A still more divided vernier is always used with the best barometers, and though a little troublesome to read at first, yet if the method of reading the simpler one just described be understood, the difficulty will be easily overcome.
Uses, &c. The barometer is employed for ascertaining the amount of atmospherical refraction in astronomical calculations, for measuring altitudes, and in prognosticating the weather. For the last purpose, on land, it sometimes proves a false prophet; but at sea, its monitions are highly trustworthy. As a mere weather-glass, the indications, as read off from the scale of the instrument, are generally sufficiently accurate; but in all observations connected with meteorology, altitudes, astronomy, &c., certain corrections must be made; the height of the mercury being influenced both by the size of the tube and by the temperature of the air by which it is surrounded, as well as by variations in the weight or pressure of the atmosphere. (See below.)
Barometrical Corrections:—
1. As to CAPILLARITY:—This applies to all cistern-barometers formed of tubes of very small diameters, owing to the mercury assuming a convex surface in the tube. As the tube increases in diameter, so the depression of the mercury lessens. Hence, the “interior diameter” of a barometer “should, in every case, exceed one-fourth of an inch.” (Brande.) Syphon barometers that have each of their legs of equal size, require no correction, as the depression is equal at both ends.
Table of Barometrical Corrections for Capillarity, from the ‘Encycl. Brit.’
| Diam. of Tube. | Depression. | ||
| ·10 | inch. | ·1403 | inch. + |
| ·15 | ” | ·0863 | ” |
| ·20 | ” | ·0581 | ” |
| ·25 | ” | ·0407 | ” |
| ·30 | ” | ·0292 | ” |
| ·35 | ” | ·0211 | ” |
| ·40 | ” | ·0153 | ” |
| ·45 | ” | ·0112 | ” |
| ·50 | ” | ·0083 | ” |
| ·60 | ” | ·0044 | ” |
| ·70 | ” | ·0023 | ” |
| ·80 | ” | ·0012 | ” |
2. As to TEMPERATURE:—These depend on the expansion of the mercury, and of the scale on which the divisions are marked. The rule for reducing an observed height to the corresponding height at the freezing-point, or 32° Fahr., the usual standard temperature, is—Subtract 1·10000th part of the observed height of the barometer for every degree of Fahr. above 32° at the time of the observation. Or—
(obs. t. - 32) × obs. h. × ·0001 = corr. req.
Measurement of Heights by the Barometer.—When a barometer is at the foot of a mountain, the pressure it sustains is greater than that to which it is subjected at the top, by the weight of the column of air intervening between the top and the bottom.
The height can be obtained from the following table by calculating the number of feet which must have been ascended to cause the observed fall; and then making a correction for temperature by multiplying the number obtained from the table, which may be called A, by the following formula: t is the temperature of the lower and t′ of the upper station:—