umes, 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
at a temperature of zero, it will reach about 100 degrees when the gauge
pressure is 14.7 pounds. We find this by following down the first line
intersected by the adiabatic curve to the point where the zero heat
curve intersects this same line, the reading being given in figures to
the left immediately opposite. If the air had been admitted to the
compressor at 60 degrees, it would register about 176 degrees at 14.7
pounds gauge pressure. If the air were 100 degrees before compression,
it would go up to about 230 degrees at this pressure. Following this
adiabatic curve until it intersects line No. 5, representing a pressure
of five atmospheres above a vacuum (58.8 lb. gauge pressure), we see
that the total increase of temperature on the zero heat curve is about
270 degrees, for the 60 degree curve it is about 370 degrees, and for
the 100 degree curve it is about 435 degrees. The diagram shows that
when a volume of air is compressed adiabatically to 21 atmospheres (294
lb. gauge pressure), it will occupy a volume a little more than
one-tenth; the total increase of temperature with an initial temperature
of zero is about 650 degrees; with 6