The law relating to the compressibility of gases was discovered by Boyle. It is, that if temperature be kept constant, the volume of all gases is inversely as the pressure. Thus, if the pressure of the atmosphere, which is equal to 1033 grams on each square centimeter of the earth's surface at sea-level, or approximately i 5 lbs. on each square inch, be doubled, the volume of a given weight of air, or of any other gas, will be halved ; on trebling the pressure the volume is reduced to one-third, and so on. As the length of a column of mercury, one square centimeter in cross-section, must be 76 centimeters in order that its weight shall be 1033 grams, 76 centimeters is taken as the " normal" height of the barometer. And if the height of the mercury in a gauge or ¹' manometer" is 152 centimeters, the pressure which produces that rise in the mercurial column will halve the volume of a gas exposed to it.