AND ITS SELF-CONSERVATION. 175 



But, again, as we have seen, a mass in motion pre- 

 sents a case of actual energy energy of motion. And 

 since this energy of motion is directed, not in a straight 

 line toward the center, but in a curved line about the 

 center, it is evident that what began as a tendency 

 toward the center has developed into a tendency away 

 from the center. It is this latter tendency that is 

 properly termed centrifugal force. 



And, now, let us note that the nearer the smaller 

 bodies approach the central one the greater becomes their 

 velocity, and, hence, the greater becomes their energy of 

 motion. But since this increasing energy of motion is 

 directed in a curve about the center, it must (the bodies 

 being of the given relative mass) attain at length a 

 degree of intensity that is not merely sufficient to bal- 

 ance the tendency toward the center, but which will 

 be even sufficient to overbalance the centripetal ten- 

 dency and thus actually carry the lesser masses away 

 from the center. 



At the same time the gravitative strain or tendency 

 toward the center is a continuously impressed force. 

 And at the maximum point reached in its intensity 

 (which is, of course, at the moment of nearest approach 

 of the bodies to the center) the gravitative strain is ade- 

 quate to overcome the heightened degree of the energy 

 of motion (here developed as centrifugal force) acquired 

 by the revolving bodies so far as to cause a rapid change 

 in the direction of their motion. 



Thus, while they do indeed pass the center, it is not 

 until, with their extreme velocity, they have passed so far 

 around it as to make a swift retrograde movement toward 

 the distant point from which their fall began. Their fall 



