press, and squeezed forcibly. Although the
plates of the press are also cooler than the air
of the room, after compression the wood,
brought in contact with the pile, declares that
heat has been developed by the act of
compression. Precisely the same thing occurs
when a block of lead is put between the plates
of the press, and squeezed to flatness.

And now for the effect of percussion. A cold
lead bullet, placed upon a cold anvil, is struck
with a cold sledge-hammer. The sledge descends
with a certain mechanical force, and its motion
is suddenly destroyed by the bullet and anvil;
apparently, the force of the sledge is lost. But
on examining the lead, we find it is heated;
and we shall by-and-by learn that if we could
gather up all the heat generated by the shock
of the sledge, and apply it without loss
mechanically, we should be able, by means of it, to lift
the hammer to the height from which it fell.

Another experiment leads to a like conclusion.
Cold mercury is put into one of two
glasses, which are thickly swathed round with
listing, to prevent the warmth of the lecturer's
hands from reaching the mercury. He pours
the cold mercury from the one glass into the
other, and back. It falls with a certain
mechanical force; its motion is destroyed, but heat is
developed. The amount of heat generated by
a single pouring out is extremely small—the
exact amount might be easily determined—so
the mercury is poured from glass to glass ten
or fifteen times. Now mark the result when
the pile is plunged into the liquid. The needle
moves, and its motion declares that the
mercury, which at the beginning of the experiment
was cooler, is now warmer than the pile. An
effect is thus introduced into the lecture-room,
which occurs at the base of every waterfall.
There are many who have stood amidst the
foam of Niagara. Had they, when there,
dipped sufficiently sensitive thermometers into
the water at the top and bottom of the cataract,
they would have found the latter warmer than
the former. The sailor's tradition, also, is
theoretically correct; the sea is rendered
warmer by a storm, the mechanical dash of its
billows being ultimately converted into heat.

The co-relation of Friction with Heat may be
illustrated thus:—All the force of our locomotives
is derived from heat, and all of it eventually
becomes heat. To maintain the proper speed, the
friction of the train must be continually overcome,
and the force spent in overcoming it is entirely
converted into heat. An eminent writer, Dr.
Mayer, has compared the process to one of
distillation. The energy of heat passes from the
furnace into the mechanical motion of the train; and
this reappears as heat in the wheels, axles, and
rails. When a station is approached, say at the
rate of thirty miles an hour, a brake is applied,
and smoke and sparks issue from the wheel on
which it presses. The train is brought to rest.
How? Simply by converting the entire moving
force which it possessed at the moment
the brake was applied, into heat.

Aristotle refers to the heating of arrows by
the friction of the air. A rifle-bullet, in passing
through the air, is also warmed by friction. The
most probable theory of shooting stars is, that
they are small planetary bodies revolving round
the sun, which are caused to swerve from their
orbits by the attraction of the earth, and are
raised to incandescence by friction against our
atmosphere. Mr. Joule has shown that the
atmospheric friction is competent to produce the
effect. These bodies move at planetary rates.
The reader may be reminded that, in round
numbers, Mars travels along his orbit at fifteen
miles per second; the Earth runs round hers at
eighteen; while Venus does twenty-two, and
Mercury thirty miles per second. The velocity
of the aërolites varies from eighteen to thirty-six
miles a second. The friction engendered by
this enormous speed is certainly competent to
produce the effects ascribed to it.

More than sixty-four years ago, Count
Rumford executed a series of experiments on the
generation of heat by friction. By means of an
iron cylinder turned in a box by horse-labour,
he actually caused water to boil in two hours
and thirty minutes from the commencement
of the work. His delight at the astonishment
of the bystanders was great. "I fairly acknowledge,"
he says, "that it afforded me a degree
of childish pleasure, which, were I ambitious of
the reputation of a grave philosopher, I ought
most certainly to hide rather than discover."
Professor Tyndall, not having two hours and a
half to devote to a single experiment, produced
the same effect in two minutes and a half.

The new philosophy accounts for the light
and heat emitted by the sun, in a way which, a
hundred years ago, would not have been
imagined by the wildest dreamer. This hypothesis
was propounded, in 1848, by Dr. Mayer, in his
Essay on Celestial Dynamics, and is called the
Meteoric Theory of the Sun.

Take a cold iron hammer; with it beat a cold
anvil; and, with sufficient strength and
perseverance, you can beat till both hammer and
anvil are hot. In like manner, the sun is an
enormous anvil, and his heat is maintained by a
succession of blows. The reader, to whom the
idea is new, will naturally ask, "But where and
what are the hammers which give the blows?"
Before replying to the question, it will be well
to state a few preliminaries.

The heat emitted by the sun has been
measured by Sir John Herschel at the Cape of Good
Hope, and by M. Pouillet in Paris. The agreement
between the measurement is very remarkable,
and the mean of the determination cannot
be far from the truth. This assigns to the direct
heat of a vertical sun, at the level of the sea, the
power of melting nearly half an inch of ice per
hour. The mode of measurement and the
instrument employed—called by M. Pouillet a
pyrheliometer—are clearly described by Mr.
Tyndall. The observations were made at different
hours of the day, and consequently through
different thicknesses of the earth's atmosphere;
augmenting from the minimum thickness at
noon, up to the maximum at six P.M. It was