THE CONSTITUTION OF NATURE 21 



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 concentra- 

 tion the heat developed becomes 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 

 gunpowder which urges the cannon-ball. This is com- 

 posed 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 con- 

 nected. 



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 

 impossible to create force and to annihilate it. But in 

 what sense are we to understand this assertion ? It would 

 be manifestly inapplicable to the force of gravity as de- 

 fined by Newton; for this is a force varying inversely as 

 the square of the distance; and to affirm the constancy of 

 a varying force would be self-contradictory. Yet, when 

 the question is properly understood, gravity forms no ex- 

 ception to the law of conservation. Following the method 

 pursued by Helmholtz, I will here attempt an elementary 

 expositio'n of this law. Though destined in its applica- 

 tions to produce momentous changes in human thought, 

 it is not difficult of comprehension. 



