weariness, and sighed heavily. There were
three persons observing her; but her thoughts
were very bitter at that moment, and she
was quite unconscious of their scrutiny. Those
persons were Lady Castletowers; Signor
Colonna, who had but just arrived, and was
leaning on the back of her chair; and Miss
Hatherton—and neither the look of pain, nor
the sigh, was lost on either of them.

HEAT AND WORK.*
* See IS HEAT MOTION? page 534 in the last volume.

IN his treatise, Heat considered as a mode of
Motion, Professor Tyndall shows that heat is
expended whenever work is done. After
demonstrating by experiment that, where mechanical
force is expended, heat is produced, he brings
before us the converse experiment, and shows
us the consumption of heat in mechanical work.

He exhibits to his audience a strong vessel
filled with compressed air. It has been so
compressed for some hours, in order that the
temperature of the air within the vessel may
be the same as that of the air in the room
without. At that moment, then, the inner air
was pressing against the sides of the vessel;
and, if he opened the tap, a portion of the
air would rush violently out of the vessel.
The word "rush," however, but vaguely
expresses the true state of things. The air which
issues, is driven out by the air behind it; this
latter accomplishes the work of urging forward
the stream of air. And what will be the condition
of the working air during this process? It
will be chilled. It performs mechanical work;
and the only agent it can call upon to perform
it, is the heat which it possesses, and to which
the elastic force with which it presses against
the sides of the vessel, is entirely due. A portion
of this heat will be consumed, and the air will
be chilled. It is so, on carrying out the experiment.
The tap is turned, and the current of
air from the vessel is allowed to strike against
the face of the thermo-electric pile—the most
delicate and demonstrative of thermometers. The
magnetic needle instantly responds, declaring
that the pile has been chilled by the current of air.

The effect is different when air is urged from
the nozzle of a common bellows against the pile.
In the last experiment, the mechanical work of
urging the air forward was performed by the air
itself, and a portion of its heat was consumed in
the effort. In the case of the bellows, it is the
experimenter's muscles which perform the work.
He raises the upper board of the bellows, and
the air rushes in; he presses the boards with a
certain force, and the air rushes out. The
expelled air, striking the face of the pile, has its
motion stopped; and an amount of heat equivalent
to the destruction of this motion is instantly
generated. When a current of air is directed
with the bellows against the pile, the motion of
the needle shows that the face of the pile has, in
this instance, been warmed by the air.

Again: to prove the chilling effect of work
done, even by so slightly-built a labourer as gas,
the Professor takes a bottle of soda-water, which
is shown to be a trifle warmer than the pile. He
cuts the string which holds the cork, and it is
driven out by the elastic force of the carbonic
acid gas. The gas performs work; in so doing,
it consumes heat; and the deflection of the
needle produced by the bottle shows that it has
become colder. A simple detail of daily life,
an operation with which every child is familiar,
allows the lecturer, to illustrate principles from
which all material phenomena flow. That it is
not the expansion, but the work, which
produces the chill, is proved by allowing compressed
air, from one vessel, to pass into another from
which the air has been exhausted. No work
having to be done, there is no change of
temperature. Mere rarefaction, therefore, is
not of itself sufficient to produce a lowering
of the mean temperature of a mass of air.
It was, and still is, a current notion that the
mere expansion of a gas produces refrigeration,
no matter how that expansion may be effected.
The coldness of the higher atmospheric regions
was accounted for by reference to the expansion
of the air. But the refrigeration which
accompanies expansion is really due to the
consumption of heat in the performance of work.
Where no work is performed, there is no
absolute refrigeration. The simple experiment of
allowing a leaden ball to fall from the ceiling to
the floor, shows that heat is generated by the
sudden stoppage of the motion. This affords an
opportunity of telling how the "mechanical
equivalent" of heat has been calculated.

It is found that the quantity of heat which
would raise one pound of water one degree
Fahrenheit in temperature, is exactly equal to what
would be generated if a pound weight, after
having fallen through a height of seven hundred
and seventy-two feet, had its moving force
destroyed by collision with the earth.
Conversely, the amount of heat necessary to raise
a pound of water one degree in temperature,
would, if all applied mechanically, be competent
to raise a pound weight seven hundred and
seventy-two feet high; or, it would raise seven
hundred and seventy-two pounds, one foot high.
The term "foot-pound" has therefore been
introduced to express in a convenient way the
lifting of one pound to the height of a foot.
Thus, the quantity of heat necessary to raise
the temperature of a pound of water one degree
Fahrenheit being taken as a standard, seven
hundred and seventy-two foot-pounds constitute
what is called the mechanical equivalent of heat.

For every stroke of work done by the
steam-engine, for every pound that it lifts, and for
every wheel that it sets in motion, an equivalent
quantity of heat disappears.  A ton of coal
furnishes by its combustion a certain definite
amount of heat.  Let this quantity of coal be
applied to work a steam engine; and let all the
heat communicated to the machine and the
condenser, and all the heat lost by radiation and
by contact with the air, be collected; it will
fall short of the quantity produced by the simple