable changes would be visible.

Notwithstanding these difficulties, it is believed that changes have been detected in Linné, Marius, Messier, and several other craters. An observation of mine seems to indicate that changes have recently taken place in the great crater Eudoxus. On February 20th, 1877, between 9h. 30m. and 10h. 30m., I observed a straight, narrow wall crossing this crater from east to west, a little to the south of its centre. This wall had a considerable elevation, as was proved by the shadow it cast on its northern side. Towards its western end this wall appeared as a brilliant thread of light on the black shadow cast by the western wall of the crater. The first time I had occasion to observe this crater again, after this observation, was a year later, on February 17th, 1878; no traces of the wall were then detected. Many times since I have tried to find this narrow wall again, when the Moon presented the same phase and the same illumination, but always with negative results. It seems probable that this structure has crumbled down, yet it is very singular that so prominent a feature should not have been noticed before.

PLATE VI.—MARE HUMORUM.

From a study made in 1875

The "Mare Humorum," or sea of moisture, as it is called, which is represented on Plate VI., is one of the smaller gray lunar plains. Its diameter, which is very nearly the same in all directions, is about 270 miles, the total area of this plain being about 50,000 square miles. It is one of the most distinct plains of the Moon, and is easily seen with the naked eye on the left-hand side of the disk. The floor of the plain is, like that of the other gray plains, traversed by several systems of very extended but low hills and ridges, while small craters are disseminated upon its surface. The color of this formation is of a dusky greenish gray along the border, while in the interior it is of a lighter shade, and is of brownish olivaceous tint. This plain, which is surrounded by high clefts and rifts, well illustrates the phenomena of dislocation and subsidence. The double-ringed crater Vitello, whose walls rise from 4,000 to 5,000 feet in height, is seen in the upper left-hand corner of the gray plain. Close to Vitello, at the east, is the large broken ring-plain Lee, and farther east, and a little below, is a similarly broken crater called Doppelmayer. Both of these open craters have mountainous masses and peaks on their floor, which is on a level with that of the Mare Humorum. A little below, and to the left of these objects, is seen a deeply embedded oval crater, whose walls barely rise above the level of the plain. On the right-hand side of the great plain, is a long fault, with a system of fracture running along its border. On this right-hand side, may be seen a part of the line of the terminator, which separates the light from the darkness. Towards the lower right-hand corner, is the great ring-plain Gassendi, 55 miles in diameter, with its system of fractures and its central mountains, which rise from 3,000 to 4,000 feet above its floor. This crater slopes southward towards the plain, showing the subsidence to which it has been submitted. While the northern portion of the wall of this crater rises to 10,000 feet, that on the plain is only 500 feet high, and is even wholly demolished at one place where the floor of the crater is in direct communication with the plain. In the lower part of the mare, and a little to the west of the middle line, is found the crater Agatharchides, which shows below its north wall the marks of rills impressed by a flood of lava, which once issued from the side of the crater. On the left-hand side of the plain, is seen the half-demolished crater Hippalus, resembling a large bay, which has its interior strewn with peaks and mountains. On this same side can be seen one of the most important systems of clefts and fractures visible on the Moon, these clefts varying in length from 150 to 200 miles.

[ECLIPSES OF THE MOON]
PLATE VII

Since the Moon is not a self-luminous body, but shines by the light which it borrows from the Sun, it follows that when the Sun's light is prevented from reaching its surface, our satellite becomes obscured. The Earth, like all opaque bodies exposed to sunlight, casts a shadow in space, the direction of which is always opposite to the Sun's place. The form of the Earth's shadow is that of a long, sharply-pointed cone, which has our globe for its base. Its length, varying with the distance of the Earth from the Sun, is, on an average, 855,000 miles, or 108 times the terrestrial diameter. This conical shadow of the Earth, divided longitudinally by the plane of the ecliptic, lies half above and half below that plane, on which the summit of the shadow describes a whole circumference in the course of a year. If the Moon's orbit were not inclined to the ecliptic, our satellite would pass at every Full Moon directly through the Earth's shadow; but, owing to that inclination, it usually passes above or below the shadow. Twice, however, during each of its revolutions, it must cross the plane of the ecliptic, the points of its orbit where this happens being called nodes. Accordingly, if it is near a node at the time of Full Moon, it will enter the shadow of the Earth, and become either partly or wholly obscured, according to the distance of its centre from the plane of the ecliptic. The partial or total obscuration of the Moon's disk thus produced constitutes a partial or total eclipse of the Moon. The essential conditions for an eclipse of the Moon are, therefore, that our satellite must not only be full, but must also be at or very near one of its nodes.

Although inferior in importance to the eclipses of the Sun, the eclipses of the Moon are, nevertheless, very interesting and remarkable phenomena, which never fail to produce a deep impression on the mind of the observer, inasmuch as they give him a clear insight into the silent motions of the planetary bodies.

At the mean distance of the Moon from the Earth, the diameter of the conical shadow cast in space by our globe is more than twice as large as that of our satellite. But, besides this pure dark shadow of the Earth, its cone is enveloped by a partial shadow called "Penumbra," which is produced by the Sun's light being partially, but not wholly, cut off by our globe.

While the Moon is passing into the penumbra, a slight reduction of the light of that part of the disk which has entered it, is noticeable. As the progress of the Moon continues, the reduction becomes more remarkable, giving the impression that rare and invisible vapors are passing over our satellite. Some time after, a small dark-indentation, marking the instant of first contact, appears on the eastern or left-hand border of the Moon, which is always the first to encounter the Earth's shadow, since our satellite is moving from west to east. The dark indentation slowly and gradually enlarges with the onward progress of the Moon into the Earth's shadow, while the luminous surface of its disk diminishes in the same proportion. The form of the Earth's shadow on the Moon's disk clearly indicates the rotundity of our globe by its circular outline. Little by little the dark segment covers the Moon's disk, and its crescent, at last reduced to a mere thread of light, disappears at the moment of the second contact. With this the phase of totality begins, our satellite being then completely involved in the Earth's shadow.

The Moon remains so eclipsed for a period of time which varies with its distance from the Earth, and with the point of its orbit where it crosses the conical shadow. When it passes through the middle of this shadow, while its distance from our globe is the least, the total phase of an eclipse of the Moon may last nearly two hours. The left-hand border of our satellite having gone first into the Earth's shadow, is also the first to emerge, and, at the moment of doing so, it receives the Sun's light, and totality ends with the third contact. The lunar crescent gradually increases in breadth after its exit from the shadow, and finally the Moon recovers its fully illuminated disk as before, at the moment its western border leaves the Earth's shadow. Soon after, it passes out of the penumbra, and the eclipse is over. In total eclipses, the interval of time from the first to last contact may last 5h. 30m, but it is usually shorter.

Soon after the beginning of an eclipse, the dark segment produced by the Earth's shadow on the Moon's disk generally appears of a dark grayish opaque color, but with the progress of the phenomenon, this dark tint is changed into a dull reddish color, which, gradually increasing, attains its greatest intensity when the eclipse is total. At that moment the color of the Moon is of a dusky, reddish, coppery hue, and the general features of the Moon's surface are visible as darker and lighter tints of the same color. It sometimes happens, however, that our satellite does not exhibit this peculiar coppery tint, but appears either blackish or bluish, in which case it is hardly distinguishable from the sky.

It is very rare for the Moon to disappear completely during totality, and even when involved in the deepest part of the Earth's shadow, our satellite usually remains visible to the naked eye, or, at least, to the telescope. This phenomenon is to be attributed to the fact that the portion of the solar rays which traverse the lower strata of our atmosphere are strongly refracted, and bend inward in such a manner that they fall on the Moon, and sufficiently illuminate its surface to make it visible. The reddish color observed is caused by the absorption of the blue rays of light by the vapors which ordinarily-saturate the lower regions of our atmosphere, leaving only red rays to reach the Moon's surface. Of course, these phenomena are liable to vary with every eclipse, and depend almost exclusively on the meteorological conditions of our atmosphere.

In some cases the phase of totality lasts longer than it should, according to calculation. This can be attributed to the fact that the Earth is enveloped in a dense atmosphere, in which opaque clouds of considerable extent are often forming at great elevations. Such strata of clouds, in intercepting the Sun's light, would have, of course, the effect of increasing the diameter of the Earth's shadow, in a direction corresponding to the place they occupy, and, if the Moon were moving in this direction, would increase the phase of total obscuration.

The eclipses of the Moon, like those of the Sun, as shown above, have a cycle of 18 years, 11 days and 7 hours, and recur after this period of time in nearly the same order. They can, therefore, be approximately predicted by adding 18y. 11d. 7h. to the date of the eclipses which have occurred during the preceding period. During this cycle 70 eclipses will occur—41 being eclipses of the Sun and 29 eclipses of the Moon. At no time can there ever be more than seven eclipses in a year, and there are never less than two. When there are only two eclipses in a year, they are both eclipses of the Sun.

Although the number of solar eclipses occurring at some point or other of the Earth's surface is greater than that of the eclipses of the Moon, yet at any single terrestrial station the eclipses of the Moon are the more frequent. While an eclipse of the Sun is only visible on a narrow belt, which is but a very small fraction of the hemisphere then illuminated by the Sun, an eclipse of the Moon is visible from all the points of the Earth which have the Moon above their horizon at the time. Furthermore, an eclipse of the Sun is not visible at one time over the whole length of its narrow tract, but moves gradually from one end of it to the other; while, on the contrary, an eclipse of the Moon begins and ends at the very same instant for all places from which it can be seen, but, of course, not at the same local time, which varies with the longitude of the place.

PLATE VII.—PARTIAL ECLIPSE OF THE MOON.

Observed October 24, 1874

The partial eclipse of the Moon, represented on Plate VII., shows quite plainly the configuration of our satellite as seen with the naked eye during the eclipse, with its bright and dark spots, and its radiating streaks. This eclipse was observed on October 24th, 1874.

THE PLANETS

Around the Sun circulate a number of celestial bodies, which are called "Planets." The planets are opaque bodies, and appear luminous because their surfaces reflect the light they receive from the Sun.

The planets are situated at various distances from the Sun, and revolve around this body in widely different periods of time, which are, however, constant for each planet, so far as ascertained, and doubtless are so in the other cases.

The ideal line traced in space by a planet in going around the Sun, is called the orbit of the planet; while the period of time employed by a planet to travel over its entire orbit and return to its starting point, is called the sidereal revolution, or year of the planet. The dimensions of the orbits of the different planets necessarily vary with the distance of these bodies from the Sun, as does also the length of their sidereal revolution.

The distance of a planet from the Sun does not remain constant, but is subject to variations, which in certain cases are quite large. These variations result from the fact that the planetary orbits are not perfect circles having the Sun for centre, but curves called "Ellipses," which have two centres, or foci, one of which is always occupied by the Sun. This is in accordance with Kepler's first law.

The ideal point situated midway between the two foci is called the centre of the ellipse, or orbit; while the imaginary straight line which passes through both foci and the centre, with its ends at opposite points of the ellipse, is called "the major axis" of the orbit. It is also known as "the line of the apsides." The ideal straight line which, in passing through the centre of the orbit, cuts the major axis at right angles, and is prolonged on either side to opposite points on the ellipse, is called "the minor axis" of the orbit.

When a planet reaches that extremity of the major axis of its orbit which is the nearest to the Sun, it is said to be in its "perihelion;" while, when it arrives at the other extremity, which is farthest from this body, it is said to be in its "aphelion." When a planet reaches either of the two opposite points of its orbit situated at the extremities of its minor axis, it is said to be at its mean distance from the Sun.

The rapidity with which the planets move on their orbits varies with their distance from the Sun; the farther they are from this body, the more slowly they move. The rapidity of their motion is greatest when they are in perihelion, and least when they are in aphelion, having its mean rate when these bodies are crossing either of the extremities of the minor axes of their orbits.

The imaginary line which joins the Sun to a planet at any point of its orbit, and moves with this planet around the Sun, is called "the radius vector." According to Kepler's second law, whatever may be the distance of a planet from the Sun, the radius vector sweeps over equal areas of the plane of the planet's orbit in equal times.

There is a remarkable relation between the distance of the planets from the Sun and their period of revolution, in consequence of which the squares of their periodic times are respectively equal to the cubes of their mean distances from the Sun. From this third law of Kepler, it results that the mere knowledge of the mean distance of a planet from the Sun enables one to know its period of revolution, and vice versa.

The orbit described by the Earth around the Sun in a year, or the apparent path of the Sun in the sky, is called "the ecliptic." Like that of all the planetary orbits, the plane of the ecliptic passes through the Sun's centre. The ecliptic has a great importance in astronomy, inasmuch as it is the fundamental plane to which the orbits and motions of all planets are referred.

The orbits of the larger planets are not quite parallel to the ecliptic, but more or less inclined to this plane; although the inclination is small, and does not exceed eight degrees. On account of this inclination of the orbits, the planets, in accomplishing their revolutions around the Sun, are sometimes above and sometimes below the plane of the ecliptic. A belt extending 8° on each side of the ecliptic, and, therefore, 16° in width, comprises within its limits the orbits of all the principal planets. This belt is called "the Zodiac."

Since all the planets have the Sun for a common centre, and have their orbits inclined to the ecliptic, it follows that each of these orbits must necessarily intersect the plane of the ecliptic at two opposite points situated at the extremities of a straight line passing through the Sun's centre. The two opposite points on a planetary orbit where its intersections with the ecliptic occur, are called "the Nodes," and the imaginary line joining them, which passes through the Sun's centre, is called "the line of the nodes." The node situated at the point where a planet crosses the ecliptic from the south to the north, is called "the ascending node" while that situated where the planet crosses from north to south, is called "the descending node."

The planets circulating around the Sun are eight in number, but, beside these, there is a multitude of very small planets, commonly called "asteroids," which also revolve around our luminary. The number of asteroids at present known surpasses two hundred, and constantly increases by new discoveries. In their order of distance from the Sun the principal planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. The orbits of the asteroids are comprised between the orbits of Mars and Jupiter.

When the principal planets are considered in regard to their differences in size, they are separated into two distinct groups of four planets each, viz.: the small planets and the large planets. The orbits of the small planets are wholly within the region occupied by the orbits of the asteroids, while those of the large planets are wholly without this region.

When the planets are considered in regard to their position with reference to the Earth, they are called "inferior planets" and "superior planets." The inferior planets comprise those whose orbits are within the orbit of our globe; while the superior planets are those whose orbits lie beyond the orbit of the Earth.

Since the orbits of the inferior planets lie within the orbit of the Earth, the angular distances of these bodies from the Sun, as seen from the Earth, must always be included within fixed limits; and these planets must seem to oscillate from the east to the west, and from the west to the east of the Sun during their sidereal revolution. In this process of oscillation these planets sometimes pass between the Earth and the Sun, and sometimes behind the Sun. When they pass between us and the Sun they are said to be in "inferior conjunction," while, when they pass behind the Sun, they are said to be in "superior conjunction." When such a planet reaches its greatest distance, either east or west, it is said to be at its greatest elongation east or west, as the case may be, or in quadrature.

The superior planets, whose orbits lie beyond that of the Earth and enclose it, present a different appearance. A superior planet never passes between the Earth and the Sun, since its orbit lies beyond that of our globe, and, therefore, no inferior conjunction of such a planet can ever occur. When one of these planets passes beyond the Sun, just opposite to the place occupied by the Earth, the planet is said to be in "conjunction;" while, when it is on the same side of the Sun with our globe, it is said to be in "opposition." While occupying this last position, the planet is most advantageously situated for observation, since it is then nearer to the Earth. The period comprised between two successive conjunctions, or two successive oppositions of a planet, is called its "synodical period." This period differs for every planet.

It is supposed that all the planets rotate from west to east, like our globe; although no direct evidence of the rotation of Mercury Uranus, and Neptune has yet been obtained, it is probable that these planets rotate like the others. It results from the rotation of the planets that they have their days and nights, like our Earth, but differing in duration for every planet.

The axes of rotation of the planets are more or less inclined to their respective orbits, and this inclination varies but little in the course of time. From the inclination of the axes of rotation of the planets to their orbits, it results that these bodies have seasons like those of the Earth; but, of course, they differ from our seasons in duration and intensity, according to the period of revolution and the inclination of the axis of each separate planet.

[THE PLANET MARS]
PLATE VIII

Mars is the fourth of the planets in order of distance from the sun; Mercury, Venus and the Earth being respectively the first, second and third.

Owing to the great eccentricity of its orbit, the distance of Mars from the Sun is subject to considerable variations. When this planet is in its aphelion, its distance from the Sun is 152,000,000 miles, but at perihelion it is only 126,000,000 miles distant, the planet being therefore 26,000,000 miles nearer the Sun at perihelion than at aphelion. The mean distance of Mars from the Sun is 139,000,000 miles. Light, which travels at the rate of 185,000 miles a second, occupies 12½ minutes in passing from the Sun to this planet.

While the distance of Mars from the Sun varies considerably, its distance from the Earth varies still more. When Mars comes into opposition, its distance from our globe is comparatively small, especially if the opposition occurs in August, as the two planets are then as near together as it is possible for them to be, their distance apart being only 33,000,000 miles. But if the opposition occurs in February, the distance may be nearly twice as great, or 62,000,000 miles. On the other hand, when Mars is in conjunction in August, the distance between the two planets is the greatest possible, or no less than 245,000,000 miles; while, when the conjunction occurs in February, it is only 216,000,000 miles. Hence the distance between Mars and the Earth varies from .33 to 245 millions of miles; that is, this planet may be 212 million miles nearer to us at its nearest oppositions than at its most distant conjunctions.

From these varying distances of Mars from the Earth, necessarily result great variations in the brightness and apparent size of the planet, as seen from our globe. When nearest to us it is a very conspicuous object, appearing as a star of the first magnitude, and approaching Jupiter in brightness; but when it is farthest it is much reduced, and is hardly distinguishable from the stars of the second and even third magnitude. In the first position, the apparent diameter of Mars is 26", in the last it is reduced to 3" only.

The orbit of Mars has the very small inclination of 1° 51' to the plane of the ecliptic. The planet revolves around the Sun in a period of 687 days, which constitutes its sidereal year, the year of Mars being only 43 days less than two of our years.

Mars travels along its orbit with a mean velocity of 15 miles per second, being about ⁸⁄₁₀ of the velocity of our globe in its orbit. The synodical period of Mars is 2 years and 48 days, during which the planet passes through all its degrees of brightness.

Mars is a smaller planet than the Earth, its diameter being only 4,200 miles, and its circumference 13,200 miles. It seems well established that it is a little flattened at its poles, but the actual amount of this flattening is difficult to obtain. According to Prof. Young, the polar compression is ¹⁄₂₁₉.

The surface of this planet is a little over ²⁸⁄₁₀₀ of the surface of our globe, and its volume is 6½ times less than that of the Earth. Its mass is only about ⅒ while its density is about ¾ that of the Earth. The force of gravitation at its surface is nearly ¾ of what it is at the surface of our globe.

The planet Mars rotates on an axis inclined 61° 18' to the plane of its orbit, so that its equator makes an angle of 28° 42' with the same plane. The period of rotation of this planet, which constitutes its sidereal day, is 24 h. 37 m. 23 s.

The year of Mars, which is composed of 669⅔ of these Martial days, equals 687 of our days, this planet rotating 669⅔ times upon its axis during this period. But owing to the movement of Mars around the Sun, the number of solar days in the Martial year is only 668⅔, while, owing to the same cause, the solar day of Mars is a little longer than its sidereal day, and equals 24 h. 39 m. 35 s.

The days and nights on Mars are accordingly nearly of the same length as our days and nights, the difference being a little less than three-quarters of an hour. But while the days and nights of Mars are essentially the same as ours, its seasons are almost twice as long as those of the Earth. Their duration for the northern hemisphere, expressed in Martial days, is as follows: Spring, 191; Summer, 181; Autumn, 149; Winter, 147. While the Spring and Summer of the northern hemisphere together last 372 days, the Autumn and Winter of the same hemisphere last only 296 days, or 76 days less. Since the summer seasons of the northern hemisphere correspond to the winter seasons of the southern hemisphere, and vice versa, the northern hemisphere, owing to its longer summer, must accumulate a larger quantity of heat than the last. But on Mars, as on the Earth, there is a certain law of compensation resulting from the eccentricity of the planet's orbit, and from the fact that the middle of the summer of the southern hemisphere of this planet, coincides with its perihelion. From the greater proximity of Mars to the Sun at that time, the southern hemisphere then receives more heat in a given time than does the northern hemisphere in its summer season. When everything is taken into account, however, it is found that the southern hemisphere must have warmer summers and colder winters than the northern hemisphere.

Seen with the naked eye, Mars appears as a fiery red star, whose intensity of color is surpassed by no other star in the heavens. Seen through the telescope, it retains the same red tint, which, however, appears less intense, and gradually fades away toward the limb, where it is replaced by a white luminous ring.

The Trouvelot astronomical drawings manual

THE
TROUVELOT
ASTRONOMICAL DRAWINGS
MANUAL

BY

E. L. TROUVELOT,

FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE
AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL
SOCIETY OF GREAT BRITAIN; IN CHARGE OF A
GOVERNMENT EXPEDITION TO OBSERVE THE
TOTAL SOLAR ECLIPSE OF 1878.

NEW YORK

CHARLES SCRIBNER'S SONS

1882

[INTRODUCTION]

CONTENTS

THE SUN

THE AURORAL AND ZODIACAL LIGHTS

THE MOON

THE PLANETS

COMETS AND METEORS

THE STELLAR SYSTEMS

[LIST OF PLATES][1]

PLATE

Reproduced from the Original Drawings, by Armstrong & Company,
Riverside Press, Cambridge, Mass.

[GENERAL REMARKS ON THE SUN]

THE ENVELOPING LAYERS OF THE SUN

STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE

THE FACULÆ

[SUN-SPOTS AND VEILED SPOTS]
PLATE I

[SOLAR PROTUBERANCES]
PLATE II

[TOTAL ECLIPSE OF THE SUN]
PLATE III

[THE AURORA BOREALIS]
PLATE IV

[THE ZODIACAL LIGHT]
PLATE V

[THE MOON]
PLATE VI

[ECLIPSES OF THE MOON]
PLATE VII

THE PLANETS

[THE PLANET MARS]
PLATE VIII

The Trouvelot astronomical drawings manual

THETROUVELOTASTRONOMICAL DRAWINGSMANUAL

BY

E. L. TROUVELOT,

FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THEAMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICALSOCIETY OF GREAT BRITAIN; IN CHARGE OF AGOVERNMENT EXPEDITION TO OBSERVE THETOTAL SOLAR ECLIPSE OF 1878.

NEW YORK

CHARLES SCRIBNER'S SONS

1882

[INTRODUCTION]

CONTENTS

THE SUN

THE AURORAL AND ZODIACAL LIGHTS

THE MOON

THE PLANETS

COMETS AND METEORS

THE STELLAR SYSTEMS

[LIST OF PLATES][1]

PLATE

Reproduced from the Original Drawings, by Armstrong & Company,Riverside Press, Cambridge, Mass.

[GENERAL REMARKS ON THE SUN]

THE ENVELOPING LAYERS OF THE SUN

STRUCTURE OF THE PHOTOSPHERE AND CHROMOSPHERE

THE FACULÆ

[SUN-SPOTS AND VEILED SPOTS]PLATE I

[SOLAR PROTUBERANCES]PLATE II

[TOTAL ECLIPSE OF THE SUN]PLATE III

[THE AURORA BOREALIS]PLATE IV

[THE ZODIACAL LIGHT]PLATE V

[THE MOON]PLATE VI

[ECLIPSES OF THE MOON]PLATE VII

THE PLANETS

[THE PLANET MARS]PLATE VIII

THE
TROUVELOT
ASTRONOMICAL DRAWINGS
MANUAL

FORMERLY CONNECTED WITH THE OBSERVATORY OF HARVARD COLLEGE; FELLOW OF THE
AMERICAN ACADEMY OF ARTS AND SCIENCES, AND MEMBER OF THE SELENO-GRAPHICAL
SOCIETY OF GREAT BRITAIN; IN CHARGE OF A
GOVERNMENT EXPEDITION TO OBSERVE THE
TOTAL SOLAR ECLIPSE OF 1878.

Reproduced from the Original Drawings, by Armstrong & Company,
Riverside Press, Cambridge, Mass.

[SUN-SPOTS AND VEILED SPOTS]
PLATE I

[SOLAR PROTUBERANCES]
PLATE II

[TOTAL ECLIPSE OF THE SUN]
PLATE III

[THE AURORA BOREALIS]
PLATE IV

[THE ZODIACAL LIGHT]
PLATE V

[THE MOON]
PLATE VI

[ECLIPSES OF THE MOON]
PLATE VII

[THE PLANET MARS]
PLATE VIII