188 THE CONTINUITY OF MOTION. 



is the correlative of the muscular effort which moved the 

 chandelier, what becomes of this element at either extreme 

 of the oscillation? Arrest the chandelier in the middle of 

 its swing, and it gives a blow to the hand — exhibits some 

 principle of activity such as muscular effort can give. But 

 touch it at either turning point, and it displays no such 

 principle of activity. This has disappeared just as much 

 as the translation through space has disappeared. How, 

 then, can it be alleged that though the Motion through 

 space is not continuous, the principle of activity implied 

 by the Motion is continuous '. 



Unquestionably the facts show that the principle of 

 activity continues to exist under some form. TVhen not 

 perceptible it must be latent. How is it latent? A clue 

 to the answer is gained on observing that though the chan- 

 delier when seized at the turning point of its swing, gives 

 no impact in the direction of its late movement, it forth- 

 with begins to pull in the opposite direction; and on ob- 

 serving, further, that its pull is great when the swing has 

 been made extensive by a violent push. Hence the loss of 

 visible activity at the highest point of the upward motion, is 

 accompanied by the production of an invisible activity which 

 generates the subsequent motion downwards. To conceive 

 this latent activity gained as an existence equal to the per- 

 ceptible activity lost, is not easy; but we may help our- 

 selves so to conceive it by considering cases of another class. 



§ 57. AYhen one who pushes against a door that has 

 stuck fast, produces by great effort no motion, but eventually 

 by a little greater effort bursts the door open, swinging it 

 back against the wall and tumbling headlong into the room; 

 he has evidence that a certain muscular strain which did not 

 produce translation of matter through space, was yet equiva- 

 lent to a certain amount of such translation. Again, when 

 a railway-porter gradually stops a detached carriage by pull- 

 ing at the buffer, he shows us that (supposing friction, etc., 



