Thurston, in discussing "Steam and its Rivals," in the _Forum_, said:
"The science of Thermodynamics teaches that heat and mechanical energy
are only different phases of the same thing, the one being the motion of
molecules, and the other that of masses." This is the accepted theory of
heat. In other words, we do not believe that there is any such _thing_
as heat, but that what we call heat is only the sensible effect of
motion. In the cylinder of an air compressor the energy of the piston is
converted into molecular motion in the air and the result, or the
equivalent, is heat. A higher temperature means an increased speed of
vibration, and a lower temperature means that this speed of vibration is
reduced. If I hold an open cylinder in my left hand and a piston in my
right, and place the piston within the cylinder, I here have a confined
volume of air at the temperature and the pressure of this room. These
particles of air are in motion and produce heat and pressure in
proportion to that motion. Now if I press the piston to a point in the
center of the cylinder, that is, to one-half the stroke, I here decrease
the distance between the cylinder head and the piston just one-half,
hence each molecule of air strikes twice as many blows upon the piston
and head in traveling the same distance and the pressure is doubled. We
have also produced about 116 degrees of heat, because we have expended a
certain amount of work upon the air; the air has done no work in return,
but we have increased the energy of molecular vibration in the air and
the result is heat.

But what of this heat? What harm does it do? If I instantly release the
piston which I hold at one-half stroke it will return to its original
position, less only a little friction. I have, therefore, recovered all,
or nearly all, the power spent in compressing the air. I have simply
pressed a spring, and have let it recover. We see what a perfect spring
compressed air is. We see the possibility of expending one horse power