Before explaining the principle upon which the gas-engine and every
other hot-air engine depends, I shall remind you of a few data with
which most of you are already familiar. The volume of every gas
increases with the temperature; and this increase was the basis of the
air thermometer--the first ever used. It is to be regretted that it
was not the foundation of all others; for it is based on a physical
principle universally applicable. Although the volume increases with
the temperature, it does not increase in proportion to the degrees of
any ordinary scale, but much more slowly. Now, if to each of the terms
of an arithmetical series we add the same number, the new series so
formed increases or decreases more slowly than the original; and it
was discovered that, by adding 461 to the degrees of Fahrenheit's
scale, the new scale so formed represented exactly the increment of
volume caused by increase of temperature. This scale, proposed by Sir
W. Thomson in 1848, is called the "scale of absolute temperature." Its
zero, called the "absolute zero," is 461° below the zero of
Fahrenheit, or 493° below the freezing point of water; and the degree
of heat measured by it is termed the "absolute temperature." It is
often convenient to refer to 39° Fahr. (which happens to be the point
at which water attains its maximum density), as this is the same as
500° absolute; for, counting from this datum level, a volume of air
expands exactly 1 per cent. for 5°, and would be doubled at 1,000°
absolute, or 539° Fahr.

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