Part 3

Chapter 33,563 wordsPublic domain

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

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

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

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

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

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

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

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

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

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

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

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

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

[Footnote 8: Admiral Fitzroy.]

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

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

[Footnote 11: Sir Leopold McClintock.]

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

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

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

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

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

CONCISE TABLES FOR CALCULATING HEIGHTS BY MEANS OF BAROMETER OR ANEROID, AND ALSO BY THE BOILING-POINT THERMOMETER.

+-----------+--------+---------------------------------------+ |Boiling- |Bartr. | | |point of |at lower| BAROMETER AT UPPER STATION.--INCHES. | |Water for |Station.| | |pressure in| +----+----+----+----+----+----+----+----+ |next col. | In. | 22 | 21 | 20 | 19 | 18 | 17 | 16 | 15 | +-----------+--------+----+----+----+----+----+----+----+----+ | 213·78 | 31 | 998|1020|1043|1068|1095|1124|1155|1188| | 212·13 | 30 |1015|1038|1062|1087|1115|1144|1176|1210| | 210·43 | 29 |1033|1056|1081|1107|1135|1165|1198|1233| | 208·67 | 28 |1051|1075|1100|1127|1156|1187|1220|1257| | 206·87 | 27 |1073|1097|1122|1150|1180|1211|1246|1283| | 205·01 | 26 |1093|1118|1145|1173|1203|1236|1271|1309| | 203·09 | 25 |1116|1141|1169|1198|1229|1262|1299|1338| | 201·11 | 24 |1140|1166|1194|1224|1256|1290|1327|1367| | 199·05 | 23 |1164|1191|1220|1251|1284|1319|1358|1399| | 196·92 | 22 |.... 1218|1248|1280|1314|1350|1390|1433| | 194·71 | 21 |.... .... 1278|1310|1346|1383|1424|1469| | 192·41 | 20 |.... .... .... 1343|1380|1419|1461|1507| | 190·00 | 19 |.... .... .... .... 1416|1457|1500|1548| | 187·50 | 18 |.... .... Factor A. .... 1497|1542|1592| | 184·87 | 17 |.... .... .... .... .... .... 1588|1639| | 182·10 | 16 |.... .... .... .... .... .... .... 1690| | 179·20 | 15 |.... .... .... .... .... .... .... ....| +-----------+--------+---------------------------------------+

+--------------------------------------+ | Height | D. | | | | in 1,000 |additive.|Latitude.| C. +---------------------+ | feet. | | | | Mean | Factor | |----------+---------+---------+-------|Temperature.| B. | | 2 | 5 | 0° | †2·7 |------------+--------+ | 4 | 11 | 10 | †2·5 | 10° | 0·951 | | 6 | 17 | 20 | †2·0 | 20 | 0·973 | | 8 | 23 | 30 | †1·4 | 30 | 0·996 | | 10 | 30 | 40 | †0·6 | 40 | 1·018 | | 12 | 37 | 45 | 0·0 | 50 | 1·040 | | 14 | 44 | 50 | -0·5 | 60 | 1·062 | | 16 | 52 | 60 | -1·3 | 70 | 1·084 | | 18 | 60 | 70 | -2·0 | 80 | 1·127 | +----------+---------+---------+-------+------------+--------+

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

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

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

Lower Station 29·91 inches. Upper „ 26·30 ----- 3·61 Factor A 933 ----- 1083 1083 3249 ----- 3368 (neglecting decimals.) Factor B 1.055 ----- 16840 N.B.—In taking out the quantities, if accuracy 16840 is aimed at, it will be necessary to 3368 proportion for parts in the usual manner ----- with such Tables. 3553 Cor. C = 3 × 1 = -3 Cor. D +10 ---- Height 3560 feet.

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

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

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

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

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

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

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

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

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

BAROMETER PRECAUTIONS.

If vacuum suspected, cause mercury to strike top of tube. A clear metallic “click” indicates a good vacuum. A dull “thud” indicates air or moisture. In latter case return to optician, but if unable Incline _very_ gently until nearly inverted, when Air if present will ascend in a bubble into the cistern. Suspend barometer in good light out of sunshine. Let no heat of fire or lamp affect it. Let no sudden changes of temperature affect it. It _must_ hang _absolutely_ vertically. Note temperature of attached thermometer before reading barometer. Then adjust mercury in cistern to touch ivory point. Then adjust vernier and take reading _quickly_. Ascertain height above sea-level according to direction.

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

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

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

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

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

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

NOTE ON THE VERIFICATION OF INSTRUMENTS AT THE KEW OBSERVATORY.

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

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

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