Whenever any body is compressed, its specific heat is diminished; and the surplus portion is, as it were, pushed out of the body--appearing as sensible heat. And whenever any body is expanded, its specific heat is increased; and the additional quantity of heat requisite is, as it were, sucked in from surrounding bodies--so producing cold. This action may be compared to that of a wet sponge from which, when compressed, a portion of the water is forced out, and when the sponge is allowed to expand, the water is drawn back. This effect is manifested by the increase of temperature in air-compressing machines, and the cold produced by allowing or forcing air to expand in air-cooling machines. At 39 deg. Fahr., 1 lb. of air measures 121/2 cubic feet. Let us suppose that 1 lb. of air at 39 deg. Fahr. = 500 deg. absolute, is contained in a non-conducting cylinder of 1 foot area and 121/2 feet deep under a counterpoised piston. The pressure of the atmosphere on the piston = 144 square inches x 14.7 lb., or 2,116 lb. If the air be now heated up to 539 deg. Fahr. = 1,000 deg. absolute, and at the same time the piston is not allowed to move, the pressure is doubled; and when the piston is released, it would rise 121/2 feet, provided that the temperature remained constant, and the indicator would describe a hyperbolic curve (called an "isothermal") because the temperature would have remained equal throughout. But, in fact, the temperature is lowered, because expansion has taken place, and the indicator curve which would then be described is called an "adiabatic curve," which is more inclined to the horizontal line when the volumes are represented by horizontal and the pressures by vertical co-ordinates. In this case it is supposed that there is no conduction or transmission (diabasis) of heat through the sides of the containing vessel. If, however, an _additional_ quantity of heat be communicated to the air, so as to maintain the temperature at 1,000 deg. absolute, the piston will rise until it is 121/2 feet above its original position, and the indicator will describe an isothermal curve. Now mark the difference. When the piston was fixed, only a heating effect resulted; but when the piston moved up 121/2 feet, not only a heating but a mechanical, in fact, a thermodynamic, effect was produced, for the weight of the atmosphere (2,116 lb.) was lifted 121/2 feet = 26,450 foot-pounds.