THE TWO OCEANS.
(1) Aërial Ocean. (2) Greatest height attained by Messrs. Glaisher and Coxwell, being 36,960 feet, or seven miles above the sea level. (3) Aërial Alps, or stratum of clouds 15,000 feet in depth. (4) Highest bird-region.

Weather Warnings
FOR
Watchers

BY THE “CLERK”

HIMSELF.

WITH CONCISE TABLES FOR CALCULATING HEIGHTS

“The actuating force of every wind that blows; of every mighty current

that streams through ocean depths; the motive cause of every particle

of vapour in the air, of every mist and cloud and raindrop, is

Solar Radiation.”—George Warington.

LONDON

HOULSTON AND SONS

PATERNOSTER SQUARE, E.C.

1877

[The right of translation is reserved. Entered at Stationers’ Hall.]

LIST OF WORKS OF REFERENCE.

Boutan et D’Almeida. Cours Eléméntaire de Physique.

Buchan, A. Introductory Text-book of Meteorology. W. Blackwood and Sons, 1871.

Cazin, Achille. La Chaleur. Hachette and Co., 1868.

Crampton, Rev. Jos., M.A. The Three Heavens. W. Hunt and Co., 1876.

Chambers’ Encyclopædia. W. and B. Chambers, 1875.

Drew, John. Practical Meteorology. Van Voorst, 1870.

Fitzroy, The late Admiral. Weather Book and Barometer Manual.

Flammarion, Camille. L’Atmosphere.

Guillemin Amedée. Les Forces de la Nature.

Glaisher, J., F.R.S. Hygrometrical Tables. Taylor and Francis, 1869.

Hartley, W. N. Air and its Relations to Life. Longmans, 1875.

Herschel, Sir John F. W. Meteorology, from Ency. Brit. A. and C. Black, 1860.

Kaemtz, L. F. Complete Course of Meteorology. Baillière London.

Martin’s Natural Philosophy. Simpkin, Marshall and Co., 1868.

Tyndall, John, D.C.L., &c. Heat, a Mode of Motion. Fifth Edition. Longmans, 1875.

Rodwell. Dictionary of Science. E. Moxon and Co., 1871.

Proctor. Science Byways. Smith, Elder and Co., 1875.

Scott, R. H., M.A., F.R.S. Instructions in the Use of Meteorological Instruments, 1875.

Warington, George. Phenomena of Radiation.

CONTENTS.

PREFACE.

The late Admiral Fitzroy entertained the opinion that the various phenomena which go to form what we call “weather” are “measurable at any place, and that having these measurements at various places over a given area, such as the British Isles, we ought to be able to foresee the peculiar results as regards the direction and force of air currents which have their distinctive weather characteristics in relation to temperature, rainfall, and electrical manifestations.”

A conviction of the soundness of this opinion has induced the writer to make the present compilation, in the hope that many who have hitherto avoided the subject of meteorology and the weather may find interesting matter, where before all seemed dull and technical.

Any attempt at rigid mathematical accuracy is disclaimed at the outset; the leading principles involved in weather forecasting and storm prevision will, however, be stated in a sufficiently definite manner to divest the subject of the mystery in which it has hitherto seemed to be enshrined, and thus enable the unscientific reader to become weather-wise, and casual observers to note weather phenomena with some degree of method and precision.

On page [4] will be found a list of works which have proved useful aids in making the present compilation. The writer desires to acknowledge his indebtedness to the various authors and publishers, and especially to Mr. Strachan, for permission to quote from his able pamphlet on “Weather Forecasts, and Storm Prevision,” and to reproduce the valuable table on page [37], for Calculating Heights of Mountains, from the fourth edition of his handy “Pocket Meteorological Register.”

The publication of Weather Reports in the daily journals must have convinced the most indifferent that much greater importance is now attached to weather phenomena than formerly; and this conviction will be deepened when it is remembered that a Parliamentary grant of £10,000 is annually expended in support of the Meteorological Office and its seven fully organized observatories in this country, while America expends no less a sum than £80,000 annually in the pursuit of weather wisdom; and the leading nations of Europe have also established meteorological observatories in suitable localities.

The balloon ascents of Messrs. Glaisher and Coxwell attracted much attention to the instruments used in estimating atmospheric phenomena, and awakened a desire to know something of the functions of a barometer, thermometer, hygrometer, &c., and especially of the classification of those important weather-warners, clouds. These subjects will be found duly noted in their order, and every phenomenon being traced to its source, Solar Radiation, it is hoped that these pages may prove generally acceptable, and be deemed not altogether unworthy of

“The Clerk of the Weather.”

WEATHER WARNINGS.

The two great Forces of Nature are Gravitation and Heat, which always act in opposition to each other.

Weather is the result of the action of these forces on matter, and where one form of force is in excess of another, changes are produced which become apparent to our senses, or are indicated by suitable instruments.

The Matter composing the earth on which we live is of three kinds—solid, liquid, and gaseous.

The Force incessantly acting on these is the radiant heat of the sun.

The Results of this incessant action are:—

1. Calorification, or Heating, which, besides being appreciable by our senses, is indicated by the Thermometer.

2. Evaporation, which alters the weight of the air indirectly, by the diffusion of aqueous vapour through it. This alteration of weight is indicated by the Barometer, the accompanying increase of moisture being indicated by the Hygrometer.

3. Rarefaction, which alters the weight of the air directly.

4. Condensation, producing fog, dew, rain, hail, and snow; all sufficiently apparent when they occur, but estimated accurately only by the Rain Gauge, or Pluviometer.

5. Motion, producing winds, which we are able to appreciate in the gentle breeze and the awful cyclone, the force and velocity of which are indicated by the Anemometer.

6. Electrification, producing lightning, thunder, magnetic phenomena, and chemical change, respectively indicated by the Electrometer, Magnetometer, and Ozonometer.

I.—CALORIFICATION.

Before considering in detail these results of the action of solar radiation on our globe, an attempt to realize the immensity of this stupendous force will materially aid in the general comprehension of the subject.

The earth is a sphere somewhat less than 8,000 miles in diameter; and if we assume, with the gifted author[^[1]] of “The Phenomena of Radiation,”—“that it is about 91,300,000 miles from the sun, and moves around it in a slightly elliptical orbit, occupying rather more than 365 days; that its shape is globular, somewhat flattened at its two extremities; that it rotates upon its own axis in the space of 24 hours, that axis being inclined to the annual orbit at an angle of 23-1/2—if we further assume that solar radiation is of such kind and quantity as it is, we are enabled to account for the total amount of light and heat the earth receives, for the superior temperature and illumination of equatorial regions, as compared with polar, with the gradations of intermediate zones, for the alternation of day and night, and the annual progression of the seasons.

[1]. George Warington, F.C.S.

“The actuating force of every wind that blows; of every mighty current that streams through ocean depths; the motive cause of every particle of vapour in the air of every mist and cloud and raindrop, is Solar Radiation.

“The delicate tremor of the sun’s surface particles, shot hither through thirty million leagues of fine intangible æther, has power to raise whole oceans from their beds, and pour them down again upon the earth. We are apt to measure solar heat merely by the sensation it produces on our skin, and think it small and weak accordingly; a good coal fire will heat us more. But its true measure is the work it does. Judged by this standard, its immensity is overpowering. To take a single instance: the average fall of dew in England is about five inches annually; for the evaporation of the vapour necessary to produce this trifling depth of moisture, there is expended daily an amount of heat equal to the combustion of sixty-eight tons of coal for every square mile of surface, or, for the whole of England, 4,000,000 tons. Compare now the size of England with that of the whole earth—only 1/3388th part; extend the calculation to rain, as well as dew, the average fall of which on the whole earth is estimated at five feet annually, or twelve times greater; and then estimate the sum of 4,000,000 × 3,388 × 12 = 162,624,000,000 tons, or about 3,000 times as much as is annually raised in the whole world; and we have the number of tons of coal required to produce the heat expended by the sun merely in raising vapour from the sea to give us rain during a single day.”

1.
Pouillet’s Pyrheliometer. Scale about 1/8.

SOLAR RADIATION.

Seeing, then, that solar radiation plays so important a part in the production of the natural phenomena classed under the head of Meteorology, a description of the mode of estimating its amount will prove interesting, and enable the reader to realize the existence of this mighty power. M. Pouillet devised for this purpose the apparatus known as the Pyrheliometer, which registers the power of parallel solar rays by the amount of heat imparted to a disc of a given diameter in a given time. It consists of a flat circular vessel of steel A having its outside coated with lamp-black B. A short steel tube is attached to the side opposite to that covered with lamp-black, and the vessel is filled with mercury. A registering thermometer C, protected by a brass tube D, is then attached, and the whole is inverted and exposed to the sun, as shown at Fig. 1. The purpose of the second disc, E, is to aid in so placing the apparatus that it shall receive direct parallel rays. It is obvious that if the shadow of the upper disc completely covers the lower one, the sun’s rays must be perpendicular to its blackened surface.

“The surface on which the sun’s rays here fall is known; the quantity of mercury within the cylinder is also known; hence we can express the effect of the sun’s heat upon a given area by stating that it is competent, in five minutes, to raise so much mercury so many degrees in temperature.”[^[2]]

[2]. Tyndall, “Heat a Mode of Motion.”

Sir John Herschel also designed an instrument for observing the heating power of the sun’s rays in a given time, to which the title Actinometer is given. It consists of a Thermometer with a long open scale and a large cylindrical bulb, thus combining the best conditions for extreme sensibility. An observation is made by exposing the instrument in the shade for one minute and noting the temperature. It is then exposed to the sun’s rays for one minute, and a record of the temperature made. It is again placed in the shade for one minute, and the mean of the two shade readings being deducted from the solar reading shows the heating power of the sun’s rays for one minute of time.

2.
Herschel’s Actinometer. Scale about 1/8.

The stimulus imparted to the study of this class of phenomena by the publications of Professor Tyndall’s researches on Radiant Heat has induced a demand among Meteorologists for instruments capable of yielding more available indications than those just described. This demand has been most efficiently supplied by the ingenuity of scientists and instrument makers.

3.
Improved Solar Radiation Thermometer in Vacuo.
Scale about 1/3.

The early form of Solar Radiation Thermometer was a self-registering maximum thermometer, with blackened bulb, having its graduated stem, only, enclosed in an outer tube. Errors arising from terrestrial radiation and the variable cooling influences of aërial currents are all obviated in the improved and patented Solar Radiation Thermometer shown at Fig. 3, which consists of a self-registering maximum thermometer, having its bulb and stem dull-blackened, in accordance with the suggestion of the Rev. F. W. Stow, and the whole enclosed in an outer chamber of glass, from which the air has been completely exhausted. The perfection of the vacuum in the enclosing chamber is proved by the production of a pale white phosphorescent light, with faint stratification and transverse bands when tested by the spark from a Ruhmkorff coil. Due provision is made for this by the attachment of platinum wires to the lower side of the tube, and when tested by a syphon pressure gauge, the vacua have been proved to exist to within 1/50th of an inch of pressure. It will thus be seen that the indications are preserved from errors arising from atmospheric currents, and from the absorption of heat by aqueous or other vapours, the whole of the solar heat passing through the vacuum direct to the blackened bulb. The contained mercury expanding, carries the recording index to the highest point, and thus is obtained a registration of the maximum amount of solar radiation during the twenty-four hours. The great advantage accruing from the high degree of perfection to which this instrument has been brought is, uniformity of construction, which renders the observations made at different stations intercomparable. An enlarged view of the thermometer is given at Fig. 3, showing the platinum wire terminations, whereby the vacuum is tested. The Rev. Fenwick W. Stow thus directs the manner in which the solar radiation thermometer should be used:—

1. Place the instrument four feet above the ground, in an open space, Fig. 4, with its bulb directed towards the S.E. It is necessary that the globular part of the external glass should not be placed in contact with or very near to any substance, but that the air should circulate round it freely. Thus placed, its readings will be affected only by direct sunshine and by the temperature of the air.

2. One of the most convenient ways of fixing the instrument will be to allow its stem to fit into and rest upon two wooden collars fastened across the ends of a narrow slip of board, which is nailed in its centre upon a post steadied by lateral supports (Fig. 4).

3. The maximum temperature of the air in shade should be taken by a thermometer placed on a stand in an open situation. Any stand which thoroughly screens it from the sun, and exposes it to a free circulation of air, will do for the purpose.

4. The difference between the maxima in sun and shade, thus taken, is a measure of the amount of solar radiation.

4.
Solar Radiation Thermometer, black bulb and
stem in vacuo, on 4 feet stand.
Scale about 1/20.

The remarkable phenomenon recently discovered by Mr. Crookes, in which light is apparently converted into motion, has, at the suggestion of Mr. Strachan, received an interesting application to meteorology. The arrangement is shown at Fig. 5, where a Solar Radiation Thermometer has a Crookes’ Radiometer attached to it, which, in addition to forming an efficient test as to the perfection of the vacuum, will, it is hoped, aid in eventually establishing a relation between intensity of radiation, as shown by the thermometer, and the number of revolutions of the radiometer. The instrument has so recently been devised that any positive statement as to its usefulness would be premature; it may, however, prove a valuable auxiliary to the solar thermometer, and eventually be so far improved as to become a more definite exponent of solar radiation than the thermometer.

5.
Radio-Solar Thermometer. Scale about 1/4.

TERRESTRIAL RADIATION.

It is an established fact, confirmed by careful experiments, that a mutual interchange of heat is constantly going on between all bodies freely exposed to view of each other, thus tending to establish a state of equilibrium. It has further been ascertained that, as the mean temperature of the earth remains unchanged, “it necessarily follows that it emits by radiation from and through the surface of its atmosphere, on an average, the exact amount of heat it receives from the sun.” This process commences slowly at sunset, and proceeds with great rapidity at and after midnight, attaining its maximum effect in a long night, in perfect calm, under a cloudless sky, resulting in the condensation of vapour in the form of dew, or hoar-frost, when the temperature of the surface-air is reduced to the dew-point.[^[3]]

[3]. See page [47].

The extent to which heat thus escapes by radiation under varying conditions of sky is measured by a Self-registering Terrestrial Minimum Thermometer, the bulb of which is placed over short grass, and “a thermometer so exposed under a clear sky always marks several degrees below the temperature of the air, and its depression affords a rude measure of the facility for the escape of heat afforded under the circumstances of exposure.” [^[4]]

[4]. Herschel.

6.
Terrestrial Radiation Thermometer.
Scale about 1/6.

7.
Improved Cylinder Jacket Terrestrial Minimum Thermometer.
Scale about 1/12.

Fig. 6 shows the ordinary spherical bulb thermometer employed for this purpose, and Fig. 7 the improved Cylinder Jacket Thermometer, which, by exposing a larger surface of spirit to the air, gives an instrument possessing an amount of sensibility in no way inferior to that of mercury.

There is a drawback to the use of these thermometers enclosed in outer tubes, arising from moisture getting into the outer cylinder or jacket, and frequently preventing the observer from reading the thermometer. This has recently been removed by making a perfectly ground joint of glass (analogous to a glass stopper in a bottle) as a substitute for the old form of packing at the open end of the tube, the other end being fused into contact with the outer cylinder to keep it in its place. The intrusion and condensation of moisture thus becomes impossible, while the scale is protected from corrosion or abrasion. This “ground socket” arrangement is shown at Fig. 8.

8.
Ground Socket Minimum Thermometer. Scale about 1/4.

Radiation from the earth upwards proceeds with great rapidity under a cloudless sky, but a passing cloud, or the presence even of invisible aqueous vapour in the air, is sufficient to effect a marked retardation, as is beautifully illustrated by Sir John Leslie’s Æthrioscope, shown at Fig. 9, which consists of a vertical glass tube, having a bore so fine that a little coloured liquid is supported in it by the mere force of cohesion. Each end of the tube terminates in a glass bulb containing air. A scale, having its zero in the middle, is attached to the tube, and the bulb A is enclosed in a highly polished sphere of brass. The upper bulb B is blackened, and placed in the centre of a highly-gilt and polished metallic cup, having a movable cover F. These outer metallic coverings protect the bulbs from extraneous sources of heat. So long as the upper bulb is covered, the liquid in the tube stands at zero on the scale, but immediately on its removal radiation commences, the air contained in B contracts, while the elasticity of that contained in A forces the liquid up the tube to a height directly proportionate to the rapidity of the radiation.

9.
Æthrioscope.
Scale about 1/7.

SHADE TEMPERATURE.

Self-registering Maximum Thermometers are made in two ways. In the first, the index is a small portion of the mercurial column separated from it by a minute air bubble. The noontide heat expands the mercury, and the subsequent contraction as the temperature decreases affects only that portion of the mercury in connection with the bulb, leaving the disconnected portion to register the maximum temperature. In the second form the tube is ingeniously contracted just outside the bulb, so that the mercury extruded from the bulb by expansion cannot return by the mere force of cohesion, but remains to register the highest temperature.

10.
Self-registering Maximum Thermometer. Scale about 1/5.

There is a modification of this latter form produced by the addition of a supplementary chamber just outside the bulb and over the column, from which, as expansion proceeds, the mercury flows by gravitation, but into which it cannot return until, as in the other forms, the instrument is readjusted for a new observation, by unhooking the bulb end and lowering it until the mercury flows into its place.

11.
Self-registering Minimum Thermometer. Scale about 1/5.

Self-registering Minimum Thermometers are of two kinds,—spirit and mercurial. Fig. 12 shows one of Rutherford’s Alcohol Minimum Thermometers, which will be seen to consist of a bulb and tube attached to a scale, which latter may be either of wood, glass, or metal. The tube contains an index of black glass.

12.
Self-registering Minimum Thermometer.
Scale about 1/5.

The Thermometer is “set” for observation by slightly raising the bulb end until the index slides to the extreme end of the column of spirit. It is then suspended in the shade with the bulb end a little lower than the other. The contraction of the spirit consequent on a fall of temperature draws the index back, but a subsequent expansion does not carry it forward, it remains at the lowest point to which the spirit has contracted to register the minimum temperature. A very useful modification of this instrument is made for gardeners and general horticultural purposes, in which the scale is of cast zinc with raised figures, which being filed off flush after the whole has been painted of a dark colour are easily legible at a little distance.

The advantage of alcohol for the indication of very low temperatures is that it has never been frozen.[^[5]]

[5]. Mercury freezes at -39° F.

Fig. 13 shows a set of Maximum and Minimum and Wet and Dry Bulb Thermometers, with incorrodible porcelain scales, suspended on a mahogany screen. Instruments of this quality are generally engine-divided on the stem, and if, in addition to this, they are verified by comparison with standard instruments at the Kew Observatory, they may be regarded as standards, and employed for accurate scientific observations.

13.
Standard Set of Instruments on Screen. Scale about 1/6.

Six’s Self-registering Thermometer consists of a long tubular bulb, united to a smaller tube more than twice its length, and bent twice, like a syphon, so that the larger tube is in the centre, while the smaller one terminates at the top, on the right hand, in a pear-shaped bulb, as shown in the cut (Fig. 14). This bulb, and the tube in connection with it, are partly filled with spirit; the long central bulb and its connecting tube are completely filled, while the lower portion of the syphon is filled with mercury. A steel index, prevented from falling by a hair tied round it, to act as a spring, moves in the spirit in each of the side tubes. The scale on the left hand has the zero at the top, and that on the right at the bottom. When setting the instrument, the indices are brought into contact with the mercury by passing a small magnet down the outside of each tube. Then, should a rise of temperature take place, the spirit in the central bulb expands, forcing down the mercury in the left hand tube and causing it to rise in the right, and vice versa for a diminution of temperature.

It should be always used and carried upright, and the indices should be drawn gently down by the magnet into contact with the mercury; and, when a reading is taken, the ends of the indices nearest the mercury indicate the maximum and minimum temperatures which have been attained during the stated hours of observation.

14.
Six’s Thermometer.
Scale about 1/7.

Six’s form of thermometer has been extensively used for ascertaining deep sea temperatures.

15.
Deep Sea Maximum
and Minimum Registering
Thermometer.
Scale about 1/5.

Evaporation and the mechanical action of winds keep up a constant circulating motion of the ocean, the currents of which tend to equalize temperature. The most important of these is known as the Gulf Stream, taking its name from the Gulf of Mexico, out of which it flows at a velocity sometimes of five miles an hour, and in a width of not less than fifty miles. It has an important effect on the climate of Great Britain, and of all lands subject to its influence, its temperature as it leaves the Gulf of Mexico being 85° F., diminishing to 75° off the coast of Labrador, and still further as it nears northern latitudes. Observations on the temperature of the ocean are therefore included in the scope of meteorology, and are ascertained by the use of thermometers of special construction (Fig. 15). In the earlier experiments made for ascertaining the temperature of the ocean at a depth of 15,000 feet, where the pressure is equal to three tons on the square inch, it was found that a considerable error occurred in the indications in consequence of this enormous pressure; accordingly the central elongated bulb of the ordinary Six’s Thermometer (see page [19]) is shortened and enclosed in an outer bulb nearly filled with spirit, which, while effectually relieving the thermometer bulb from undue pressure, allows any change to be at once transmitted to it, and thus secures the registration of the exact temperature. The arrangement possesses the further advantage of making the instrument stronger, more compact, and more capable of resisting such comparatively rough treatment as it would receive on board ship.

The honour of constructing the first thermometer, which was an Air and Spirit Thermometer, is ascribed to Galileo; it assumed a practical shape in 1620, at the hands of Drebel, a Dutch physician. Hailey substituted mercury for spirit in 1697; Réaumur improved the instrument in 1730, and Fahrenheit in 1749. More recently the instrument has been perfected by the scales being graduated on the actual stem of the instrument. For many years it was exclusively used by chemists and men of science; it afterwards received numerous applications in the arts and manufactures; and is now considered an essential in every household.

Thermometers are instruments for measuring temperature by the contraction or expansion of fluids in enclosed tubes. The tubes, which are of glass, have spherical, cylindrical, or spiral bulbs blown on to one end; they have also an exceedingly fine bore, and when mercury or spirit is enclosed in them these fluids, in contracting and expanding with variations of temperature, indicate degrees of heat in relation to two fixed points—viz., the freezing and boiling points of water. Care is taken to exclude all air before sealing, so that the upper portion of the tube inside shall be a perfect vacuum, and thus offer no resistance to the free expansion of the mercury. In graduating, or dividing the scales, the points at which the mercury remains stationary in melting ice and boiling water are first marked on the stem, and the intervening space divided into as many equal parts as are necessary to constitute the scales of Fahrenheit, Réaumur, or Celsius, the last being known as the Centigrade (hundred steps) scale, from the circumstance of the space between the freezing and boiling points of water being divided into one hundred equal parts (Fig. 16).

16.
Comparison of Thermometer
Scales.
Scale about 1/5.

17.
“Legible”
Scale Thermometer.
Scale about 1/5.

Graduation of Thermometers.—When the fluid (either mercury or spirit) has been enclosed in the hermetically sealed tube, it becomes necessary, in order that its indications may be comparable with those of other instruments, that a scale having at least two fixed points should be attached to it. As it has been found that the temperature of melting ice or freezing water is always constant, the height at which the fluid rests in a mixture of ice and water has been chosen as one point from which to graduate the scale. It has been also found that with the barometer at 29·905 the boiling-point of water is also constant, and when a thermometer is immersed in pure distilled water heated to ebullition, the point at which the mercury remains immovable is, like the freezing-point, carefully marked, the tube is then calibrated and divided as shown in Fig. 16.

The zero of the scales of Réaumur and Centigrade is the freezing-point of water, marked, in each case, 0°, while the intervening space, up to the boiling-point of water, is divided, in the former case, into 80 parts, and in the latter to 100°.

In the Fahrenheit scale, the freezing-point is represented at 32°, and the boiling-point at 212°, the intervening space being divided into 180°, which admits of extension above and below the points named, a good thermometer being available for temperature up to 620° Fahr.

The use of the Réaumur scale is confined almost exclusively to Russia and the north of Germany, while the Centigrade scale is used throughout the rest of Europe. The Fahrenheit scale is confined to England and her colonies, and to the United States of America.

18.
Gridiron-bulb
Thermometer.
Scale
about 1/5.

Circumstances sometimes arise in which it becomes necessary to convert readings from one scale into those of the others, according to the following rules:—

1. To convert Centigrade degrees into degrees of Fahrenheit, multiply by 9, divide the product by 5, and add 32.

2. To convert Fahrenheit degrees into degrees of Centigrade, subtract 32, multiply by 5, and divide by 9.

3. To convert Réaumur degrees into degrees of Fahrenheit, multiply by 9, divide by 4, and add 32.[^[6]]

4. To convert Réaumur degrees into degrees of Centigrade, multiply by 5 and divide by 4.[^[7]]

[6]. 8 R = 50 F.

[7]. 8 R = 10 C.

For the production of continuous records, the Meteorological Committee of the Royal Society have adopted an instrument called a Thermograph, or self-recording wet and dry bulb thermometer, which is largely aided by photography. The bulbs of the thermometers are necessarily placed in the open air, and at a suitable distance from any wall or other radiating surface; the tubes are of sufficient length to admit of their being brought inside the building, in due proximity to the recording apparatus placed in a chamber from which daylight is rigidly excluded.

19.
Thermograph and Self-recording Hygrometer.
Scale about 1/18.

The essential conditions in such an apparatus are:—1. A means of denoting the height of the mercurial column in the stem of a thermometer in relation to a fixed horizontal line. 2. A time scale denoting the exact moment at which the atmosphere reached the temperature indicated by the mark. 3. As the marks are produced chemically, and not mechanically (as in the Anemograph), a dark room.

A description of the drawing on page [23] will best show how very efficiently, through the ingenuity of Mr. Beckley, these conditions have been obtained:—S, wet bulb thermometer; T, atmospheric thermometer; B, screw for adjusting thermometers; C C, paraffin lamps or gaslights; D D, condensers, concentrating the light on the mirrors R R; R R, mirrors reflecting light through air-speck in thermometers V V; E E, slits through which light passes from mirrors R R; F F, photographic lenses, producing image of air-speck from both thermometers on cylinder G; G, revolving cylinder or drum carrying photographic paper; H, clock, turning cylinder G round once in 48 hours; I, shutter to intercept light four minutes every two hours; leaving white time-line on developing latent image.

II.—EVAPORATION.

Solar heat rarefies the air by driving its particles asunder; it also vaporises water from the surface of river, lake, and ocean, diffusing the vapour through the atmosphere.

Great interest attaches to the subject of Evaporation, on account of its connection with rainfall and water supply. It is to be regretted, therefore, that the results hitherto obtained in the endeavour to measure its rate and quantity do not merit much confidence as regards their applicability to the evaporation occurring in nature, owing to the exceptional manner in which the observations have been made.

There is this uncertainty about evaporation, that all the experiments relate to that taking place from an exposed water surface of a, comparatively speaking, infinitesimally small area, and can therefore have but a very partial applicability to the conditions occurring in nature. There are two main reasons for this statement. Firstly, the proportion of the surface of the land on the earth which is covered with lakes and rivers is very limited, and the experiments above indicated throw no light on the evaporation from the soil. Secondly, the evaporation from the surface of a small atmometer erected on the ground, with comparatively dry air all around it, is certainly very different from that which would take place from an equal area in the centre of a large water surface, such as a lake.

It is of course easy to make experiments on the evaporation from the soil by means of a balance atmometer, but in order that these should possess a practical value, the investigation must be extended so as to include a wide variety of soils, &c., &c. As regards the second point which has been raised, it is recommended by the Vienna Congress to erect atmometers in the centre of water surfaces; but it is not a very easy matter to conduct such experiments with accuracy, owing to the risk of in-splashing from waves.

20.
Atmidometer.
Scale about 1/5.

Babington’s Atmidometer measures evaporation from water, ice, or snow, and in form resembles a hydrometer, with the difference that the stem bears a scale graduated to grains and half grains, and is surmounted by a light, shallow copper pan. When in use, the hydrometer-like instrument is immersed in a glass vessel having a hole in the cover, through which the stem protrudes. The copper pan is then placed on the top, and sufficient water, ice, or snow placed therein to sink the stem to the zero of the scale. As the evaporation proceeds, the stem rises; and, if the time of commencing the experiment is noted, the rate as well as the amount of evaporation is indicated in grains.

III.—RAREFACTION.

The diffusion of aqueous vapour through the air and the rarefying influence of heat jointly effect an alteration in the weight of the atmosphere. This alteration of weight is determined by the Barometer, an instrument invented by Torricelli, in 1643, and in so perfect a form that in its essential features it has not been superseded.

21. and 22.
Construction of Barometer.
Scale about 1/18.

The mode of construction is illustrated by Figs. 21 and 22. It consists in hermetically sealing a glass tube about three feet long and filling it with mercury. The finger is placed over the open end of the tube, which is then inverted and placed in a cistern of mercury and the finger withdrawn. The left-hand figure shows the result; the mercury is seen to fall some three or four inches, leaving an empty space at the top of the tube, which is called the “Torricellian vacuum.”

The mercury is prevented from falling lower than is shown, by the external pressure of the atmosphere on the cistern. The weight of this column, therefore, represents the weight or pressure of a corresponding column of air many miles in height; and so close is the relation between the column of mercury and the external air that the height of the former changes with the slightest variation in the weight of the latter, and the instrument thus becomes a measure of the weight of the air, from which property its name is derived, the Greek words baros and metron signifying respectively “weight” and “measure.”

When the mercury in the barometer tube falls, that in the cistern rises in corresponding proportion, and vice versa, so that there is an ever-varying relation between the level of the mercury in the tube and the mercury in the cistern, which affects the accuracy of the readings. In M. Fortin’s cistern this difficulty is obviated by the use of a glass, with flexible leather bottom and a brass adjusting screw, as shown in the cut. Through the top of the cistern is inserted a small ivory point, the lower end of which corresponds with the zero of the scale; and, to secure uniformity, the level of the mercury in the cistern should be adjusted by the screw at each observation, until the ivory point appears to touch its own reflection on the surface. The reading is then taken.

23.
Fortin’s
Cistern.
Scale about
1/6.

In making barometric observations for comparison with others, it is necessary that all should be reduced to the common temperature of 32° F., and for this purpose tables have been calculated which will be found to save much time.

Tables also for reducing observations of the barometer to sea level, an operation equally indispensable with the other corrections to make the readings intercomparable, have been published by direction of the Meteorological Committee.

For the British Isles the mean sea-level at Liverpool has been selected by the Ordnance Survey as their datum, and the height of any station may be ascertained by first noting the nearest Ordnance Bench Mark thus ↑, and purchasing that portion of the Ordnance map which includes the station, near to which the Bench Mark will be found with the height above sea-level duly entered. The levellings made for railways will also furnish the desired information. Failing both these, the observer should select two or more of the stations nearest his locality for which official Meteorological Reports are published daily in the Times and other journals; and taking observations of his barometer at 8 a.m., for a few weeks, should compare them with the mean of the observations at those stations. The comparison should be omitted when the barometer pressure is not steady.

24.
Error of
Capillarity.
Scale about 1/2.

25. Standard Barometer. Scale about 1/7.

A Standard Barometer is constructed on Fortin’s principle, and should have its tube about half an inch bore, enclosed in a brass body having at its upper end two vertical openings, in which the vernier works. The mercury is seen through these openings, aided by light reflected from a white opaque glass reflector let into the mahogany board behind. The scale is divided on one side into English inches and 20ths, and may have on the other French millimetres, the vernier enabling a reading to be taken, in each case respectively, of 1/500th of an inch and 1/10th of a millimetre. In making the instrument, the mercury is boiled in the tube, to ensure the complete exclusion of air and moisture; while Fortin’s principle of cistern ensures a constant level from whence to take the readings. A sensitive thermometer with scale, engine-divided on stem, is attached to the brass mount, which is perforated to admit the attenuated bulb of the thermometer into absolute contact with the glass tube of the barometer, to ensure its indicating the same temperature as the contained mercury. The instrument is suspended by a ring from a brass bracket attached to a mahogany board, and the lower end passes through a larger ring having three screws for adjusting it vertically.

A “reading” is taken in the following manner:—1. Note the temperature by the attached thermometer. 2. Raise or lower the mercury in the cistern by turning the screw underneath until the reflected image of the ivory point on the mercury seems to be in contact with the ivory itself. By the milled head at the side, the vernier is adjusted until its lower edge just touches the top of the mercurial column, the scale and vernier then indicate the height of the barometer in inches, 10ths, 100ths, and 1000ths.

High-class instruments, such as that here described, yield exact readings; but, in order to note them accurately, it is important that the eye, the zero edge of the vernier, the top of the mercurial column, and the back of the vernier should be in the same horizontal plane; conditions which may be obtained after some practice.

The accompanying illustration shows a form of barometer which, though not much used in this country, is deservedly popular on the Continent as a standard station barometer. It is called a Syphon Barometer, and was designed by Gay-Lussac. The open end of the tube is bent up in the form of a syphon, the short limb being from six to eight inches long; it is furnished with metal scales and verniers, and is mounted on a mahogany board with attached thermometer.

These barometers require no correction for capillarity or capacity, each surface of mercury being equally depressed by capillary attraction, and the quantity of mercury falling from the long limb occupies the same space in the short limb. The usual correction for temperature must, however, be applied. A scale of inches, measured from a zero point taken near the bend of the tube, furnishes the means of measuring the long and short columns. The difference of readings is the height of the barometer.

The Vernier is a movable scale for subdividing parts of a fixed scale, and was first applied to that purpose by its inventor, M. Pierre Vernier, in 1630. In the barometer the parts to be divided are inches, which by the aid of this invention are subdivided into 10ths, 100ths, and 1000ths.

Fig. 27 shows the scale of a standard barometer divided into 1/2-10ths, or ·05 of an inch. The Vernier C D is made equal to 24 of such divisions, and is divided into 25 equal parts, from whence it follows that one division on the scale is 1/25th of ·05 larger than one on the vernier, so that it shows a difference of ·002 of an inch. The vernier reads ·0, or zero, upwards; D, therefore, indicates the top of the mercurial column.

26.
Syphon
Barometer.
Scale
about 1/12.

In Fig. 27, zero on the vernier is exactly in line with 29 inches and 5/10ths of the fixed scale; the reading, therefore, is 29·500 inches. The vernier line a falls short of a division of the scale by ·002-inch; b, by ·004; c, by ·006; d, by ·008; and the succeeding line by ·010. If the vernier be adjusted to make a coincide with z on the scale, it will have moved through ·002-inch; and if 1 on the vernier be moved to coincide with y on the scale, the space measured will be ·010-inch. Consequently, the figures 1, 2, 3, 4, 5, on the vernier, measure 100ths, and the intermediate lines even 1000ths of an inch. In Fig. 28 the zero of the vernier is between 29·65 and 29·70 on the scale. Glancing up the vernier and scale, the second line above 3 will be found in a direct line with one on the scale; this gives ·03 and ·004 to add to 29·65, so that the actual reading is 29·684. In those instances where no line on the vernier is found precisely to coincide with a line on the scale, and doubt arises as to which to select from two equally coincident lines, the rule is to take the intermediate 1000th of an inch.

27. and 28.
The Vernier.

For household and marine barometers such minute subdivisions of the scale are unnecessary, and the scales of such instruments are therefore divided only to 10ths, and the verniers made only to read to 100ths of an inch, which is effected by making the vernier 9/10ths or 11/10ths of an inch long, and dividing it into 10 equal parts.

In “taking a reading” it is important that it should be done as quickly as possible, as the heat from the body and the hand is sufficient to interfere with that accuracy which is necessary where the intention is to compare the readings with those made by other observers. This facility is soon acquired by a little practice.

29. Farmer’s Barometer. Scale about 1/7.

Pediment Household Barometers, though not so imposing in appearance as the Wheel Barometer, yield direct readings without the intervention of the mechanical appliances necessary for moving a needle over an extended dial. Their mountings are for the most part in oak, walnut, and other woods, the scales are of ivory, porcelain, or enamelled glass, and in their graduation due regard is paid to the relative proportions of cistern and tube, so that the conditions essential to the production of a Standard Barometer are very closely attained. In common with other barometers, it should hang in the shade in a vertical position, so that light may be seen through the tube. As a purchaser would receive it in what is called a “portable” state, it will be necessary on first suspending it to take the pinion key, fit it on the square-headed pin at the bottom of the instrument, and turn gently to the left till the screw stops. The effect of this is to lower the base of the cistern, and allow the mercury in the tube to fall to its proper level. The key should then be replaced for use in moving the vernier. To make this kind of Barometer portable for travelling it should be unhung, very gradually sloped until the mercury is at the top of the tube, when, the instrument being upside down, the base of the cistern is screwed up by turning the pinion key gently to the right until it stops. Care should be taken to avoid concussion, and to have the cistern end always uppermost, or the instrument lying flat.

Fig. 29 shows a useful form of barometer for the farmer, combining as it does three instruments in one, for the thermometer on the right hand of the scale having its bulb covered with muslin kept moist by communication with a cistern of water enables the two thermometers to be employed as a Hygrometer, the use of which is described at page [50]. This barometer should be suspended in a place where it will be exposed as much as possible to the external air, but not in sunshine.

30.
Wheel Barometer. Scale
about 1/6.

In Wheel Barometers the varying height of a column of mercury is shown by the movement of a needle on a divided circular dial, by adopting the syphon form of barometer tube, concealed behind the dial and frame. An iron or glass float sustained by the mercury in the open branch (Fig. 31) is suspended by a counterbalance a little lighter than itself. The axis of the pulley has the needle attached to it, and consequently moves the needle with the rise and fall of the mercury. It is obvious, therefore, that if the atmospheric pressure increases the float falls and the needle turns to the right, and if it diminishes the needle turns in the opposite direction. The divisions on the scale represent inches, tenths, and hundredths in the rise and fall of a column of mercury, and these can be read with great facility, as one inch occupies the space of six or more on this very open scale, according to size of dial (Fig. 30). The wording is arbitrary, and indicates the probable weather that may be expected.

Important improvements have recently been effected in this form of household barometers, so that they may be recommended as good weather indicators where facility of reading is a desideratum.

31.
Mechanism of Wheel
Barometer.
Scale about 1/8.

Since the more scientific “Pediment” has attained so high a degree of popularity, a certain amount of unmerited obloquy has attached itself to the Dial or Wheel Barometer invented by Dr. Hooke. It must be conceded that the standard form of pediment barometers in which the height of the mercury is seen at a glance is more strictly an “instrument of precision,” but it should not be forgotten, although a delicate mechanism intervenes between the mercury and the observer, it is so arranged that a tenth of an inch rise or fall causes a movement of the index over an inch of space.

The Aneroid Barometer indicates variations in atmospheric pressure by the elevation and depression of the sides of an elastic metallic box from which the air is exhausted and which is kept from complete collapse by a powerful spring. In cases where extreme accuracy is not indispensable, the portability and sensibility of this instrument recommend it for use by tourists and fishermen. It is “quick in showing the variations of atmospheric pressure.”[^[8]] “The Aneroid readings may be safely depended upon.”[^[9]] “Its movements are always consistent.”[^[10]] “Atmospheric changes are indicated first by the Aneroid.”[^[11]] It is especially adapted for determining mountain altitudes, some being furnished with a scale of feet, enabling the observer to read off the height by direct observation, and if adjusted once a year by comparison with a mercurial standard is quite trustworthy. It is fully described in a small pamphlet entitled “The Aneroid Barometer: How to Buy, and How to Use it,” by a Fellow of the Meteorological Society.

[8]. Admiral Fitzroy.

[9]. James Glaisher, Esq., F.R.S.

[10]. James Belville, Esq., Royal Observatory, Greenwich.

[11]. Sir Leopold McClintock.

32.
Aneroid Barometer. Full size.

By a suitable arrangement of clockwork, revolving a cylinder bearing prepared paper, the aneroid barometer forms an admirable self-recording instrument, showing at a glance the height of the barometer: whether it is falling or rising, for how long it has been doing so, and at what rate the change is taking place, whether at the rate of 1/10th per hour, or 1/10th in twenty-four hours—facts which can only be obtained by very frequent and regular observations from an ordinary barometer, but which are nevertheless essential to a reliable “weather forecast.”[^[12]]

[12]. The Aneroid Barometer: How to Buy and How to Use it. By a Fellow of the Meteorological Society. Post free for six stamps, from any bookseller or optician.

The height of mountains may also be determined by the temperature at which water boils, as this depends on the pressure of the atmosphere, and according to Deschanel, “just as we can determine the boiling-point of water when the external pressure is given, so if the boiling-point be known we can determine the external pressure,” and as this varies with the elevation above sea-level, the boiling-point of water also varies.

These facts induced Wollaston to attempt the determination of heights of mountains by an apparatus which he called the Barometric Thermometer, subsequently modified by Regnault and called a Hypsometer, but now more generally known as a Boiling-point Thermometer.

33.
Boiling-point
Thermometer.
Scale about 1/3.

A portable form of boiling-point thermometer is shown at Fig. 33, which is much used by Alpine travellers, and forms a trustworthy check on the aneroid and barometer.

Rule I.—If the temperature of boiling water be observed at either or both Stations, find the equivalent pressure in the 2nd column, and calculate the height as for barometer.

Rule II.—The readings of the Barometer being corrected and reduced to 32° F., multiply the difference of pressure between the Stations by factor A, found in line with pressure at lower Station, and under that at upper Station; multiply again by factor B, corresponding to the mean temperature of the air at the Station; apply as many times C as there are thousand feet in the height, corresponding to the latitude; and add D, the correction for gravity.

Example.—At the top of Snowdon, lat. 53° N., an aneroid read 26·48, correction -0·18, the pressure at sea-level was 29·91; the temperature of the intermediate air was 57°; find the height.

The illustration (Fig. 33) shows the instrument with the telescopic tube drawn out for use, and the thermometer surrounded by the vapour of boiling water. The lamp is protected from wind by a perforated japanned tin case covered with wire gauze. When the boiler is charged and the lamp ignited the mercury ascends, and the point at which it becomes stationary shows the temperature, which will give the elevation in feet above the sea-level on reference to the table supplied by the optician from whom the instrument is purchased.

34.
Barograph. Scale about 1/6.

A highly-refined automatic arrangement is adopted at some observatories called a Barograph, which, by the aid of photography, becomes a self-recording mercurial barometer. It is simpler in its arrangement than the thermograph, and includes a clock of superior construction, causing a cylinder bearing photographic paper to make one complete revolution in forty-eight hours. A double combination of achromatic lenses brings to a focus rays passing through a slit placed in front of the mercurial column, behind which is a strong gaslight or paraffin lamp, the rays of which are condensed upon the slit by a combination of two plano-convex lenses.

Although a barometer is an instrument artificially constructed by man, it should not be forgotten that when once made the column of mercury is placed in a passive or quiescent state in direct relation with the great forces of nature, so that its indications become to some extent natural phenomena. This is aptly illustrated by what is called the “daily fluctuation” of the barometer which occurs in all countries, though the hours and extent vary with the latitude, diminishing as the latitude increases, according to a definite law. The phenomena does not admit of a satisfactory explanation, but is doubtless connected with the daily variations of temperature and of vapour in the air. The mercury falls naturally (so to speak) from nine or ten to between three and four p.m.; it then rises till between nine and ten p.m. It falls again about four a.m., and rises again about ten a.m. It is usually highest at nine a.m. and nine p.m., and lowest at three a.m. and three p.m.

These natural elevations and depressions of the mercury should be allowed for in reading the barometer, as any rise or fall in opposition to the natural rise and fall possesses for that reason increased importance. For instance, fine weather may be expected if the mercury rises between nine a.m. and three p.m.; in like manner rain may be expected should a fall take place between three p.m. and nine p.m.

It will be inferred from the preceding facts that there are certain hours better suited for “taking a reading” than others. When one observation only is made daily, noon is the best time, two observations should be made at nine a.m. and nine p.m., and for three the best hours are nine a.m. (maximum), noon (mean), and three p.m. (minimum).

The opinion generally entertained that a high barometer is an indication of fine weather, and a low one a warning of bad weather, is open to exception, and an increased value would attach to the indications of the instrument in proportion as the following points are noted and allowed for:—

1. The actual height of the mercury. 2. Whether it is rising or falling. 3. The rate of rise and fall. 4. Whether the rise or fall has been long continued.

The state of the barometer foretells coming weather, and when the present weather disagrees with the barometer a change will soon take place. A fall of half a tenth, or more, in an hour is a sure warning of a storm, a rapid rise is a warning of unsettled weather.

The barometer is generally lowest with wind from the S.W., and highest with wind N.E., or with a calm. N.E. and S.W. may be called the wind’s poles, and the difference of height due to direction only from one of these bearings to another amounts to about half an inch.

BAROMETER PRECAUTIONS.

If vacuum suspected, cause mercury to strike top of tube.

A clear metallic “click” indicates a good vacuum.

A dull “thud” indicates air or moisture.

In latter case return to optician, but if unable

Incline very gently until nearly inverted, when

Air if present will ascend in a bubble into the cistern.

Suspend barometer in good light out of sunshine.

Let no heat of fire or lamp affect it.

Let no sudden changes of temperature affect it.

It must hang absolutely vertically.

Note temperature of attached thermometer before reading barometer.

Then adjust mercury in cistern to touch ivory point.

Then adjust vernier and take reading quickly.

Ascertain height above sea-level according to direction.

The Storm Glass (Fig. 36) is a glass bottle, ten inches long, containing a mixture of camphor, nitre, sal-ammoniac, alcohol, and water. As “temperature affects the mixture much,” an arrangement has recently been designed in which the stem of a thermometer is immersed in the fluid, as shown at Fig. 37, thus imparting a higher value to its indications. The late Admiral Fitzroy says—

“Since 1825, we have generally had some of these glasses, as curiosities rather than otherwise; for nothing certain could be made of their variations until lately, when it was fairly demonstrated that if fixed undisturbed in free air, not exposed to radiation, fire, or sun, but in the ordinary light of a well-ventilated room, or, preferably, in the outer air, the chemical mixture in a so-called storm glass varies in character with the direction of the wind—not its force.”

The quarter from which the wind or storm is blowing is indicated by the substance adhering more closely to the bottom of the glass opposite to the point whence the wind or tempest arises.

The Sympiesometer is an instrument used chiefly at sea for purposes of comparison with the mercurial and aneroid barometers. Its indications result partly from the pressure and partly from the temperature of the atmosphere; it would, therefore, be more correctly named a Thermo-Barometer.

35. 36. 37.
Storm Glass, or Chemical Weather Glass. Scale about 1/5.

The height of the atmosphere has been variously estimated:—By Bravais, from the duration of twilight, at 66 to nearly 100 miles; by Dalton, in 1819, from observations of the auroral light, at 102 miles; by Sir John Herschel, from similar observations in 1861, at 83 miles; from observations of meteors, from 100 to 200 miles; by Liais, in 1859, from observations on the polarisation of the sky, at no less than 212 miles.

The density of the atmosphere diminishes with distance from the earth’s surface, in accordance with the following rule:—“At a height of seven miles the density of the atmosphere is reduced to one-fourth the density at the sea-level, and for every additional seven miles, the rarity of the air is similarly quadrupled.”

NOTE ON THE VERIFICATION OF INSTRUMENTS AT THE
KEW OBSERVATORY.

The Kew Committee of the Royal Society receive, for verification and comparison with the standard instruments of the Kew Observatory, barometers, thermometers, and other instruments intended for meteorological observation or scientific investigations.

Any persons ordering instruments of opticians may direct them to be previously forwarded to the observatory for verification.

A scale of charges is issued by the Committee which is exclusive of packing and carriage, or of rail expenses, when a special messenger is sent out. The Meteorological Office, Victoria Street, London, also receives and forwards instruments for verification to the Kew Observatory.

The Committee wish it to be understood that they cannot undertake the verification of an inferior class of instruments (such as barometers mounted upon wooden frames, and thermometers not graduated on the stem), and that the superintendent of the observatory may at his discretion decline to receive such instruments as he may consider unfit for scientific observation.

BAROMETER WARNINGS.

IV.—CONDENSATION.

Dew is a deposition of moisture from the air, resulting from the condensation of the aqueous vapour of the atmosphere on substances which have become cooled by the radiation of their heat. This is, in fact, the substance of Dr. Wells’s famous Theory of Dew, enunciated in 1814, and which, according to Dr. Tyndall, “has stood the test of all subsequent criticism, and is now universally accepted,” and by which all the phenomena of dew may be explained.

Dr. Wells’s experiments were interesting and conclusive. He exposed definite weights (10 grains) of wool to the air on clear nights, one on a four-legged stool, the other under it, the upper portion gained 14 grains in weight, the lower only 4 grains. On an evening when one portion of wool, protected by a curved pasteboard roof, gained only 2 grains, a similar portion on the top of the miniature roof gained 16 grains. A little reflection will suggest the explanation: radiation from the wool was arrested by the pasteboard cover, while the portion fully exposed to the sky lost all its heat, and thus condensation ensued. Dr. Wells speaks with such candour, and so pointedly, on this fact and its consequences, that his words may be advantageously quoted: “I had often, in the pride of half-knowledge, smiled at the means frequently employed by gardeners to protect tender plants from cold, as it appeared to me impossible that a thin mat, or any such flimsy substance, could prevent them from attaining the temperature of the atmosphere, by which alone I thought them liable to be injured. But when I had learned that bodies on the surface of the earth become during a still and serene night colder than the atmosphere, by radiating their heat to the heavens, I perceived immediately a just reason for the practice I had before deemed useless.”

Familiar instances of the formation of dew will have been noted by many “watchers;” e. g., breathing on a cold pane of glass, a tumbler of cold water becoming dew-covered on being brought into a warm room, the outside of a tankard of iced claret cup, &c. When, radiation is so free and rapid that the temperature is below the freezing point, the dew freezes as it forms, producing hoar-frost.

In our climate the air is never completely dry, nor completely saturated with moisture, and the amount of aqueous vapour held in suspension is very variable. This fact has important bearings on many branches of industry, as also on the hygienic qualities of the atmosphere. The consideration that a certain amount of moisture in the air is necessary to the continuance of health will suggest the importance of maintaining that due proportion in the atmosphere of sick rooms, where the artificial heat so injudiciously used, often disturbs the healthful hygrometric condition of the air. Mr. Glaisher is of opinion that the medical profession should enforce, as far as lies in their power, the use of this simple and effective instrument, which gives indications so important to the comfort of the patient.

The amount of moisture in the air is estimated by the use of instruments called Hygrometers, which may be thus classified:—

1. Hygrometers of Absorption.—Made with hair, oatbeard, catgut, seaweed, grass, chloride of calcium.

2. Hygrometers of Condensation.—Regnault’s, Daniell’s, Leslie’s, Dyne’s.

3. Hygrometers of Evaporation.—Mason’s Psychrometer, or Wet and Dry Bulb Thermometers.

By an ingenious application of the affinity of the oatbeard for moisture, Damp Detectors are constructed for tourists, commercial travellers, &c., to test moisture and avoid the consequences of sleeping in damp beds. They are strongly gilt, and resemble in size and shape a lady’s watch.

38.
Damp Detector.
Scale about 2/3.

In Saussaure’s Hygrometer the frame is of brass, and the scale of the same metal silvered. It has an attached thermometer, and the indications are the result of the contraction and expansion of a prepared human hair, consequent upon its absorbing or yielding moisture. The scale is divided on the arc of a circle, and an index needle, working on an enlarged arc, multiples the indications.

Regnault’s Hygrometer (Fig. 39) consists of a thin and highly polished silver tube or bottle, into the neck of which is inserted a delicate thermometer. The bottle has a lateral tubular opening, to which is attached a flexible tube with an ivory mouthpiece.

Ether is poured into the silver tube in sufficient quantity to cover the bulb of the thermometer. The ether is then agitated by breathing through the flexible tube until, by the rapid evaporation thus produced, a condensation of moisture takes place, readily observable on the bright polished silver surface, and the temperature indicated by the thermometer at that moment is the dew-point.

39.
Regnault’s Hygrometer. Scale
about 1/10.

Daniell’s Hygrometer, or Dew-point Thermometer (Fig. 40), consists of a glass tube, bent twice at right angles, each extremity terminating in a bulb about 1-1/2 inch in diameter, supported on a brass stand, to which a thermometer is attached to indicate the temperature of the surrounding air. The lower bulb is of blackened glass, to facilitate the observation of the dew-point; it is about three parts filled with pure ether, and contains a very delicate thermometer. The upper bulb at the extremity of the short stem is transparent, but covered with thin muslin, upon which, when an observation is made, pure ether is slowly dropped. The evaporation rapidly lowers the temperature, until a moment arrives at which dew condenses on the black bulb. A quick eye is necessary to note this and the temperature shown by the thermometer simultaneously, the latter showing the degree at which the atmosphere is saturated with moisture at the time of observation. To avoid error, it is usual to note the temperature at which the dew disappears, and take the mean of the two temperatures.

40.
Daniell’s Hygrometer.
Scale about 1/5.

Dyne’s Hygrometer, for showing the dew-point by direct observation, by means of iced water and black glass, enables the observer to dispense with the use of ether, and shows the dew-point with great distinctness.

41.
Mason’s Hygrometer.
Scale about 1/6.

The hygrometer in most general use is the wet and dry bulb thermometer, and for which Mr. Glaisher has calculated an elaborate set of tables, a brief abstract of which sufficient for general purposes is subjoined.