AEROSTATICS. This branch of science treats of the equilibrium and pressure of air and other gases, and of the methods of measuring it by the barometer and other instru ments. The expansive force or pressure of atmospheric air varies with time and place. In a medium condition of the atmosphere, and near the sea-level, barometrical observa tions give the pressure or weight equal to that of a column of mercury, 30 inches high. or of a column of water about 34 feet high. This makes the mean pressure of the atmos phere nearly 15 lbs. on every sq.in. This mean pressure of the atmosphere is generally taken as the unit or measure of expansive or elastic forces generally; any particular pres sura is said to be equal to so many atmospheres. A. also investigates the phenom ena of the compression of eases; in other words, the relation between the elasticity and the density or volume of a gas. According to the law of 3Iariotte, the expansive force of one and the same body of gas is proportional to its density; or, which is the same thing, the expansive force of a body of gas under different degrees of compression varies inversely as the space which it occupies. If its elastic force, at one stage, lie measured
by 50 lbs., when compressed into half the space that force will be 100 Ills. Connected with this is the investigation of the variation of density and pressure in the several vertical strata of the atmosphere. It is obvious that the weight of the atmosphere must diminish as we ascend, as part of it is left below; and it results from Mariotte's law that, at dif ferent distances from the earth's surface, increasing in arithmetical progression, the atmos pheric pressure diminishes in geometrical progression. This principle furnishes the means of measuring heights by the barometer (q.v.).
The elastic force of air and other gases is very much increased by heat, and, con sequently when allowed, they expand. It is found that a rise of temperature of 1° of Fahrenheit causes any gas to expand of its own bulk; and this expansion is uniform.