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 3 CO 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 the conservation 



