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THE CONSTITUTION OF NATURE. 21

raise its own weight of water one degree Fahrenheit in
temperature. We have here the mechanical equivalent of
heat. Now, a body falling from a height of 772 feet, has,
upon striking the earth, a velocity of 223 feet a second;
and if this velocity were imparted to a body, by any other
means, the quantity of heat generated by the stoppage of
its motion would be that stated above. Six times that ve-
locity, or 1,338 feet, would not be an inordinate one for a
cannon-ball as it quits the gun; but if animated by six
times the velocity, thirty-six times the heat will be gener-
ated by the stoppage of its motion. Hence a cannon-ball
moving with a velocity of 1,338 feet a second, would, by
collision, generate an' amount of heat competent to raise its
own weight of water 36 degrees Fahrenheit in tempera-
ture. If composed of iron, and if all the heat generated
were concentrated in the ball itself, its temperature would
be raised about 360 degrees Fahrenheit; because one de-
gree in the case of water is equivalent to about ten de-
grees in the case of iron. In artillery practice the heat
generated is usually concentrated upon the front. of the
bolt, and on the portion of the target first struck. By this
concentration the heat developed may become sufficiently
intense to raise the dust of the metal to incandescence, a
flash of light often accompanying collision with the target.

Let us now fix our attention for a moment on the gun-
powder which urges the cannon-ball. This is composed of
combustible matter, which if burnt in the open air would
yield a certain amount of heat. It will not yield this
amount if it performs the work of urging a ball. The heat
then generated by the gunpowder will fall short of that
produced in the open air, by an amount equivalent to the
vis viva of the ball; and this exact amount is restored by
the ball on its collision with the target. In this perfect
way are heat and mechanical motion connected.

Broadly enunciated, the principle of theconservation

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