ity of
expression and notwithstanding the fact that heat is vibration.]

Thirteen cubic feet of free air at normal temperature and barometric
pressure weigh about one pound. We have seen that 116 degrees of heat
have been liberated at half stroke. The gauge pressure at this point
reaches 24 pounds. According to Mariotte's law, "The temperature
remaining constant, the volume varies inversely as the pressure," we
should have 15 pounds gauge pressure. The difference, 9 pounds,
represents the effect of the heat of compression in increasing the
relative volume of the air.

[Illustration: FIG. 1.: CURVES OF COMPRESSION ILLUSTRATEDIN VOLUMES,
PRESSURES, AND TEMPERATURES.]

The specific heat of air under constant pressure being 0.238, we have
0.238 x 116 = 27.6 heat units produced by compressing one pound or
thirteen cubic feet of free air into one-half its volume. 27.6 x 772
(Joule's equivalent) = 21,307 foot pounds. We know that 33,000 foot
pounds is one horse power, and we see how easily about two-thirds of a
horse power in heat units may be produced and lost in compressing one
pound of air. I would mention here that exactly this same loss is
suffered when compressed air does work in an engine and is expanded down
to its original pressure. In other words, _the heat of compression and
the cold of expansion are in degree equal_.

Experiments made by M. Regnault and others on the influence of heat on
pressures and volumes of gases have enabled us to fix the absolute zero
of temperature as -461 degrees Fahrenheit. This point, 461 degrees below
zero, is the theoretical point at which a volume of air is reduced to
nothing. The volume of air at different temperatures is in proportion to
the absolute temperature, and on this basis Box gives us the following
table:

TABLE l.--OF THE VOLUME AND WEIGHT OF DRY AIR AT DIFFERENT TEMPERATURES
UNDER A CONSTANT ATMOSPHERIC PRESSURE OF 29.92 INCHES OF MERCURY IN THE
BAROMETER (ONE ATMOSPHERE), THE VOLUME AT 32 deg. FAHRENHEIT BEING 1.

Temperature Volume in Weight of a
in degrees. cubic feet. cubic foot in lb.
32 1.000 0.0807
42 1.020 0.0791
52 1.041 0.0776
62 1.061 0.0761
72 1.082 0.0747
82 1.102 0.0733
92 1.122 0.0720
102 1.143 0.0707
112 1.163 0.0694