THE CONSTITUTION OF NA TURE. 1 1 
the velocity, it will generate nine times the heat, and so 
on. 
Mr. Joule has shown that a pound weight falling from 
a height of 772 feet, or 772 pounds falling through one 
foot, will generate by its collision with the earth an 
amount of heat sufficient to raise a pound of water one de- 
gree Fahrenheit in temperature. 772 " foot-pounds" 
constitute the mechanical equivalent of heat. Now, a 
body falling from a height of 772 feet, has, npon striking 
the earth, a velocity of 223 feet a second; and if this 
velocity were imparted to the body, by any other means, 
the quantity of heat generated by the stoppage of its 
motion would be that stated above. Six times that 
velocity, or 1,388 feet, would not be an inordinate one for a 
cannon-ball as it quits the gun. Hence, a cannon-ball 
moving with a verocity 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 
temperature. If composed of iron, and if all the heat 
generated were concentrated in the ball itself, its tempera- 
ture would be raised about 360 degrees Fahrenheit; 
because one degree in the case of water is equivalent to 
about ten degrees in the case of iron. In artillery prac- 
tice, 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 be- 
comes sufficiently intense to raise the dust of the metal to 
incandescence, a flash of light often accompanying colli- 
sion 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 burned in the open air 
would yield a certain amount of heat. It will not yield 
this amount if it perform 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 equiva- 
lent 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 the conservation 
of force asserts, that the quantity of force in the universe 
is as unalterable as the quantity of matter; that it is alike 
