+--------------------------------------+ | 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.