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FIG. 19.

about thirty-two inches in length, the shorter eight or nine inches; to both upper and lower limbs scales are fixed, and the difference of level in the two gives the height of the barometer.

The best and most accurate form of mercurial barometer is that known as Fortin's; in this what is called the cistern-error is got rid of. When mercury falls in the tube the mercury in the cistern in which the tube is immersed necessarily rises, and if the mercury rises in the tube of course the mercury in the cistern falls: hence there is a constant source of error from the height of the mercury in the cistern varying. In Fortin's there is an inner cistern containing the mercury, the bottom of the cistern is of a flexible material (leather) and by means of a small screw the cistern capacity is either contracted or enlarged, and in this way the level adjusted before each observation.

The one figured (Fig. 19) was made by Mr. Casella. The mercury in the cistern is adjusted at each observation to a fixed ivory point which thus forms the zero of the scale.

The cistern is made partly of glass, to admit of the zero of the scale being visible, and the mercury is adjustable to the zero or ivory point by means of a thumb-screw acting upon the flexible base. The vernier reads to 0.002 of an inch, or, by estimation, to 0.001 inch, and is adjusted by a rack and pinion motion. In front of the barometer a thermometer is attached, in contact with the tube, with divisions etched on the stem. For facility of reading, a slab of white porcelain is placed behind the scale. The barometer is mounted in a brass frame, and suspended from a bracket at the top of a mahogany board, so as to insure perpendicularity. At the bottom of the board

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is a socket, with clamping screws for steadying the barometer in a vertical position, when an observation is made. The instrument is so mounted that it can be turned at pleasure to any source of light.

(76) Reading a Mercurial Barometer.

The attached thermometer must first be read. Next the cistern error must be got rid of; this is done by the bottom screw, the mercury is raised or depressed until it barely touches the ivory point which, with its reflection, will then appear as a double cone. The height of the column is then taken by adjusting the lower edge of the vernier, so that it shall exactly form a tangent to the convex surface of the mercury in the tube just excluding the light from the apex when the eye is in the same plane with the back and front lower edges of the vernier. In adjusting the level in the cistern, the mercury should always be screwed up to the cone, and if the point of the cone is submerged the screw must be turned down until it is quite clear of the surface before the final adjustment is made. This precaution is necessary in order to preserve the same form of surface of mercury in the tube at different readings.

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A

B

30

15

10

05

29

•00

How to Read the Vernier.-By means of the annexed diagram, the use of the vernier in insuring accurate measurement is readily understood. CD represents part of the fixed scale of the barometer, and A B is the sliding scale or vernier. The scale CD is divided into inches, tenths, and half-tenths of an inch, so that each division of the scale is 05; A B is made equal to 24 divisions of the scale, and is divided into 25 equal parts. It follows, therefore, that each division of the vernier is smaller than each division of the scale by the 25th part of 05; which is 002 inch. The lower edge of the vernier A, is set to the top of the barometrical column, and hence we have to find the height of A. First we read on the scale, say 29:15; next we look along the vernier until we find one of its lines which lies evenly with

C

C

FIG. 20.

a line of the scale. As shown in the figure, this line is the second above 3. Now each of the figures engraved on the vernier counts as hundredths, and each intermediate division counts as two thousandths (002); hence the vernier shows 034, and this added to the scale reading 29-15, gives the reading sought, 29 184.

In the Kew pattern barometers the inches on the scale are not really inches, but are so contracted as to thoroughly correct for the variation of the level of the mercury in the cistern.

In the old barometers this was effected by calculating the socalled "capacity correction;" in the Fortin barometers it is done. by raising or lowering the mercury to the fiducial ivory point, and in the Observatory standard by moving the scales to the mercury.

The instructions given above for using the Fortin barometer apply equally to the Kew pattern, except that the cistern adjustment is unnecessary.

(77) Correction of Barometer Readings.

In order to ascertain the true pressure of the atmosphere, and to render comparable observations made in different localities, it is usual to reduce all barometric observations to what they would have been had the instrument been at the temperature of melting ice, and the barometer itself at the mean level of the sea. These corrections are fully explained, and all necessary tables are given in Marriott's Hints to Meteorological Observers, and in Scott's Instructions for the Use of Meteorological Instruments, and in many other meteorological books, but some formulæ are given below which may be useful to those who may happen to be without a set of tables or text-book when they wish to reduce their observations.

A special correction for temperature must be made in the case of barometers of the Kew pattern, a full account of which appears in the Philosophical Magazine for 1861.

These formulæ assume that the scale is brass, and also that the expansion of mercury is uniform for the range of temperature employed, which latter assumption, though not absolutely true, will give no appreciable error.

Barometric readings must be reduced to sea-level, and to 32 degrees of the Fahrenheit thermometer (0°C.) These are most conveniently obtained from tables, but the principle should be thoroughly understood.

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There is also a correction for capillarity, and one for differences of gravity, the latter depending on the latitude, that is the different distances from the centre of the earth. The two latter are neglected save in very refined observations. These correc

tions are simplest when the French system of millimetres is in For moderate elevations the following formulæ for both the English and metrical barometer are sufficiently accurate.

use.

(a.) Formula for the reduction of the English barometer to the freezing point and sea-level.

H = h (1 — 96-256) +

E

100000 812-86-1945 t

Where H is the corrected reading in inches.

h is the observed reading in inches.

t is the temperature in degrees Fahrenheit.

E is the elevation of the station above the sea-level in feet.

(b.) Formula for the reduction of the metrical barometer to the freezing point and sea-level.

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Where H is the corrected reading in millimetres.

h is the observed reading in millimetres.

t is temperature in degrees centigrade.

E is the elevation of the station above the sea-level in

metres.

(78) The Aneroid Barometer.

The aneroid barometer is of special value to medical officers of health in country districts, for from its portability it may be used for the determination of heights and contours, matters which are often necessary to know, whether for the purposes of passing judgment on a drainage scheme; the initiation of a water supply

1 The force of gravity is usually symbolised by the letter g, at Paris g = 980-94 cm, at Greenwich it equals 981 17. 30 inches or 76 cm. of mercury at Paris represent therefore a little less pressure than at Greenwich. If h equals the height of the mercurial column, d the specific gravity of mercury, the measure of the pressure of air is ghd in absolute units of force; taking the barometer to be for instance at Greenwich as 76 cm, then 981.17 x 76 x 13 596 = 1013800 grms. per square centimetre. The value of g for any latitude and any height h in centimetres above sea-level may be found from the following formula :

:

g = 980 6056 (100257 cos 2 1-1.96h × 109).

or for studying the general configuration of the district. Aneroid barometers are now made of very great accuracy, but they must be compared with the standard mercury or glycerin barometer occasionally, otherwise mistakes may occur.

An aneroid barometer is a thin metallic air-tight box, deeply corrugated to increase its elasticity, from which the air has been entirely removed: the upper and lower surfaces of the box are held in a state of tension or separation from each other by means of a strong spring; the atmosphere pressing with varying intensity on the box, the two walls respond being driven in or relaxed; this action communicates motion to smaller springs which are connected with a needle on the dial.

The diminution of pressure in measuring heights with this or

any other form of barometer is not equal for equal differences of altitude, but it follows known laws, and altitude scales have been computed, taking the varying ratio between pressure and altitude into account, besides this the temperature of the air is another cause of variation, for as temperature affects airdensity it also affects the ratio of pressure to altitude. Mr. Rogers Field has, however, invented an "Engineering Aneroid" (see Fig. 21) in which there is an adjustable scale for temperature.

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FIG. 21.

The altitude scale adopted in this aneroid is that of the late Astronomer-Royal (which has been compared with the formulæ of Laplace, Guyot, Baily, Plantamour and other authorities, and found to give results lying between them), and as the instrument is intended for the accurate measurement of moderate altitudes, the range is purposely limited so as to give an open graduation. The adjustment for the temperature of the air is applied by shifting the scale in accordance with the figures engraved on the outside of the instrument. The rim which holds the glass should be slightly raised, so as to be free from the lockingpin, and then turned until the figures corresponding to the air temperature are opposite to the pin, when the glass should be depressed so as to re-lock it.

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