A Treatise on Meteorological Instruments / Explanatory of Their Scientific Principles, Method of Construction, and Practical Utility

A TREATISE

ON

METEOROLOGICAL INSTRUMENTS.

LONDON:
PRINTED BY WILLIAMS AND STRAHAN,
7 LAWRENCE LANE, CHEAPSIDE, E.C.

A TREATISE

ON

METEOROLOGICAL
INSTRUMENTS:

EXPLANATORY OF

THEIR SCIENTIFIC PRINCIPLES,

METHOD OF CONSTRUCTION, AND PRACTICAL UTILITY.

BY

NEGRETTI & ZAMBRA,

METEOROLOGICAL INSTRUMENT MAKERS TO THE QUEEN, THE ROYAL OBSERVATORY, GREENWICH,
THE BRITISH METEOROLOGICAL SOCIETY, THE BRITISH AND FOREIGN GOVERNMENTS,
ETC. ETC. ETC.

LONDON:
PUBLISHED AND SOLD AT NEGRETTI & ZAMBRA’S ESTABLISHMENTS:
1 HATTON GARDEN, E.C., 59 CORNHILL, E.C., 122 REGENT STREET W.,
AND 153 FLEET STREET, E.C.

1864.

Price Five Shillings.

PREFACE.

The national utilisation of Meteorology in forewarning of storms, and the increasing employment of instruments as weather indicators, render a knowledge of their construction, principles, and practical uses necessary to every well-informed person. Impressed with the idea that we shall be supplying an existing want, and aiding materially the cause of Meteorological Science, in giving a plain description of the various instruments now in use, we have endeavoured, in the present volume, to condense such information as is generally required regarding the instruments used in Meteorology; the description of many of which could only be found in elaborate scientific works, and then only briefly touched upon. Every Meteorological Instrument now in use being fully described, with adequate directions for using, the uninitiated will be enabled to select those which seem to them best adapted to their requirements. With accounts of old or obsolete instruments we have avoided troubling the reader; on the other hand, we were unwilling to neglect those which, though of no great practical importance, are still deserving of notice from their being either novel or ingenious, or which, without being strictly scientific, are in great demand as simple weather-glasses and articles of trade.

We trust, therefore, that the work (however imperfect), bearing in mind the importance of the subject, will be acceptable to general readers, as well as to those for whose requirements it has been prepared.

The rapid progress made in the introduction of new apparatus of acknowledged superiority has rendered the publication of some description absolutely necessary. The Report of the Jurors for Class XIII. of the International Exhibition, 1862, on Meteorological Instruments, fully bears out our assertion, as shown by the following extract:—

“The progress in the English department has been very great;—in barometers, thermometers, anemometers, and in every class of instruments. At the close of the Exhibition of 1851, there seemed to have arisen a general anxiety among the majority of makers to pay every attention to all the essentials necessary for philosophical instruments, not only in their old forms, but also with the view of obtaining other and better forms. This desire has never ceased; and no better idea can be given of the continued activity in these respects, than the number of patents taken out for improvements in meteorological instruments in the interval between the recent and preceding exhibitions, which amount to no less than forty-two.” * * * “In addition to numerous improvements patented by Messrs. Negretti and Zambra, there is another of great importance, which they did not patent, viz. enamelling the tubes of thermometers, enabling the makers to use finer threads of mercury in the construction of all thermometers; for the contrast between the opaque mercury and the enamel back of the tubes is so great, that the finest bore or thread of mercury, which at one time could not be seen without the greatest difficulty, is now seen with facility; and throughout the British and Foreign departments, the makers have availed themselves of this invention, the tubes of all being made with enamelled backs. It is to be hoped that the recent exhibition will give a fresh stimulus to the desire of improvement, and that the same rate of progress will be continued.”

To fulfil the desire of the International Jury in the latter portion of the above extract will be the constant study of

NEGRETTI & ZAMBRA.

1st January, 1864.

TABLE OF CONTENTS.

TABLES.

ADDENDA.

METEOROLOGICAL INSTRUMENTS.

In the pursuits and investigations of the science of Meteorology, which is essentially a science of observation and experiment, instruments are required for ascertaining, 1. the pressure of the atmosphere at any time or place; 2. the temperature of the air; 3. the absorption and radiation of the sun’s heat by the earth’s surface; 4. the humidity of the air; 5. the amount and duration of rainfall; 6. the direction, the horizontal pressure, and the velocity of winds; 7. the electric condition of the atmosphere, and the prevalence and activity of ozone.

CHAPTER I.

INSTRUMENTS FOR ASCERTAINING THE ATMOSPHERIC PRESSURE.

Fig. 1.

1. Principle of the Barometer.—The first instrument which gave the exact measure of the pressure of the atmosphere was invented by Torricelli, in 1643. It is constructed as follows:—A glass tube, CD (fig. 1), about 34 inches long, and from two to four-tenths of an inch in diameter of bore, having one end closed, is filled with mercury. In a cup, B, a quantity of mercury is also poured. Then, placing a finger securely over the open end, C, invert the tube vertically over the cup, and remove the finger when the end of the tube dips into the mercury. The mercury in the tube then partly falls out, but a column, AB, about 30 inches in height, remains supported. This column is a weight of mercury, the pressure of which upon the surface of that in the cup is precisely equivalent to the corresponding pressure of the atmosphere which would be exerted in its place if the tube were removed. As the atmospheric pressure varies, the length of this mercurial column also changes. It is by no means constant in its height; in fact, it is very seldom stationary, but is constantly rising or falling through a certain extent of the tube, at the level of the sea, near which the above experiment is supposed to be performed. It is, therefore, an instrument by which the fluctuations taking place in the pressure of the atmosphere, arising from changes in its weight and elasticity, can be shown and measured. It has obtained the name Barometer, or measurer of heaviness,—a word certainly not happily expressive of the utility of the invention. If the bore of the barometer tube be uniform throughout its length, and have its sectional area equal to a square inch, it is evident that the length of the column, which is supported by the pressure of the air, expresses the number of cubic inches of mercury which compose it. The weight of this mercury, therefore, represents the statical pressure of the atmosphere upon a square inch of surface. In England the annual mean height of the barometric column, reduced to the sea-level and to the temperature of 32° Fahrenheit, is about 29·95 inches. A cubic inch of mercury at this temperature has been ascertained to weigh 0·48967 lbs. avoirdupois. Hence, 29·95 × 0·48967= 14·67 lbs., is the mean value of the pressure of the atmosphere on each square inch of surface, near the sea-level, about the latitude of 50 degrees. Nearer the equator this mean pressure is somewhat greater; nearer the poles, somewhat less. For common practical calculations it is assumed to be 15 lbs. on the square inch. When it became apparent that the movements of the barometric column furnished indications of the probable coming changes in the weather, an attempt was made to deduce from recorded observations the barometric height corresponding to the most notable characteristics of weather. It was found that for fine dry weather the mercury in the barometer at the sea-level generally stood above 30 inches; changeable weather happened when it ranged from 30 to 29 inches, and when rainy or stormy weather occurred it was even lower. Hence, it became the practice to place upon barometer scales words indicatory of the weather likely to accompany, or follow, the movements of the mercury; whence the instruments bearing them obtained the name “Weather Glasses.”

2. Construction of Barometers.—In order that the instrument may be portable, it must be made a fixture and mounted on a support; and, further, to render it scientifically or even practically useful, many precautions are required in its construction. The following remarks apply to the construction of all barometers:—Mercury is universally employed, because it is the heaviest of fluids, and therefore measures the atmospheric pressure by the shortest column. Water barometers have been constructed, and they require to be at least 34 feet long. Oil, or other fluids, might be used. Mercury, however, has other advantages: it has feeble volatility, and does not adhere to glass, if pure. Oxidised, or otherwise impure mercury, may adhere to glass; moreover, such mercury would not have the density of the pure metal, and therefore the barometric column would be either greater or less than it should be. The mercury of commerce generally contains lead; sometimes traces of iron and sulphur. It is necessary, therefore, for the manufacturer to purify the mercury; and this is done by washing it with diluted acetic, or sulphuric acid, which dissolves the impurities. No better test can be found for ascertaining if the mercury be pure than that of filling a delicate thermometer tube; if, on exhausting the air from this thermometer, the mercury will freely run up and down the bore, which is probably one thousandth of an inch in diameter, the mercury from which this thermometer was made will be found fit for any purpose, and with it a tube may be filled and boiled, not only of one inch, but even of two inches diameter. In all barometers it is requisite that the space above the mercurial column should be completely void of air and aqueous vapour, because these gases, by virtue of their elasticity, would depress the column. To exclude these the mercury is introduced, and boiled in the tube, over a charcoal fire, kept up for the purpose. In this manner the air and vapour which adhere to the glass are expanded, and escape away. One can tell whether a barometer has been properly “boiled,” as it is termed, by simply holding the tube in a slanting direction and allowing the mercury to strike the top. If the boiling has been well performed, the mercury will give a clear, metallic sound; if not, a dull, flat sound, showing some air to be present.

When the mercury in a barometer tube rises or falls, the level of the mercury in the cup, or cistern, as it is generally termed, falls or rises by a proportionate quantity, which depends upon the relative areas of the interior of the tube and of the cistern. It is necessary that this should be taken into consideration in ascertaining the exact height of the column. If a fixed scale is applied to the tube, the correct height may be obtained by applying a correction for capacity. A certain height of the mercury is ascertained to be accurately measured by the scale, and should be marked on the instrument as the neutral point. Above this point the heights measured are all less, and below, all more, than they should be. The ratio between the internal diameters of the tube and cistern (which should also be stated on the instrument, as, for instance, capac. ¹⁄50) supplies the data for finding the correction to be applied. This correction is obviated by constructing the cistern so as to allow of the surface of the mercury in it being adjustable to the commencement of the fixed scale, as by Fortin’s or Negretti’s plan. It is also unnecessary in barometers constructed on what is now called the “Kew method.” These will all be detailed in their proper place. The tube, being fixed to the cistern, may have a moveable scale applied to it. But such an arrangement requires the utmost care and skill in observing, and is seldom seen except in first-class Observatories.

[Larger Image]
Fig. 2.

3. Fortin’s Barometer.—Fortin’s plan of constructing a barometer cistern is shown in fig. 2. The cistern is formed of a glass cylinder, which allows of the level of the mercury within being seen. The bottom of the cylinder is made of sheep-skin or leather, like a bag, so as to allow of being pushed up or lowered by means of a screw, D B, worked from beneath. This screw moves through the bottom of a brass cylinder, C C, which is fixed outside, and protects the glass cylinder containing the mercury. At the top of the interior of the cistern is fixed a small piece of ivory, A, the point of which exactly coincides with the zero of the scale. This screw and moveable cistern-bottom serve also to render the barometer portable, by confining the mercury in the tube, and preventing its coming into the cistern, which is thus made too small to receive it.

Fig. 3.

4. STANDARD BAROMETER.

Fig. 3 represents a Standard Barometer on Fortin’s principle. The barometer tube is enclosed and protected by a tube of brass extending throughout its whole length; the upper portion of the brass tube has two longitudinal openings opposite each other; on one side of the front opening is the barometrical scale of English inches, divided to show, by means of a vernier, ¹⁄500th of an inch; on the opposite side is sometimes divided a scale of French millimetres, reading also by a vernier to ¹⁄10th of a millimetre (see directions for reading the vernier, [page 7]). A thermometer, C, is attached to the frame, and divided to degrees, which can be read to tenths; it is necessary for ascertaining the temperature of the instrument, in order to correct the observed height of the barometer.

As received by the observer, the barometer will consist of two parts, packed separately for safety in carriage,—1st, the barometer tube and cistern, filled with mercury, the brass tube, with its divided scale and thermometer; and 2nd, a mahogany board, with bracket at top, and brass ring with three adjusting screws at bottom.

Directions for fixing the Barometer.—In selecting a position for a barometer, care should be taken to place it so that the sun cannot shine upon it, and that it is not affected by direct heat from a fire. The cistern should be from two to three feet above the ground, which will give a height for observing convenient to most persons. A standard barometer should be compared with an observatory standard of acknowledged accuracy, to determine its index error; which, as such instruments are graduated by micrometrical apparatus of great exactitude, will be constant for all parts of the scale. It should be capable of turning on its axis by a movement of the hand, so that little difficulty can ever be experienced in obtaining a good light for observation. Having determined upon the position in which to place the instrument, fix the mahogany board as nearly vertical as possible, and ascertain if the barometer is perfect and free from air, in the following manner:—lower the screw at the bottom of the cistern several turns, so that the mercury in the tube, when held upright, may fall two or three inches from the top; then slightly incline the instrument from the vertical position, and if the mercury in striking the top elicit a sharp tap, the instrument is perfect. Supposing the barometer to be in perfect condition, as it is almost sure to be, it is next suspended on the brass bracket, its cistern passing through the ring at bottom, and allowed to find its vertical position, after which it is firmly clamped by means of the three thumb-screws.

To Remove the Instrument when fixed to another Position.—If it should be necessary to remove the barometer,—first, by means of the adjusting screw, drive the mercury to the top of the tube, turning it gently when it is approaching the top, and cease directly any resistance is experienced; next, remove from the upper bracket or socket; lift the instrument and invert it, carrying it with its lower end upwards.

Directions for taking an Observation.—Before making an observation, the mercury in the cistern must be raised or lowered by means of the thumb-screw, F, until the ivory point, E, and its reflected image in the mercury, D, are just in contact; the vernier is then moved by means of the milled head, until its lower termination just excludes the light from the top of the mercurial column; the reading is then taken by means of the scale on the limb and the vernier. The vernier should be made to read upward in all barometers, unless for a special object, as this arrangement admits of the most exact setting. In observing, the eye should be placed in a right line with the fore and back edges of the lower termination of the vernier; and this line should be made to form a tangent to the apex of the mercurial column. A small reflector placed behind the vernier and moving with it, so as to assist in throwing the light through the back slit of the brass frame on to the glass tube, is advantageous; and the observer’s vision may be further assisted by the aid of a reading lens. The object is, in these Standard Barometers, to obtain an exact reading, which can only be done by having the eye, the fore part of the zero edge of the vernier, the top of the mercurial column, and the back of the vernier, in the same horizontal plane.

Uniformity of Calibre.—The diameter of that part of the tube through which the oscillations of the mercury will take place is very carefully examined to insure uniformity of calibre, and only those tubes are used which are as nearly as possible of the same diameter throughout. The size of the bore should be marked on the frame of the barometer in tenths and hundredths of an inch. A correction due to capillary action, and depending on the size of the tube, must be applied to the readings.

5. Correction due to Capillarity.—When an open tube of small bore is plunged into mercury, the fluid will not rise to the same level inside as it has outside. Hence, the effect of capillary action is to depress the mercurial column; and the more so the smaller the tube. The following table gives the correction for tubes in ordinary use:—

This correction is always additive to the observed reading of the barometer.

6. Correction due to Temperature.—In all kinds of mercurial barometers attention must be given to the temperature of the mercury. As this metal expands and contracts very much for variations of temperature, its density alters correspondingly, and in consequence the height of the barometric column also varies. To ascertain the temperature of the mercury, a thermometer is placed near the tube, and is sometimes made to dip into the mercury in the cistern. The freezing point of water, 32°F., is the temperature to which all readings of barometers must be reduced, in order to make them fairly comparable. The reduction may be effected by calculation, but the practical method is by tables for the purpose; and for these tables we refer the reader to the works mentioned at the end of this book.

7. Correction due to Height above the Half-tide Level.—Further, in order that barometrical observations generally may be made under similar circumstances, the readings, corrected for capacity, capillarity, and temperature, should be reduced to what they would be at the sea-level, by adding a correction corresponding to the height above the mean level of the sea, or of half-tide. For practical purposes of comparison with barometric pressure at other localities, add one-tenth of an inch to the reading for each hundred feet of elevation above the sea. For scientific accuracy this will not suffice, but a correction must be obtained by means of Schuckburg’s formula, or tables computed therefrom.

Fig. 4.

Fig. 5.

8. The Barometer Vernier.—The vernier, an invaluable contrivance for measuring small spaces, was invented by Peter Vernier, about the year 1630. The barometer scale is divided into inches and tenths. The vernier enables us to accurately subdivide the tenths into hundredths, and, in first-class instruments, even to thousandths of an inch. It consists of a short scale made to pass along the graduated fixed scale by a sliding motion, or preferably by a rack-and-pinion motion, the vernier being fixed on the rack, which is moved by turning the milled head of the pinion. The principle of the vernier, to whatever instrumental scale applied, is that the divisions of the moveable scale are to those in an equal length of the fixed scale in the proportion of two numbers which differ from each other by unity.

The scales of standard barometers are usually divided into half-tenths, or ·05, of an inch, as represented, in fig. 5, by AB. The vernier, CD, is made equal in length to twenty-four of these divisions, and divided into twenty-five equal parts; consequently one space on the scale is larger than one on the vernier by the twenty-fifth part of ·05, which is ·002 inch, so that such a vernier shows differences of ·002 inch. The vernier of the figure reading upwards, the lower edge, D, will denote the top of the barometer column; and is the zero of the vernier scale. In fig. 4, the zero being in line exactly with 29 inches and five-tenths of the fixed scale, the barometer reading would be 29·500 inches. It will be seen that the vernier line, a, falls short of a division of the scale by, as we have explained, ·002 inch; b, by ·004; c, by ·006; d, by ·008; and the next line by one hundredth. If, then, the vernier be moved so as to make a coincide with z, on the scale, it will have moved through ·002 inch; and if 1 on the vernier be moved into line with y on the scale, the space measured will be ·010. Hence, the figures 1, 2, 3, 4, 5 on the vernier measure hundredths, and the intermediate lines even thousandths of an inch. In fig. 5, the zero of the vernier is intermediate 29·65 and 29·70 on the scale. Passing the eye up the vernier and scale, the second line above 3 is perceived to lie evenly with a line of the scale. This gives ·03 and ·004 to add to 29·65, so that the actual reading is 29·684 inches. It may happen that no line on the vernier accurately lies in the same straight line with one on the scale; in such a case a doubt will arise as to the selection of one from two equally coincident, and the intermediate thousandth of an inch should be taken.

For the ordinary purposes of the barometer as a “weather-glass,” such minute measurement is not required. Hence, in household and marine barometers the scale need only be divided to tenths, and the vernier constructed to measure hundredths of an inch. This is done by making the vernier either 9 or 11-10ths of an inch long, and dividing it into ten equal parts. The lines above the zero line are then numbered from 1 to 10; sometimes the alternate divisions only are numbered, the intermediate numbers being very readily inferred. Hence, if the first line of the vernier agrees with one on the scale, the next must be out one-tenth of a tenth, or ·01 of an inch from agreement with the next scale line; the following vernier line must be ·02 out, and so on. Consequently, when the vernier is set to the mercurial column, the difference shown by the vernier from the tenth on the scale is the hundredths to be added to the inches and tenths of the scale.

A little practice will accustom a person to set and read any barometer quickly; an important matter where accuracy is required, as the heat of the body, or the hand, is very rapidly communicated to the instrument, and may vitiate, to some extent, the observation.

Fig. 6.

9. SELF-COMPENSATING STANDARD BAROMETER.

This barometer has been suggested to Messrs. Negretti and Zambra by Wentworth Erk, Esq. It consists of a regular barometer; but attached to the vernier is a double rack worked with one pinion, so that in setting or adjusting the vernier in one position, the second rack moves in directly the opposite direction, carrying along with it a plug or plunger the exact size of the internal diameter of the tube dipping in the cistern, so that whatever the displacement that has taken place in the cistern, owing to the rise or fall of the mercury, it is exactly compensated by the plug being more or less immersed in the mercury, so that no capacity correction is required.

A barometer on this principle is, however, no novelty, for at the Royal Society’s room a very old instrument may be seen reading somewhat after the same manner.

Fig. 6 is an illustration of the appearance of this instrument. The cistern is so constructed that the greatest amount of light is admitted to the surface of the mercury.

10. BAROMETER WITH ELECTRICAL ADJUSTMENT.

This barometer is useful to persons whose eyesight may be defective; and is capable of being read off to greater accuracy than ordinary barometers, as will be seen by the following description:—The barometer consists of an upright tube dipping into a cistern, so contrived, that an up-and-down movement, by means of a screw, can be imparted to it. In the top of the tube a piece of platina wire is hermetically sealed. The cistern also has a metallic connection, so that by means of covered copper wires (in the back of the frame) a circuit is established; another connection also exists by means of a metallic point dipping into the cistern. The circuit, however, can be cut off from this by means of a switch placed about midway up the frame; on one side of the tube is placed a scale of inches; a small circular vernier, divided into 100 parts, is connected with the dipping point, and works at right angles with this scale.

To set the instrument in action for taking an observation, a small battery is connected by means of two small binding screws at the bottom of the frame. The switch is turned upwards, thereby disconnecting the dipping point; the cistern is then screwed up, so that the mercury in the tube is brought into contact with the platina wire at the top; the instant this is effected the magnetic needle seen on the barometer will be deflected. The switch is now turned down; by so doing the connection with the upper wire or platina is cut off, and established instead only between the dipping point carrying the circular vernier and the bottom of the cistern; the point is now screwed by means of the milled head until the needle is again deflected. We may now be sure that the line on the circular vernier that cuts the division on the scale is the exact height of the barometer. Although the description here given may seem somewhat lengthy, the operation itself is performed in less time than would be taken in reading off an ordinary instrument.

11. PEDIMENT BAROMETERS.

These Barometers, generally for household purposes, are illustrated by figs. 7 to 11. They are intended chiefly for “weather glasses,” and are manufactured to serve not only a useful, but an ornamental purpose as well. They are usually framed in wood, such as mahogany, rosewood, ebony, oak or walnut, and can be obtained either plain or handsomely and elaborately carved and embellished, in a variety of designs, so as to be suitable for private rooms, large halls, or public buildings. The scales to the barometer and its attached thermometer may be ivory, porcelain, or silvered metal. It is not desirable that the vernier should read nearer than one-hundredth of an inch. Two verniers and scales may be fitted one on either side of the mercurial column, so that one can denote the last reading, and thus show at a glance the extent of rise or fall in the interval. The scale and thermometer should be covered with plate glass. A cheap instrument has an open face and plain frame, with sliding vernier instead of rack-and-pinion motion. The barometer may or may not have a moveable bottom to the cistern, with screw for the purpose of securing the mercury for portability. The cistern should not, however, require adjustment to a zero or fiducial point. It should be large enough to contain the mercury, which falls from 31 to 27 inches, without any appreciable error on the height read off on the scale.

12. The Words on the Scale.—The following words are usually engraved on the scales of these barometers, although they are not now considered of so much importance as formerly:—

The French place upon their barometers a similar formula:—

Manufacturers of barometers have uniformly adopted these indications for all countries, without regard to the elevation above the sea, or the different geographical conditions; and as it can readily be shown that the height and variations of the barometer are dependent on these, it follows that barometers have furnished indications which, under many circumstances, have been completely false. Even in this country, and near the sea-level, storms are frequent with the barometer not below 29; rain is not uncommon with the glass at 30; even fine weather sometimes occurs with a low pressure; while it is evident that at an elevation of a few thousand feet the mercury would never rise to 30 inches; hence, according to the scale, there should never be fair weather there. If tempests happened as seldom in our latitude as the barometer gets down to 28 inches, the maritime portion of the community at least would be happy indeed. These words have long been ridiculed by persons acquainted with the causes of the barometric fluctuations; nevertheless opticians continue to place them on the scales, evidently only because they appear to add to the importance of the instrument in the eyes of those who have not learned their general inutility. In different regions of the world, the indications of the barometer are modified by the conditions peculiar to the geographical position and elevation above the sea, and it is necessary to take account of these in any attempt to found rules of general utility in connection with the barometer as a weather guide. All that can be said in favour of these words is, that within a few hundred feet of the sea-level, when the column rises or falls gradually during two or three days towards “Fair” or “Rain,” the indications they afford of the coming weather are generally extremely probable; but when the variations are quick, upward or downward, they presage unsettled or stormy weather.

Admiral FitzRoy writes:—“The words on the scales of barometers should not be so much regarded, for weather indications, as the rising or falling of the mercury; for if it stands at Changeable, and then rises a little towards Fair, it presages a change of wind or weather, though not so great as if the mercury had risen higher; and, on the contrary, if the mercury stands above Fair and falls, it presages a change, though not to so great a degree as if it had stood lower; besides which, the direction and force of wind are not in any way noticed. It is not from the point at which the mercury stands that we are alone to form a judgment of the state of the weather, but from its rising or falling; and from the movements of immediately preceding days as well as hours, keeping in mind effects of change of direction and dryness, or moisture, as well as alteration of force or strength of wind.”[1]

13. Correction due to Capacity of Cistern.—These barometers, having no adjustment for the zero of the scale, require a correction for the varying level of the mercury in the cistern, when the observations are required for strict comparison with other barometric observations, or when they are registered for scientific purposes; but for the common purpose of predicting the weather, this correction is unnecessary. The neutral point, and the ratio of the bore of the tube to the diameter of the cistern, must be known (see [p. 3]). Then the capacity correction, as it is termed, is found as follows:—Take the fractional part, expressed by the capacity ratio, of the difference between the observed reading and the height of the neutral point; then, if the mercury stand below the neutral point, subtract this result from the reading; if it stand above, add it to the reading.

For example, suppose the neutral point to be 29·95 inches, and the capacity ratio ¹⁄50, required the correction when the barometer reads 30·78.

Of course the correction could as easily be found to three decimal places, if desirable. It is evident that the correction is more important the greater the distance of the top of the mercury from the neutral point.

14. PUBLIC BAROMETERS.

Since the increased attention paid to the signs of forthcoming weather of late years, and the good which has resulted therefrom to farmers, gardeners, civil engineers, miners, fishermen, and mariners generally, by forewarning of impending wet or stormy weather, the desirability of having good barometers exposed in public localities has become evident.

Barometers may now be seen attached to drinking fountains, properly protected, and are frequently consulted by the passers-by. But it is among those whose lives are endangered by sudden changes in the weather, fishermen especially, that the warning monitor is most urgently required. Many poor fishing villages and towns have therefore been provided by the Board of Trade, at the public expense, and through the humane effort of Admiral FitzRoy, with first-class barometers, each fixed in a conspicuous position, so as to be easily accessible to all who desire to consult it. Following this example, the Royal National Life Boat Institution has supplied each of its stations with a similar storm warner; the Duke of Northumberland and the British Meteorological Society have erected several on the coast of Northumberland; and many other individuals have presented barometers to maritime places with which they are connected.

These barometers have all been manufactured by Messrs. Negretti and Zambra. The form given to the instrument seems well adapted for public purposes.

Fig. 12.

15. Fishery or Sea-coast Barometers.—Fig. 12 gives a representation of these coast and fishery barometers. The frame is of solid oak, firmly screwed together. The scales are very legibly engraved on porcelain by Negretti and Zambra’s patent process. The thermometer is large, and easily read; and as this instrument is exposed, it will indicate the actual temperature sufficiently for practical purposes. The barometer tube is three-tenths of an inch in diameter of bore, exhibiting a good column of mercury; and the cistern is of such capacity, in relation to the tube, that the change of height in the surface of the mercury in the cistern corresponding to a change of height of three inches of mercury in the tube, is less than one-hundredth of an inch, and therefore, as the readings are only to be made to this degree of accuracy, this small error is of no importance. The cistern is made of boxwood, which is sufficiently porous to allow the atmosphere to influence the mercurial column; but the top is plugged with porous cane, to admit of free and certain play.

16. Admiral FitzRoy’s Scale Words.—The directions given on the scales of these barometers were drawn up by Admiral FitzRoy, F.R.S. They appear to be founded on the following considerations:—

Supposing a compass diagram, with the principal points laid down, the N.E. is the wind for which the barometer stands highest; for the S.W. wind it is lowest. This is found to be so in the great majority of cases; but there are exceptions to this, as to all rules. The N.E. and S.W. may therefore be regarded as the poles of the winds, being opposite each other. When the wind veers from the S.W. through W. and N. to N.E., the barometer gradually rises; on the contrary, when the wind veers from N.E. and E. to S.E., S. and S.W., the mercury falls. A similar curious law exists in relation to the veering of the wind, and the action of the thermometer. As the wind veers from the S.W. to W. and N., the thermometer falls; as it veers from N.E. to E. and S., it rises, because the wind gets from a colder to a warmer quarter. The polar winds are cold, dry, and heavy. Those from the equatorial regions are warm, moist, and comparatively light.

These laws have been clearly developed and expressed by Professor Dové in his work on the “Law of Storms.” The warm winds of Europe are those which bring the greatest quantity of rain, as they blow from the ocean, and come heavily laden with moisture. The cold winds, besides containing less moisture, blow more from the land. The weight of the vapour of the warm winds tends to raise the barometric column; but, at the same time, the increased dilatation of the air tends to lower it. This latter influence being the stronger, the barometer always falls for these winds; and in regions where they traverse a large extent of land, retain their heat, and become necessarily very dry, the fall in the barometer will be greater. Admiral FitzRoy’s words for the scales of barometers for use in northern latitudes, then, are as follows:—

It will be perceived that the exception in each case applies to N.E. winds. The barometer may fall with north-easterly winds, but they will be violent and accompanied with rain, hail, or snow; again, it will rise with these winds accompanied with rain, when they are light, and bring only little rain. It rises, however, highest with the dry and light N.E. winds.

These directions are very practically useful; they provide for geographical position—also for elevation above the sea—since they are not appended to any particular height of the column. They are suited to the northern hemisphere generally, as well as around the British Isles. The same directions are adapted to the southern hemisphere, by simply substituting for the letter N the letter S, reading south for north, and vice versa. South of the equator the cold winds come from the south; the warm, from the north. The S.E. wind in the southern hemisphere corresponds to the N.E. in the northern. The laws there are, while the wind veers from S.E. through E. to N. and N.W., the barometer falls and the thermometer rises. As the wind veers from N.W. through W. and S. to S.E., the barometer rises and the thermometer falls.

17. Instructions for the Sea-coast Barometer.—The directions for fixing the barometer, and making it portable when it has to be removed, should be attended to carefully. The barometer should be suspended against a frame or piece of wood, so that light may be seen through the tube. Otherwise a piece of paper, or a white place, should be behind the upper or scale part of the tube.

When suspended on a hook, or stout nail, apply the milled-head key (which will be found just below the scales) to the square brass pin at the lower end of the instrument, and turn gently toward the left hand till the screw stops; then take off the key and replace it for use, near the scale, as it was before. The cistern bottom being thus let down, the mercury will sink to its proper level quickly.

In removing this barometer it is necessary to slope it gradually, till the mercury is at the top of the tube, and then, with the instrument reversed, to screw up the cistern bottom, or bag, by the key, used gently, till it stops. It will then be portable, and may be carried with the cistern end upwards, or lying flat; but it must not be jarred, or receive a concussion.

18. French Sea-coast Barometer.—The French have imitated this form of barometer for coast service, and have translated Admiral FitzRoy’s indications for the scale as follows:—

Fig. 13.

Fig. 14.

MARINE BAROMETERS.

19. The Common Form.—The barometer is of great use to the mariner, who, by using it as a “weather glass,” is enabled to foresee and prepare for sudden changes in the weather. For marine purposes, the lower portion of the glass tube of the barometer must be contracted to a fine bore, to prevent oscillation in the mercurial column, which would otherwise be occasioned by the movements of the ship. This tube is cemented to the cistern, which is made of boxwood, and has a moveable leathern bottom, for the purpose of rendering the instrument portable, by screwing up the mercury compactly in the tube. The tube is enclosed in a mahogany frame, which admits of a variety of style in shape, finish, and display, to meet the different fancies and means of purchasers. The frame is generally enlarged at the upper part to receive the scales and the attached thermometer, which are covered by plate glass. The cistern is encased in brass for protection, the bottom portion unscrewing to give access to the portable screw beneath the cistern. Figs. 13 and 14 illustrate this form of barometer. Marine barometers require to be suspended, so that they may remain in a vertical position under the changeable positions of a vessel at sea. To effect this they are suspended in gimbals by a brass arm. The gimbals consist of a loose ring fastened by thumb-screws to the middle part of the frame of the barometer, in front and back. The forked end of the arm supports this ring at the sides, also by the aid of thumb-screws. Hence the superior weight of the cistern end is always sufficient to cause the instrument to move on its bearing screws, so as always to maintain a perpendicular position; in fact, it is so delicately held that it yields to the slightest disturbance in any direction. The other end of the arm is attached to a stout plate, having holes for screws, or fitted to slip into a staple or bracket, by which it may be fixed to any part of the cabin of a ship; the arm is hinged to the plate, for the purpose of turning the arm and barometer up whenever it is desirable.

Other forms of barometer (to be immediately described) have superseded this in the British Marine, but the French still give the preference to the wooden frames. They think the barometer can be more securely mounted in wood, is more portable, and less liable to be broken by a sudden concussion than if fitted in a metal frame. The English deem the ordinary wooden barometers not sufficiently accurate, owing to the irregular expansion of wood, arising from its hygrometric properties. Some of the English opticians have shown that very portable, and really accurate barometers can be made in brass frames, and therefore the preference is now given to this latter material.

20. The Kew Marine Barometer.—The form of barometer so-called, is that recommended by the Congress of Brussels, held in 1853, for the purpose of devising a systematic plan of promoting meteorological observations at sea.

The materials employed in its construction are mercury, glass, iron, and brass. The upper part of the tube is carefully calibrated to ensure uniformity of bore, as this is a point upon which the accuracy of the instrument to some extent depends. At sea, the barometer has never been known to stand above 31 inches, nor below 27. These extremes have been attained with instruments of undoubted accuracy, but they are quite exceptional. It is not necessary, therefore, to carry the scales of marine barometers beyond these limits, but they should not be made shorter. If the vernier is adjusted to read upward, the scale should extend to 32 inches, to allow room for the vernier to be set to 31 inches at least. Cases have occurred in which this could not be done, and rare, but valuable observations have been lost in consequence. If the scale part of the tube be not uniform in bore, the index error will be irregular throughout the scale. Whether the bore of the rest of the tube varies in diameter, is of no moment. From two to three inches below the measured part, the bore is contracted very much, to prevent the pulsations in the mercurial column—called “pumping”—which, otherwise, would occur at sea from the motion of the ship. In ordinary marine barometers, this contraction extends to the end of the tube. Below the contracted part is inserted a pipette—or Gay Lussac air-trap—which is a little elongated funnel with the point downwards. Its object is to arrest any air that may work in between the glass and the mercury. The bubble of air lodges at the shoulder, and can go up no farther. It is one of those simple contrivances which turn out remarkably useful. If any air gets into the tube, it does not get to the top, and therefore does not vitiate the performance of the barometer; for the mercury itself works up and down through the funnel. Below this, the tube should not be unnecessarily contracted.

Fig. 15.

The open end of the tube is fixed into an iron cylinder, which forms the cistern of the barometer. Iron has no action upon mercury, and is therefore used instead of any other metal. One or two holes are made in the top of the cistern, which are covered on the inside with strong sheep-skin leather, so as to be impervious to mercury, but sufficiently porous for the outer air to act upon the column. The cistern is of capacity sufficient to receive the mercury which falls out of the tube until the column stands lower than the scale reads; and when the tube is completely full, there is enough mercury to cover the extremity so as to prevent access of air. There is no screw required for screwing up the mercury.

The glass tube thus secured to the cistern is protected by a brass tubular frame, into which the iron cistern fits and screws compactly. Cork is used to form bearings for the tube. A few inches above the cistern is placed the attached thermometer. Its bulb is enclosed in the frame, so as to be equally affected by heat with the barometric column. The upper end of the frame is fitted with a cap which screws on, and embraces a glass shield which rests in a gallery formed on the frame below the scale, and serves to protect the silvered scale, as well as the inner tube, from dust and damp. A ring, moveable in a collar fixed on the frame above the centre of gravity of the instrument, is attached to gimbals, and the whole is supported by a brass arm in the usual manner; so that the instrument can be moved round its axis to bring any source of light upon it, and will remain vertical in all positions of the ship. The vernier reads to five-hundredths of an inch. No words are placed upon the scale, as the old formulary was deemed misleading. The vernier can be set with great exactness, as light is admitted to the top of the mercury by a front and a back slit in the frame. The lower edge of the vernier should be brought to the top of the mercury, so as just to shut out the light.

It is evident that this form of barometer must be more reliable in its indications than those in wooden frames. The graduations can be accurately made, and they will be affected only by well-known alterations due to temperature. Some think the tube is too firmly held, and therefore liable to be broken by concussion more readily than that of an inferior instrument. This, however, appears a necessary consequence of greater exactness. It is an exceedingly good portable instrument, and can be put up and taken down very readily. These barometers are preferred to marine barometers in wood, wherever they have been used. In merchant ships, and under careful treatment, they have been found very durable. They may be sent with safety by railway, packed carefully in a wooden box.

Directions for Packing.—In removing this barometer it is necessary to slope it gradually till the mercury reaches the top of the tube. It is then portable, if carried cistern end upwards or lying flat. If carried otherwise, it will very probably be broken by the jerking motion of the heavy mercury in the glass tube. Of course it must not be jarred, or receive concussion.

Position for Marine Barometer.—Admiral FitzRoy, to whose valuable papers we are much indebted, writes in his “Barometer Manual”:—“It is desirable to place the barometer in such a position as not to be in danger of a side blow, and also sufficiently far from the deck above to allow for the spring of the metal arm in cases of sudden movements of the ship.

“If there is risk of the instrument striking anywhere when the vessel is much heeled, it will be desirable to put some soft padding on that place, or to check movement in that direction by a light elastic cord; in fixing which, attention must be paid to have it acting only where risk of a blow begins, not interfering otherwise with the free swing of the instrument: a very light cord attached above, when possible, will be least likely to interfere injuriously.”

21. Method of verifying Marine and other Barometers.—“In nearly all the barometers which had been employed at sea till recently the index correction varied through the range of scale readings, in proportion to the difference of capacity between the cistern and the tube. To find the index correction for a land barometer, comparison with a standard, at any part of the scale at which the mercury may happen to be, is generally considered sufficient. To test the marine barometer is a work of much more time, since it is necessary to find the correction for scale readings at about each half inch throughout the range of atmospheric pressure to which it may be exposed; and it becomes necessary to have recourse to artificial means of changing the pressure of the atmosphere on the surface of the mercury in the cistern.

“The barometers to be thus tested are placed, together with a standard, in an air-tight chamber, to which an air-pump is applied, so that, by partially exhausting the air, the standard can be made to read much lower than the lowest pressure to which marine barometers are likely to be exposed; and by compressing the air it can be made to read higher than the mercury ever stands at the level of the sea. The tube of the standard is contracted similarly to that of the marine barometer, but a provision is made for adjusting the mercury in its cistern to the zero point. Glass windows are inserted in the upper part of the iron air-chamber, through which the scales of the barometers may be seen; but as the verniers cannot be moved in the usual way from outside the chamber, a provision is made for reading the height of the mercury independent of the verniers attached to the scales of the respective barometers. At a distance of some five or six feet from the air-tight chamber a vertical scale is fixed. The divisions on this scale correspond exactly with those on the tube of the standard barometer. A vernier and telescope are made to slide on the scale by means of a rack and pinion. The telescope has two horizontal wires, one fixed and the other moveable by a micrometer screw, so that the difference between the height of the column of mercury and the nearest division on the scale of the standard, and also of all the other barometers placed by the side of it for comparison, can be measured either with the vertical scale and vernier or the micrometer wire. The means are thus possessed of testing barometers for index error in any part of the scale, through the whole range of atmospheric pressure to which they are likely to be exposed; and the usual practice is to test them at every half inch from 27·5 to 31 inches.

“In this way barometers of various other descriptions have been tested, and some errors found to be so large that a few barometers read half an inch and upwards too high, while others read as much too low. In some cases those which were correct in one part of the scale were found to be from half an inch to an inch wrong in other parts. These barometers were of an old and ordinary, not to say inferior, construction. In some the mercury would not descend lower than about 29 inches, owing to a fault very general in the construction of many common barometers till lately in frequent use:—the cistern was not large enough to hold the mercury which descended from the tube in a low atmospheric pressure.

“When used on shore, this contraction of the tube causes the marine barometer to be sometimes a little behind an ordinary land barometer, the tube of which is not contracted. The amount varies according to the rate at which the mercury is rising or falling, and ranges from 0·00 to 0·02 of an inch. As the motion of the ship at sea causes the mercury to pass more rapidly through the contracted tube, the readings are almost the same there as they would be if the tube were not contracted, and in no case do they differ enough to be of importance in maritime use.”

The cistern of this marine barometer is generally made an inch and a quarter in diameter, and the scale part of the tube a quarter of an inch in bore. The inches on the scale, instead of being true, are shortened by ·04 of an inch, in order to avoid the necessity of applying a correction due to the difference of capacity of the tube and cistern. This is done with much perfection, and the errors of the instruments, when compared with a standard by the apparatus used at Kew and Liverpool Observatories, are determined to the thousandth of an inch, and are invariably very uniform and small. The error so determined includes the correction due to capillarity, capacity, and error of graduation, and forms a constant correction, so that only one variable correction, that due to temperature, need be applied, when the barometer is suspended near the water line of the ship, to make the observations comparable with others. With all the advantages of this barometer, however, it has recently been superseded, to some extent, because it was found to require more care than could ordinarily be expected to be given to it by the commander of a ship. Seamen do not exactly understand the value of such nice accuracy as the thousandth part of an inch, but prefer an instrument that reads only to a hundredth part.

22. THE FITZROY MARINE BAROMETER.

Admiral FitzRoy deemed it desirable to construct a form of barometer as practically useful as possible for marine purposes. One that should be less delicate in structure than the Kew barometer, and not so finely graduated. One that could be set at a glance and read easily; that would be more likely to bear the common shocks unavoidable in a ship of war. Accordingly, the Admiral has devised a barometer, which he has thus described:—

“This marine barometer, for Her Majesty’s service, is adapted to general purposes.

“It differs from barometers hitherto made in points of detail, rather than principle:—1. The glass tube is packed with vulcanised india-rubber, which checks vibration from concussion; but does not hold it rigidly, or prevent expansion. 2. It does not oscillate (or pump), though extremely sensitive. 3. The scale is porcelain, very legible, and not liable to change. 4. There is no iron anywhere (to rust). 5. Every part can be unscrewed, examined, or cleaned, by any careful person. 6. There is a spare tube, fixed in a cistern, filled with boiled mercury, and marked for adjustment in this, or any similar instrument.

“These barometers are graduated to hundredths, and they will be found accurate to that degree, namely the second decimal of an inch.

“They are packed with vulcanised caoutchouc, in order that (by this, and by a peculiar strength of glass tube) guns may be fired near these instruments without causing injury to them by ordinary concussion.

“It is hoped that all such instruments, for the public service at sea, will be quite similar, so that any spare tube will fit any barometer.

Fig. 16.

“To Shift a Tube.—Incline the barometer slowly, and then take it down, after allowing the mercury to fill the upper part. Lay the instrument on a table, unscrew the outer cap at the joining just below the cistern swell, then unscrew the tube and cistern, by turning the cistern gently, against the sun, or to the left, and draw out the tube very carefully without bending it in the least, turning it a little, if required, as moved. Then insert the new tube very cautiously, screw in, and adjust to the diamond-cut mark for 27 inches. Attach the cap, and suspend the barometer for use.

“If the mercury does not immediately quit the top of the tube, tap the cistern end rather sharply. In a well-boiled tube, with a good vacuum, the mercury hangs, at times, so adhesively as to deceive, by causing a supposition of some defect.

“In about ten minutes the mercurial column should be nearly right; but as local temperature affects the brass, as well as the mercury, slowly and unequally, it may be well to defer any exact comparisons with other instruments for some few hours.”

Messrs. Negretti and Zambra are the makers of these barometers for the Royal Navy. Fig. 16 is an illustration.

The tube is fixed to a boxwood cistern, which is plugged with very porous cane at the top, to allow of the ready influence of a variation in atmospheric pressure upon the mercury. Round the neck of the cistern is formed a brass ring, with a screw thread on its circumference. This screws into the frame, and a mark on the tube is to be adjusted to 27 inches on the scale, the cistern covering screwed on, and the instrument is ready to suspend. The frame and all the fittings are brass, without any iron whatever; because the contact of the two metals produces a galvanic action, which is objectionable. The spare tube is fitted with india-rubber, and ready at any time to replace the one in the frame. The ease with which a tube can be replaced when broken is an excellent feature of the instrument. The spare tube is carefully stowed in a box, which can also receive the complete instrument when not in use. All the parts are made to a definite gauge; the frames are, therefore, all as nearly as possible similar to each other, and the tubes—like rifle bullets—are adjustible to any frame. If, then, the tube in use gets broken, the captain can replace it by the other; but, as it is securely packed with india-rubber, there is very little liability of its being broken by fair usage. Every person who knows the importance of the barometer on board ship, will acknowledge that the supplementary tube is a decided improvement. Many instruments of this description are afloat in the Royal Navy, and in a short time it may be expected that all the frames and tubes of barometers in the public service at sea will be similar in size and character; so that should a captain have the misfortune to get both his tubes broken, he would be able to borrow another from any ship he fell in with that had one to spare, which would be perfectly accurate, because it would have been verified before it was sent out.

23. Admiral FitzRoy’s Words for the Scale.—The graduation of inches and decimals are placed in this barometer on the right-hand side of the tube; and on a similar piece of porcelain, on the left-hand, are engraved, as legibly as they are expressed succinctly, the following words, of universal application in the interpretation of the barometer movements:—

Reverting to the explanation of the words on the “Coast” barometers (at [page 14]), and comparing and considering them as given for northern latitudes, and as they must be altered for southern latitudes, it will be perceived, that for all cold winds the barometer rises; and falls for warm winds. The mercury also falls for increased strength of wind; and rises as the wind lulls. Likewise before or with rain the column of mercury falls; but it rises with fine dry weather. Putting these facts together, and substituting for the points of the compass the terms “cold” and “warm,” the appropriateness of the words on the scale of this barometer is readily perceived. These concise and practical indications of the movements in the barometer are applicable for instruments intended for use in any region of the world, and are in perfect accordance with the laws of winds and weather deduced by Dové and other meteorologists. There is nothing objectionable in them, and being founded upon experience and the deductions made from numerous recorded observations of the weather in all parts of the world, as well as confirmed by the theories of science, they may consequently be considered as generally reliable. They involve no conjecture, but express succinctly scientific principles.

24. Trials of the FitzRoy Marine Barometer under Fire of Guns.—Some of the first barometers made by Messrs. Negretti and Zambra on Admiral FitzRoy’s principle were severely tried under the heaviest naval gun firing, on board H.M.S. Excellent; and under all the circumstances, they withstood the concussion. The purpose of the trials was “to ascertain whether the vulcanized india-rubber packing round the glass tube of a new marine barometer did check the vibration caused by firing, and whether guns might be fired close to these instruments without causing injury to them.” In the first and second series of experiments, a marine barometer on Admiral FitzRoy’s plan was tried against a marine barometer on the Kew principle, both instruments being new, and treated in all respects similarly. They were “hung over the gun, under the gun, and by the side of the gun, the latter both inside and outside a bulkhead,—in fact, in all ways that they would be tried in action with the bulkheads cleared away.” The result was that the Kew barometer was broken and rendered useless, while the new pattern barometer was not injured in the least. In a third series of experiments, Mr. Negretti being present, five of the new pattern barometers were subjected to the concussion produced by firing a 68-pounder gun with shot, and 16 lbs. charge of powder. They were suspended from a beam immediately under the gun, then from a beam immediately over the gun, and finally they were suspended by the arm to a bulkhead, at a distance of only 3 ft. 6 in. from the axis of the gun; and the result was, according to the official report, “that all these barometers, however suspended, would stand, without the slightest injury, the most severe concussion that they would ever be likely to experience in any sea-going man-of-war.” These trials were conducted under the superintendence of Captain Hewlett, C.B., and the guns were fired in the course of his usual instructions. His reports to Admiral FitzRoy, giving all the particulars of the trials, are published in the “Ninth Number of Meteorological Papers,” issued by the Board of Trade.[2]

25. NEGRETTI AND ZAMBRA’S FARMER’S BAROMETER AND DOMESTIC WEATHER-GLASS.

It is a well-known fact that the barometer is as much, or even more affected by a change of wind as it is by rain; and the objection raised against a simple barometer reading, as leaving the observer in doubt whether to expect wind or rain, is removed by the addition of the Hygrometer, an instrument indicating the comparative degree of dryness or dampness of the air;—a most important item in the determination of the coming weather.

The farmer should not be content to let his crops lie at the mercy, so to speak, of the weather, when he has within his command instruments which may be the means of preventing damage to, and in cases total loss of, his crops.

The farmer hitherto has had to depend for his prognostication of the weather on his own unassisted “Weather Wisdom;” and it is perfectly marvellous how expert he has become in its use. Science now steps in, not to ignore this experience, but on the contrary, to give it most valuable assistance by extending it, and enabling it to predict, with an accuracy hitherto unknown, the various changes that take place in this most variable of climates.

To the invalid, the importance of predicting with tolerable accuracy the changes that are likely to occur in the weather, cannot be over-rated. Many colds would be prevented, if we could know that the morning so balmy and bright, would subside into a cold and cheerless afternoon. Even to the robust, much inconvenience may be prevented by a due respect to the indications of the hygrometer and the barometer, and the delicate in health will do well to regard its warnings.

Fig. 17.

Description of the Instrument.—The farmer’s barometer, as figured in the margin, consists of an upright tube of mercury inverted in a cistern of the same fluid; this is secured against a strong frame of wood, at the upper end of which is fixed the scale, divided into inches and tenths of an inch. On either side of the barometer, or centre tube, are two thermometers—that on the left hand has its bulb uncovered and freely exposed, and indicates the temperature of the air at the place of observation; that on the right hand has its bulb covered with a piece of muslin, from which depend a few threads of soft lamp cotton; this cotton is immersed in the small cup situated just under the thermometer, this vessel being full of water; the water rises by capillary attraction to the muslin-covered bulb, and keeps it in a constantly moist state.

These two thermometers, which we distinguish by the names “Wet Bulb” and “Dry Bulb,” form the Hygrometer; and it is by the simultaneous reading of these two thermometers, and noting the difference that exists between their indications, that the humidity in the atmosphere is determined.

Admiral FitzRoy’s words (see [p. 22]) are placed upon the scale of the barometer, as the value of a reading depends, not so much on the actual height of the mercury in the tube, as it does on whether the column is rising, steady, or falling.

The moveable screw at the bottom of the cistern is for the purpose of forcing the mercury to the top of the tube when the instrument is being carried from place to place, and it must always be unscrewed to its utmost limit when the barometer is hung in its proper place. After this it should never be touched.

The manner in which the Hygrometer acts is as follows: It is a pretty well-known fact that water or wine is often cooled by a wet cloth being tied round the bottle, and then being placed in a current of air. The evaporation that takes place in the progressive drying of the cloth causes the temperature to fall considerably below that of the surrounding atmosphere, and the contents of the bottle are thus cooled. In the same manner, then, the covered wet bulb thermometer will be found invariably to read lower than the uncovered one; and the greater the dryness of the air, the greater will be the difference between the indications of the two thermometers; and the more moisture that exists in the air, the more nearly they will read alike.

The cup must be kept filled with pure water, and occasionally cleaned out, to remove any dirt. The muslin, or cotton-wick, should also be renewed every few weeks. The hygrometer may be had separate from the barometer, if the combined instruments cannot be sufficiently exposed to the external air, this being essential for the successful use of the hygrometer.

This farmer’s weather-glass, then, consists of three distinct instruments: the barometer, the thermometer, and the hygrometer. He has thus at command the three instrumental data necessary for the prediction of the weather. And now to describe—

How to Use the Instrument.—The observations should be taken twice a day, say at 9 A.M. and 3 P.M.; and should be entered on a slip of paper, or a slate hung up by the barometer. The observer will then be able to see the different values of the readings from time to time, and to draw his conclusions therefrom.

The thermometer on the left hand should first be read, and a note made of its indication, which is the temperature of the air. The wet bulb thermometer should now be read, and also noted; and the difference should be taken of these two readings. Next read the barometer by moving the small index at the side of the tube until it is on a level with the top of the mercury. Having noted the number of inches at which the column stands, compare with the last observation, and see immediately whether the barometer is rising, steady, or falling.

Now, having taken the observations as above, we naturally ask the question, What are we to predict from them?

And, probably, the best way of answering this query will be by giving an example. We will suppose that our readings yesterday were as follows:—Temperature, 70°; Wet Bulb, 69°; Difference, 1°; =very moist air. Barometer, 29·5, and that rain has fallen.

To-day, we read:—Temperature, 60°; Wet Bulb, 55°; Difference, 5°; =dryer air. Barometer, 30. We may safely predict that the rain will cease, and probably we may have wind from the northward.

In spring or autumn, if the barometric height be steady any where between 29·5 and 30 inches, with the temperature about 60°, fresh to moderate south-westerly winds, with cloudy sky, will probably characterize the weather; the indications of the hygrometer being then specially serviceable in enabling us to foretell rain; but if the mercury become steady at about 30·5 inches, with temperature about 40°, north-easterly winds, dry air, and clear sky, may be confidently expected.

Many cases will doubtless suggest themselves to the observer where these figures do not occur, and where he might find a difficulty in interpreting the indications of his instruments. We have, therefore, drawn up some concise rules for his guidance; and although they will not prove absolutely infallible guides to this acknowledged most difficult problem, still, they will be found of much service in foretelling the weather, when added to an intelligent observation of ordinary atmospheric phenomena, as force and direction of wind, nature of any particular season, and the time of year.

26. RULES FOR FORETELLING THE WEATHER.

A RISING BAROMETER.

A “Rapid” rise indicates unsettled weather.

A “Gradual” rise indicates settled weather.

A “Rise,” with dry air, and cold increasing in summer, indicates wind from northward; and if rain has fallen, better weather is to be expected.

A “Rise,” with moist air and a low temperature, indicates wind and rain from northward.

A “Rise,” with southerly wind, indicates fine weather.

A STEADY BAROMETER,

With dry air and a seasonable temperature, indicates a continuance of very fine weather.

A FALLING BAROMETER.

A “Rapid” fall indicates stormy weather.

A “Rapid” fall, with westerly wind, indicates stormy weather from northward.

A “Fall,” with a northerly wind, indicates storm, with rain and hail in summer, and snow in winter.

A “Fall,” with increased moisture in the air, and the heat increasing, indicates wind and rain from southward.

A “Fall,” with dry air, and cold increasing (in winter), indicates snow.

A “Fall,” after very calm and warm weather, indicates rain with squally weather.

27. Causes which may bring about a Fall or a Rise in the Barometer.[3]—As heat produces rarefaction, a sudden rise of temperature in a distant quarter may affect the weight of the atmosphere over our heads, by producing an aerial current outwards, to supply the place of the lighter air which has moved from its former position; in which case the barometer will fall. Now such a movement in the atmosphere is likely to bring about an intermixture of currents of air of different temperatures, and from this intermixture rain is likely to result.

On the other hand, as cold produces condensation, any sudden fall of temperature causes the column of air over the locality to contract and sink to a lower level, whilst other air rushes in from above to supply the void; and, accordingly, the barometer rises. Should this air, as often happens, proceed from the north, it will contain in general but little moisture; and hence, on reaching a warmer latitude, will take up the vapour of the air, so that dry weather will result.

It is generally observed, that wind causes a fall in the instrument; and, indeed, in those greater movements of the atmosphere which we denominate storms or hurricanes, the depression is so considerable as to forewarn the navigator of his impending danger. It is evident, that a draught of air in any direction must diminish the weight of the column overhead, and consequently cause the mercury in the barometer to sink.

The connection, therefore, of a sinking of the barometric column with rain is frequently owing to the wind causing an intermixture of the aerial currents which, by their motion, diminish the weight of the atmosphere over our heads; whilst a steady rise in the column indicates the absence of any great atmospheric changes in the neighbourhood, and a general exemption from those causes which are apt to bring about a precipitation of vapour.

28. Use of the Barometer in the management of Mines.—The inflammable and suffocating gases, known to coal-miners as fire-damp and choke-damp, are specifically heavier than air; and as they issue from the fissures of the mine, or are released from the coal, the atmospheric pressure tends to drive them into the lowest and least ventilated galleries. Consequently a greatly reduced atmospheric pressure will favour a sudden outflow or advance of gas; whence may result cases of explosion or suffocation. It has been found that these accidents occur for the most part about the time of a low barometric column. A reliable barometer should, therefore, be systematically consulted by those entrusted with the management or control of coal-mines, so that greater vigilance and caution may be enjoined on the miners whenever the mercury falls low, especially after it has been unusually high for some days.

29. Use of the Barometer in estimating the Height of Tides.—The pressure of the atmosphere affects the height of the tide, the water being in general higher as the barometer is lower. The expressions of seamen, that “frost nips the tide,” and “fog nips the tide,” are explained by the high barometer which usually accompanies frost and fog. M. Daussy, Sir J. C. Ross, and others, have established that a rise of one inch in the barometer will have a corresponding fall in the tide of about one foot. Therefore navigators and pilots will appreciate the following suggestion of Admiral FitzRoy:—

“Vessels sometimes enter docks, or even harbours, where they have scarcely a foot of water more than their draught; and as docking, as well as launching large ships, requires a close calculation of height of water, the state of the barometer becomes of additional importance on such occasions.”

CHAPTER II.

SYPHON TUBE BAROMETERS.

30. Principle of.—If some mercury, or any other fluid, be poured into a tube of glass, bent in the form of ∪, and open at both ends, it will rise to the same height in both limbs, the tube being held vertically. If mercury be poured in first, and then water upon it at one end, these liquids will not come to the same level; the water will stand much higher than the mercury. If the height of the mercury, above the line of meeting of the fluids, be one inch, that of the water will be about thirteen-and-a-half inches. The explanation of this is, that the two columns balance each other. The pressure of the atmosphere in each limb is precisely similar; but the one column stands so much higher than the other, because the fluid of which it is composed is so much lighter, bulk for bulk, than the other. If one end of the tube be hermetically closed, the other limb be cut off within a few inches of the bend, and the tube carefully filled with mercury; by placing it in a vertical position, the mercury will fall, if the closed limb be long enough, until it is about thirty inches higher than that in the exposed limb, where it will remain. Here the atmosphere presses upon the short column; but not upon the long one. It is this pressure, therefore, which maintains the difference of level. In fact, it forms a barometer without a cistern, the short limb answering the purpose of a cistern. The first barometers on this principle were devised by the celebrated philosopher, Dr. Hook, as described in the next section.

31. DIAL, OR WHEEL BAROMETERS.

The familiar household “Weather Glasses” are barometers on the syphon principle. The portions of the two limbs through which the mercury will rise and fall with the varying pressure of the atmosphere are made of precisely the same diameter; while the part between them is contracted. On the mercury, in the exposed limb, rests a round float of ivory or glass; to this a string is attached and passed over and around a brass pulley, the other end carrying another lighter weight. The weight resting on the mercury rises and falls with it. On the spindle of the pulley, which passes through the frame and centre of the dial-plate, is fixed a light steel hand, which revolves as the pulley turns round. When the mercury falls for a decrease of atmospheric pressure, it rises by the same quantity in the short tube, and pushes up the float, the counterpoise falls, and thus moves the hand or pointer to the left. When the pressure increases, the pointer is drawn in a similar manner to the right.

The dials are generally made of metal silvered over or enamelled, but porcelain may be used. If the circumference of the pulley, or “wheel,” be two inches, it will revolve once for an alteration of level amounting to two inches in each tube, or four inches in the height of the barometric column; and as the dial may be from twenty to thirty-six inches in circumference, five to nine inches on the graduated scale corresponds to one inch of the column; and hence the sub-divisions are distinctly perceptible, and a vernier is not necessary.

The motion of the pointer alone is visible; and a mahogany, or rosewood, frame, supports, covers, and renders the instrument ornamental and portable. In the back of the frame is a hinged door, which covers the cavity containing the tube and fixtures. The dial is covered by a glass in a brass rim, similar to a clock face. A brass index, working over the dial, moveable by a key or button, may be applied, and will serve to register the position of the hand when last observed. These instruments are usually fitted with a thermometer, and a spirit level; the latter for the purpose of getting the instrument perfectly vertical. They sometimes have, in addition, a hygrometer, a sympiesometer, an aneroid, a mirror, or a clock, &c., singly or combined. The frame admits of much variety of style and decoration. It may be carved or inlaid. The usual adjustment of scale is suited for localities at no considerable elevation above the sea. Accordingly, being commercial articles, they have been found frequently quite out of place. When intended for use at high elevations, they should have a special adjustment of scale. As household instruments they are serviceable, and ornamental. But the supply-and-demand principle upon which they are sold, has entailed upon those issued by inferior makers a generally bad adjustment of scale. The illustrations are those of ordinary designs.

Dial barometers required for transmission to distant parts, as India and the Colonies, are furnished with a steel stop-cock, to render them portable more effectually than can be done by the method of plugging the tube.

Fig. 24.

32. STANDARD SYPHON BAROMETER.

Fig. 24 represents the most accurate form of the Gay Lussac barometer. The short limb is closed at the top, after the mercury is introduced, and a small lateral puncture is made at a, which is covered over with a substance which permits the access of air, but prevents the escape of any mercury when the instrument is packed for travelling. The bent part of the tube is contracted to a capillary bore; and just above this, in the long limb, is placed the air-trap, already described (see [p. 17]), and here illustrated (fig. 25). Fig. 25.

When reversed, as it must be for portability, the capillary attraction keeps the mercury in the long branch. Should the mercury of the short column get detached, some small quantity of air may pass; but it will be arrested at the pipette, and will not vitiate the length of the barometric column. It can be easily expelled by gently shaking or tapping the instrument before suspending it for observation. In the illustration, the zero of the scale is placed at Z, near the middle of the tube; and the graduations extend above and below. In making an observation, it is necessary to take the reading ZA on the long branch, and ZB on the short one. The sum of the two gives the height of the barometer. The zero of the scale in some instruments is placed low down, so as to require the difference of the two readings to be taken. A thermometer is attached to the frame as usual.

These instruments can be very accurately graduated, and are very exact in their indications, provided great care has been exercised in selecting the tubes, which must be of the same calibre throughout the parts destined to measure the variations of atmospheric pressure. They should be suspended so as to insure their hanging vertically.

The syphon barometer does not require correction for capillarity nor for capacity, as each surface of the mercury is equally depressed by capillary attraction, and the quantity of mercury which falls from the long limb of the tube occupies the same length in the short one. The barometric height must, however, be corrected for temperature, as in the cistern barometer. Tables containing the temperature corrections to be applied to barometer readings for scales engraved on the glass tube, or on brass or wood frames, are published.

CHAPTER III.

BAROGRAPHS, OR SELF-REGISTERING BAROMETERS.

33. Milne’s Self-Registering Barometer.—For a long time a good and accurate self-recording barometer was much desired. This want is now satisfactorily supplied, not by one, but by several descriptions of apparatus. The one first to be described was the design of Admiral Sir A. Milne, who himself constructed, in 1857, we believe, the original instrument, which he used with much success. Since that time several of these instruments have been made, and have performed satisfactorily. The barometer tube is a syphon of large calibre, provided with a Gay Lussac pipette, or air-trap; and fitted with a float, a wheel, and a pointer, as in the “Dial” barometer. The float is attached to a delicate watch-chain, which passes over the wheel and is adequately counterpoised. Behind the indicating extremity of the pointer or hand is a projecting point, which faces the frame of the instrument, and is just within contact with the registering paper. A clock is applied, and fitted with auxiliary mechanism, so as to be able to move the mounted paper with regularity behind the pointer, and at designed equal intervals of time to release a system of levers and springs, so as to cause the marker to impress a dot on the paper, either by puncture or pencil-mark. The paper is ruled with horizontal lines for the range of the mercurial column, and parallel arcs of circles for the hours. Thus the barometer is rendered self-recording, by night or day, for a week or more; hence the great value of the instrument. The clock, index, and registering mechanism are protected from dust and interference by a glass front, hinged on and locked. As the temperature of the mercury is not registered, there is fixed to the frame a Sixe’s thermometer to record the maximum and minimum temperatures, which should be noted at least every twenty-four hours.

Admiral FitzRoy has suggested the name “Atmoscope” for Admiral Milne’s barometer; and he has also termed it a “Barograph.” This latter word appears to be applicable to all kinds of self-registering barometers hitherto designed. Of the arrangement under consideration Admiral FitzRoy writes:—“It shows the alterations in tension, or the pulsations, so to speak, of atmosphere, on a large scale, by hourly marks; and the diagram expresses, to a practised observer, what the ‘indicator card’ of a steam-cylinder shows to a skilful engineer, or a stethescope to a physician.”

Fig. 26.

[Larger Image]

34. Modification of Milne’s Barometer.—The great difficulty to be overcome in Milne’s barometer, is to adjust the mechanism for obtaining registration so that the action of the striker upon the pointer should not in the slightest degree move it from its true position. A different mode of registration, capable of recording accurately the least appreciable movement of the mercurial column, has been effected. In this instrument the registering paper is carried upon a cylinder or drum. By reference to the illustration, Fig. 26, the details of construction will be readily understood. It should, however, be mentioned, that it is not a picture of the outward appearance of the instrument. The position of the barometer should be behind the clock; it is represented on one side merely for the purpose of clearly illustrating the arrangement and principles. The instrument has a large syphon barometer tube, in which the mercurial column is represented. On the mercury at A, in its open end, rests a glass float, attached to a watch-chain, or suitable silken cord, the other end of which is connected to the top of the arched head on the short arm of a lever-beam. The long arm of the beam is twice the length of the short arm, for the following reason. As the mercury falls in the long limb, it rises through an equal space in the short limb of the tube, and vice versa. But the barometric column is the difference of height of the mercury in the two limbs; hence the rise or fall of the float through half-an-inch will correspond to a decrease or an increase of the barometric column of one inch. In order, then, to record the movements of the barometric column, and not those of the float, the arm of the beam connected with the float is only half the radius of the other arm. Both arms of the beam carry circular-arched heads, which are similar portions of the complete circles, the centre of curvature being the fulcrum, or axis. This contrivance maintains the leverage on each extremity of the beam always at the same distance from the fulcrum. From the top of the large arched head a piece of watch-chain descends, and is attached to the marker, B, which properly counterpoises the float, A, and is capable of easy movement along a groove in a brass bar, so as to indicate the barometric height on an ivory scale, C, fixed on the same vertical framing. On the opposite side of the marker, B, is formed a metallic point, which faces the registration sheet and is nearly in contact with it. The framing, which carries the scale and marker, is an arrangement of brass bars, delicately adjusted and controlled by springs, so as to permit of a quick horizontal motion, in a small arc, being communicated to it by the action of the hammer, E, of the clock, whereby the point of the marker is caused to impress a dot upon the paper. The same clock gives rotation to the hollow wooden cylinder, D, upon which is mounted the registering paper. The clock must be rewound when a fresh paper is attached to the cylinder, which may be daily, weekly, or monthly, according to construction; and the series of dots impressed upon the paper shows the height of the barometric column every hour by day and night. The space traversed by the marker is precisely equal to the range of the barometric column.

Fig. 27.

35. King’s Self-Registering Barometer.—Mr. Alfred King, Engineer of the Liverpool Gas-light Company, designed, so long ago as 1854, a barometer to register, by a continuous pencil-tracing, the variations in the weight of the atmosphere; and a highly-satisfactory self-recording barometer, on his principle and constructed under his immediate superintendence, has quite recently been erected at the Liverpool Observatory.

Fig. 27 is the front elevation of this instrument. A, the barometer tube, is three inches in internal diameter, and it floats freely (not being fixed as usual) in the fixed cistern, B, guided by friction-wheels, W. The top end of the tube is fastened to a peculiar chain, which passes over a grooved wheel turning on finely-adjusted friction rollers. The other end of the chain supports the frame, D, which carries the tracing pencil. The frame is suitably weighted and guided, and faces the cylinder, C, around which the tracing paper is wrapped, and which rotates once in twenty-four hours by the movement of a clock. Mr. Hartnup, Director of the Liverpool Observatory, in his Annual Report, 1868, says:—“For one inch change in the mercurial column the pencil is moved through five inches, so that the horizontal lines on the tracing, which are half an inch apart, represent one-tenth of an inch change in the barometer. The vertical lines are hour lines, and being nearly three-quarters of an inch apart, it will be seen that the smallest appreciable change in the barometer, and the time of its occurrence, are recorded.”

“It has been remarked by persons in the habit of reading barometers with large tubes, that, in squally weather, sudden and frequent oscillations of the mercurial column are sometimes seen. Now, to register these small oscillations must be a very delicate test of the sensitiveness of a self-registering barometer, as the time occupied by the rise and fall of the mercury in the tube in some cases does not exceed one minute.” Mr. Hartnup affirms that the tracing of this instrument exhibits such oscillations whenever the wind blows strong and in squalls.

As the barometer in this instrument is precisely similar to the “Long Range Barometer” invented by Mr. McNeild (and which will be found described at page 48), it may be desirable to quote the following, from Mr. Hartnup’s Report:—“Mr. King constructed a small model instrument to illustrate the principle. This instrument was entrusted to my care for examination, and it was exhibited to the scientific gentlemen who visited the Observatory in 1854, during the meeting of the British Association for the Advancement of Science.”

36. Syphon, with Photographic Registration.—A continuous self-registering barometer has been constructed, in which photography is employed. Those who may wish to adopt a similar apparatus, or thoroughly to understand the arrangements and mode of observation, should consult the detailed description given in the Greenwich Magnetical and Meteorological Observations, 1847. As the principles are applicable to photographic registration of magnetic and electric as well as meteorologic variations in instrumental indications, it would be beside our purpose to describe fully the apparatus.

The barometer is a large syphon tube; the bore of the upper and lower extremities, through which the surfaces of the mercury rise and fall, is 1¹⁄10 inch in diameter. The glass float in the open limb is attached to a wire, which moves a delicately-supported light lever as it alters its elevation. The fulcrum of the lever is on one side of the wire; the extremity on the other side, at four times this distance from the fulcrum, carries a vertical plate of opaque mica, having a small aperture. Through this hole the light of a gas-jet shines upon photographic paper wrapped round a cylinder placed vertically, and moved round its axis by a clock fixed with its face horizontal. The cylinder is delicately supported, and revolves in friction rollers. A bent wire on the axis is embraced by a prong on the hour hand of the time-piece; therefore the cylinder is carried round once in twelve hours. It might be arranged for a different period of rotation.

As the cylinder rotates, the paper receives the action of the light, and a photographic trace is left of the movements of the barometer four times the extent of the oscillations of the float, or twice the length of the variations in the barometric column. Certain chemical processes are required in the preparation of the paper, and in developing the trace. The diagram which we give on the next page, with the explanation, taken from Drew’s Practical Meteorology, will enable the above description to be better understood:

Fig. 28.

“Q e is a lever whose fulcrum is e, the counterpoise f nearly supporting it; s is an opaque plate of mica, with a small aperture at p, through which the light passes, having before been refracted by a cylindrical lens into a long ray, the portion only of which opposite the aperture p impinges on the paper; d is a wire supported by a float on the surface of the mercury; G H is the barometer; p, the vertical cylinder charged with photographic paper; r, the photographic trace; I, the timepiece, carrying round the cylinder by the projecting arm t. It is evident that the respective distances of the float and the aperture p from the fulcrum may be regulated so that the rise and fall of the float may be multiplied to any extent required.” When only the lower surface of the mercury in a syphon barometer is read, as in the instrument just described, a correction for temperature is strictly due to the height of the quicksilver in the short tube; but this in so short a column will rarely be sensible.

CHAPTER IV.

MOUNTAIN BAROMETERS.

37. The Syphon Tube Mountain Barometer, on Gay Lussac’s principle, constructed as described at [page 31], and fixed in a metallic tubular frame, forms a simple and light travelling instrument. The graduations are made upon the frame, and it is suspended for reading by a ring at the top, from beneath an iron tripod stand, which is usually supplied with it. Considerable care is requisite in adjusting the verniers, so as to keep the instrument steady and vertical. A drawback to the convenience of this barometer is the movement of the mercury in the short limb, which is generally not confined, and hence has every facility for becoming quickly oxidised in travelling. To remedy this, Messrs. Negretti and Zambra so construct the Mountain Syphon Barometer that by a simple half turn of a screw the mercury can be confined for portability, while the lower limb can be taken out for cleaning whenever found requisite.

38. Mountain Barometer on Fortin’s principle.—This barometer, with Fortin’s cistern, as arranged by Messrs. Negretti and Zambra, is an elegant, manageable, and very accurate instrument for travelling purposes, and well adapted for careful measurement of heights. The cistern is made large enough to receive all the mercury that will fall from the tube at the highest attainable elevation. The screw at the bottom confines the mercury securely for carriage, and serves to adjust the surface of the mercury to the zero of the scale when making an observation. The vernier reads to ·002 of an inch, and slides easily on the brass frame, which is made as small in diameter as is compatible with the size of the tube. The tube in this barometer should be altogether without contractions, so that the mercury will readily fall when it is set up for observation. It must be carefully calibrated, and its internal diameter ascertained, in order that correction may be made for capillarity. This correction, however, should be combined with the error of graduation, and form a permanent index error, ascertainable at any time by comparison with an acknowledged standard barometer.

The barometer is supported in the tripod stand (furnished as part of the instrument) when used for observation. It is suspended by placing two studs, in the ring on the frame, in slots formed on the top of the stand, so that it hangs freely and vertically in gimbals. To the metal top of the stand, mahogany legs are hinged. To make the barometer portable, it must be lifted out of the stand, sloped gently until the mercury reaches the top, turning the screw at the bottom meanwhile; then invert and screw until the mercury is made tight. The inverted instrument packs in the stand, the legs being formed to fit round the frame; and receptacles are scooped out for the cistern, thermometer, gimbals, and vernier; so that the instrument is firmly surrounded by the wooden legs, which are held fast together by brass rings passed over them.

Fig. 29.

39. Newman’s Mountain Barometer.—Fig. 29 is an illustration of the mountain barometer known as Newman’s. The cistern consists of two separate compartments;—the top of the lower and the bottom of the upper, being perfectly flat, are pivoted closely together at the centres, so that the lower can move through a small arc, when turned by the hand. This movement is limited by two stops. The top of the lower compartment and the bottom of the upper have each a circular hole, through which the mercury communicates. When the instrument is required for observation, the cistern is turned close up to the stop marked “open” or “not portable.” When it is necessary to pack it for travelling, the mercurial column must be allowed to fill the tube by sloping the barometer gently; then invert it, and move the cistern to the stop marked “shut” or “portable.” In this condition, the upper compartment is completely filled with mercury, and consequently that in the tube cannot move about, so as to admit air or endanger the tube. Nor can the mercury pass back to the lower compartment, as the holes are not now coincident, and the contact is made too perfect to allow the mercury to creep between the surfaces. The tube does not enter the lower compartment, which is completely full of mercury when the instrument is arranged for observation. The spare capacity of the upper cistern is sufficient to receive the mercury which descends from the tube to the limit of the engraved scale, which in these barometers generally extends only to about 20 inches. A lower limit could of course be given by increasing the size of the cisterns, which it is not advisable to do unless for a special purpose. This barometer may be had mounted in wood, or in brass frame. If in wood, it has a brass shield, which slides round the scale part of the frame, so as to be easily brought in front of the tube and scale as a protection in travelling; the vernier screw, in this case, being placed at the top of the instrument. When the scale is graduated with true inches, the neutral point, the capacity and capillarity corrections should be marked on the frame. The graduated scales, however, placed on these barometers in brass frames, are usually artificial inches, like the Kew plan of graduation; the advantage being that one simple correction only is required, viz. one for index error and capillarity combined, which can always be readily determined by comparison with a standard barometer; moreover, as no adjustment of cistern is required in reading, the instrument can be verified by artificial pressure throughout the scale, by the plan practised at Kew, Liverpool, &c., and already described (see [p. 18]).

40. NEGRETTI & ZAMBRA’S PATENT MOUNTAIN AND OTHER BAROMETERS.

Fig. 30.

This invention is intended to make mountain and other barometers of standard accuracy stronger, more portable, and less liable to derangement, when being carried about, than heretofore, by dispensing with the ordinary flexible cistern containing the mercury at the bottom of the instrument, and adapting in lieu thereof a rigid cistern constructed of glass and iron. The cistern is composed of a glass cylinder, which is secured in a metallic tube or frame. In order to render the cistern mercury-tight at top and bottom, metal caps are screwed into the tube or frame, and bear against leather washers placed between them and the edges of the glass cylinder. The upper cap of the cistern is tapped with a fine threaded screw to receive the iron plug or socket, into which the barometer tube is securely fixed. The whole length of this plug has a fine screw cut upon it by which the cistern can be screwed up or down. At the side of this plug or socket, extending from the lower end to within half an inch of the top, is cut a groove for admitting the air to the surface of the mercury within the cistern when the barometer is in use. An ivory point is screwed into the under surface of the plug, carrying the barometer tube. This ivory point is very carefully adjusted by measurement to be the zero point of the instrument, from which the barometer scale of inches is divided. The surface of the mercury in the cistern is adjusted to the zero point by screwing the cistern up or down until the ivory point and its reflected image are in contact.

The instrument (fig. 30) is shown in a state of adjustment, ready to take an observation; but when it is desired to render it portable, it must be inclined, until mercury from the cistern fills the tube; the cistern must then be screwed up on the socket, so as to bring the face of the upper cap against the under side of the shoulder of the cover immediately above it; the instrument may then be carried without being liable to derangement.

Precautions necessary in using the Mountain Barometer.—On removing the barometer from its case after a journey, allow it to remain with its scale end downward, whilst the cistern is unscrewed to the extent of one turn of the screw, after which slightly shake the cistern; the mercury in it will then completely fill the end of the barometer tube, should any portion of it have escaped therefrom.

The barometer is then inverted, and if it be desired to make an observation, suspend it vertically from its stand by the ring at top. The cistern must then be unscrewed, until the surface of the mercury is brought just level with the extreme end of the ivory or zero point fixed to the iron plug on which the glass cistern moves up and down.

Should the elevation of the place where the barometer is to be used be considerably above the sea level, it will be well—after suspending it from the stand—to unscrew the cistern several turns, holding the barometer in an oblique position, as at great heights the mercury will fall considerably quicker than the cistern can be unscrewed, thereby filling it to overflowing; but by partly unscrewing the cistern first, room is given for the reception of a fall of mercury to the extent of several inches.

The cistern must not be unscrewed when the Instrument is INVERTED more than two turns of the screw, otherwise the mercury will flow out through the groove.

It is found safer when travelling to carry the barometer in a horizontal position, or with its cistern end uppermost.

To clean the Barometer.—Should at any time the mercury in the cistern become oxidised, and reading from its surface be difficult, it can be readily cleaned by removing the cistern and its contained mercury from the barometer frame by unscrewing it when in a horizontal position; this precaution is necessary that the mercury in the tube may not escape, and thereby allow air to enter. The cistern must then be emptied, and with a dry clean leather, or silk handkerchief, well cleaned.

The operation of cleaning being performed, return the cistern to the frame, and screw it until the face is brought up against the under side of the shoulder, still keeping the instrument horizontal. The cistern is now ready for re-filling, to do which stand the barometer on end head downwards, and remove the small screw at bottom; through the aperture thus opened, pour in mercury, passing it through a paper funnel with a very small aperture. It is well to pass the mercury through a very small funnel two or three times before returning it to the barometer cistern, as by this process all particles of dust or oxide adhere to the paper, and are effectually removed.

Should any small quantity of the mercury be lost during the operation of cleaning, it is of no importance so long as sufficient remains to allow of adjustment to the zero point. This latter constitutes one of the great advantages of this new instrument over the ordinary barometer; for, in the majority of cases, after an instrument has been compared carefully with a standard, should mercury be lost, there is no means of correcting the error unless a standard barometer be at hand; the new barometer is, in this respect, independent, a little mercury more or less being unimportant.

41. Short Tube Barometer.—This is simply a tube shorter, as may be required, than that necessary to show the atmospheric pressure at the sea level. It is convenient for balloon purposes, and for use at mountain stations, being of course a special construction.

42. Method of Calculating Heights by the Barometer.—The pressure of the atmosphere being measured by the barometer, it is evident that as the instrument is carried up a high mountain or elevated in a balloon, the length of the column must decrease as the atmospheric pressure decreases, in consequence of a stratum of air being left below. The pressure of air arises from its weight, or the attraction of gravitation upon it, and therefore the quantity of air below the barometer cistern cannot influence the height of the column. Hence it follows that a certain relation must exist between the difference of the barometric pressure at the foot and at the top of a hill or other elevation, and the difference of the absolute heights above the sea. Theoretical investigation, abundantly confirmed by practical results, has determined that the strata of air decrease in density in a geometrical proportion, while the elevations increase in an arithmetical one. Hence we have a method of determining differences of level, by observations made on the density of the air by means of the barometer. It is beyond our purpose to explain in detail the principles upon which this method is founded, or to give its mathematical investigation. We append Tables, which will be useful to practical persons,—surveyors, engineers, travellers, tourists, &c.,—who may carry a barometer as a travelling companion.

Table I. is calculated from the formula, height in feet = 60,200 (log. 29·922 - log. B) + 925; where 29·922 is the mean atmospheric pressure at 32° F., and the mean sea-level in latitude 45°; and B is any other barometric pressure; the 925 being added to avoid minus signs in the Table.

Table II. contains the correction necessary for the mean temperature of the stratum of air between the stations of observation; and is computed from Regnault’s co-efficient for the expansion of air, which is ·002036 of its volume at 32° for each degree above that temperature.

Table III. is the correction due to the difference of gravitation in any other latitude, and is found from the formula, x = 1 + ·00265 cos. 2 lat.

Table IV. is to correct for the diminution of gravity in ascending from the sea-level.

To use these Tables: The barometer readings at the upper and lower stations having been corrected and reduced to temperature 32° F., take out from Table I. the numbers opposite the corrected readings, and subtract the lower from the upper. Multiply this difference successively by the factors found in Tables II. and III. The factor from Table III. may be neglected unless precision is desired. Finally, add the correction taken from Table IV.

Table I.

Approximate Height due to Barometric Pressure.

Table I.—continued.

Approximate Height due to Barometric Pressure.

Table II.

Correction due to Mean Temperature of the Air.

Table III.

Table IV.

Example 1. On October 21st, 1852, when Mr. Welsh ascended in a balloon, at 3h. 30m. p.m., the barometer, corrected and reduced, was 18·85, the air temperature 27°, while at Greenwich, 159 feet above the sea, the barometer at the same time was 29·97 inches, air temperature 49°, the balloon not being more than 5 miles S.W. from over Greenwich; required its elevation.

The following examples, from the balloon ascents of J. Glashier, Esq., F.R.S., will serve for practice.[4]

2. Ascended from Wolverhampton, 18th August, 1862, at 2h. 38m. p.m.; barometer (in all cases corrected and reduced to 32° F) was 14·868, the temperature of the air 26°; at the same time, at Wrottesley Hall, 531 feet above the sea, in latitude 52½° N, the barometer was 29·46, and the temperature of the air 65°·4; find the elevation of the balloon above the sea.

Height, 18,959 feet.

3. From the same place an ascent was made 5th September, 1862, when at 1h. 48m. p.m. barometer was 11·954, air O°; at Wrottesley Hall 29·38, air 56°.

Height, 23,923 feet.

4. From the Crystal Palace a balloon ascent was made 20th August, 1862. At 6h. 47m. p.m. barometer was 25·55, air 50°·5; and at the same time at Greenwich Observatory, at 159 feet above the sea, the barometer was 29·81, air 63°.

Height, 4,406 feet.

5. From the same place an ascent was made 8th September, 1862. At 5 p.m., the balloon being over Blackheath, barometer was 25·60, and the air 49°·5, while at Greenwich, barometer was 29·92, air 66°·4.

Height, 4,461 feet.