average man will pump or shovel in a lifetime; so that if a man
proposes to do nothing but work with his muscles, he had better dig
three tons of coal and set that to do his work and then die, because
his work will be better done, and without any cost for the
maintenance of the doer.

Come down below the color vibrations, and we shall find that those
which are too infrequent to be visible, manifest as heat. Naturally
there will be as many different kinds of heat as tints of color,
because there is as great a range of numbers of vibration. It is
our privilege to sift them apart and sort them over, and find what
kinds are best adapted to our various uses.

Take an electric lamp, giving a strong beam of light and heat, and
with a plano-convex lens gather it into a single beam and direct
it upon a thermometer, twenty feet away, that is made of glass
and filled with air. The [Page 32] expansion or contraction of this
air will indicate the varying amounts of heat. Watch your
air-thermometer, on which the beam of heat is pouring, for the
result. There is none. And yet there is a strong current of heat
there. Put another kind of test of heat beyond it and it appears;
coat the air-thermometer with a bit of black cloth, and that will
absorb heat and reveal it. But why not at first? Because the glass
lens stops all the heat that can affect glass. The twenty feet of
air absorbs all the heat that affects air, and no kind of heat is
left to affect an instrument made of glass and air; but there are
kinds of heat enough to affect instruments made of other things.

A very strong current of heat may be sent right through the heart
of a block of ice without melting the ice at all or cooling off