to twenty-five atmospheres, will register 367.5 pounds above a vacuum
(352.8 pounds gauge pressure), will occupy a volume of 0.1020 cubic
foot, will have a temperature of 864 degrees, and the total increase of
temperature is 804 degrees.

The thermal results of air compression and expansion are shown by the
accompanying diagram.

The horizontal and vertical lines are the measures of volumes, pressures
and temperatures. The figures at the top indicate pressures in
atmospheres above a vacuum, the corresponding figures at the bottom
denote pressures by the gauge. At the right are volumes from one to
one-tenth. At the left are degrees of temperatures from zero to 1,000
Fahrenheit. The two curves which begin at the upper left hand corner and
extend to the lower right are the lines of compression or expansion.

The upper one being the _Adiabatic_ curve, or that which represents the
pressure at any point on the stroke with the heat developed by
compression remaining in the air; the lower is the _Isothermal_, or the
pressure curve uninfluenced by heat. The three curves which begin at the
lower left hand corner and rise to the right are heat curves and
represent the increase of temperature corresponding with different
pressures and volumes, assuming in one case that the temperature of the
air before admission to the compressor is zero, in another sixty
degrees, and in another one hundred degrees.

Beginning with the adiabatic curve, we find that for one volume of air
when compressed without cooling the curve intersects the first vertical
line at a point between 0.6 and 0.7 volume, the gauge pressure being
14.7 pounds. If we assume that this air was admitted to the compressor