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° Fahr., 1 lb. of air measures 12½ cubic
feet. Let us suppose that 1 lb. of air at 39° Fahr. = 500° absolute,
is contained in a non-conducting cylinder of 1 foot area and 12½ feet
deep under a counterpoised piston. The pressure of the atmosphere on
the piston = 144 square inches × 14.7 lb., or 2,116 lb. If the air be
now heated up to 539° Fahr. = 1,000° 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 12½ 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° absolute, the piston will rise
until it is 12½ 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 12½ 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 12½ feet = 26,450 foot-pounds.