168 THE WORLD-ENERGY 



all such motion must be unified into a perfectly consistent 

 system. 



In the aggregations of force-centers into masses, then, 

 we see the same tendency as that which, at a more 

 advanced stage, cannot fail to produce still larger aggre- 

 gations. Masses attracting one another must move toward 

 one another, and this with increasing velocity, both from 

 the continuous action of the mutual attraction between 

 them, and also from the increasing intensity of the gravi- 

 tative pull, as the distance separating them diminishes. 



Thus masses, which are but groups of force-centers, 

 come to have velocity as such. And this combination of 

 masses and velocities is the development of that phase of 

 force technically known as " molar motion," and which, 

 regarded simply as energy, is called kinetic energy. 



It is true that, in popular language, motion is distin- 

 guished from bodies. Thus bodies are said to be "in 

 motion." And it must doubtless be something of a 

 shock to the ordinary consciousness to be assured that 

 bodies are "in motion" only in the sense that they are 

 themselves one of the necessary factors of motion. And 

 yet this is universally recognized, at least in words, in the 

 treatises dealing with this subject. 



As to the decrease of molar motion, and the special 

 phase of energy which it embodies, this is effected either 

 gradually, in which case there is friction or compression ; 

 or suddenly, in which case there is .percussion. But 

 these may be better considered in connection with their 

 effects under the subject of heat. 



It is to be observed that our consideration of the laws 

 of falling bodies has confined us to the simplest cases of 

 molar motion. We have abstracted or withdrawn our 



