Regnault ascertained that at the freezing point of water (320 F.) a cubic centimetre of perfectly pure, dry air had a weight of 0.0012932 of a gramme when the barometer stood at 76 centimetres at, Paris. Of course, the earth attracts bodies more strongly at the poles than at the equator, though the slight difference can in ordinary practice be ignored. In English equivalents, a cubic foot of air has a weight of 0.080681 lb., or 1.29 oz., at 32° F. and at ordinary atmospheric- pressure - that is, 14.7 lb. per sq. in. at sea level. The density, and consequently the weight, of air vary with its pressure and temperature. In ascertaining the weight of air exceedingly delicate apparatus is necessary, or there will be a large percentage of error in the result. The usual method is to weigh a bulb of glass or other material tilled with air; the air is exhausted, and the bulb weighed again, the difference in the two weighings being the weight of the quantity of air that is sufficient just to fill the bulb. By ascertaining the cubical contents of the bull), it is an easy matter to calculate the weight of any given quantity of air. The table below gives the absolute weights of a cubic foot of air under varying conditions of temperature and pressure.

The weights given are those that would be obtained by weighing the air subject to the given conditions in an air-tight case surrounded by a vacuum; if the case were surrounded by the ordinary atmosphere, the case of air would appear to have a less weight. For example, 1 cub. ft. of air, temperature 700 F., pressure 801b. per sq. in. above the atmosphere, has an actual, absolute weight of 0.4825 lb.; weighed in air having a temperature of 70° F., the weight would appear to be only 0..4070 lb. The table printed above is on the authority of the Locomotive.