270 THE PRINCIPLES OF SCIENCE. 



ball expelled, all simultaneously to our senses. But there 

 is no doubt that time is occupied by every part of the 

 process, and that the ball begins to move at first with 

 indefinite slowness. Captain Noble is able to measure 

 by his chronoscope the progress of the shot in a 300- 

 pounder gun, and finds that the whole motion within the 



barrel takes something less than of a second. It is 



an invariable principle of nature that no finite force can 

 produce motion, except in a finite space of time. The 

 amount of momentum communicated to a body is pro- 

 portional to the accelerating force multiplied by the time 

 through which it acts uniformly. Thus a slight force 

 produces a great velocity only by long continued action. 

 In a powerful shock, like that of a railway collision, the 

 stroke of a hammer on a hard anvil, or the discharge of a 

 gun, the time is very short, and therefore the accelerating 

 forces brought into play are exceedingly great, but never 

 infinite. In the case of a large gun the powder in ex- 

 ploding is said to exert for a moment a force equivalent 

 to at least 2,800,000 horses. 



Our belief in some of the most fundamental laws of 

 nature rests upon the principles of continuity. Galileo is 

 held to be the first philosopher who consciously employed 

 this principle in his arguments concerning the nature of 

 motion, and it is certain that w T e can never by pure ex- 

 perience assure. ourselves of the truth even of the first law 

 of motion. A material particle, we are told, when not 

 acted on by extraneous forces will continue in the same 

 state of rest or motion. This may be true, but as we can 

 find no body which is free from the action of extraneous 

 causes, how are we to prove it 1 Only by observing that 

 the less the amount of those forces the more nearly is the 

 law found to be true. A ball rolled along rough ground 

 is soon stopped ; along a smooth pavement it continues 



