CHAPTEE VI. 



THE MOTIVE POWERS OF THE UNIVEESE. 



WE have seen that matter and energy are incapable of 

 being either produced or destroyed, and capable only 

 of being transformed ; and it is an obvious inference from 

 this that the universe has existed from an eternal past, and Theoiy 

 will continue to exist through an eternal future. It ap- and future 

 pears at first sight a probable theory that all changes that ^^""^^^g"! 

 go on in the universe are like the motions of the planets in order of 

 their orbits, which, after a shorter or longer period, end at ^grse'"' 

 the point where they began. It appears as if we had good 

 ground for the belief, which was that of many ancient 

 philosophers, that cosmic changes (to use a mathematical 

 metaphor) move in closed curves ; and that at the end of a 

 finite though very long cycle, or annus maximus, aU. things 

 will return to their state at its beginning. 



But a profounder knowledge of the laws of energy 

 shows that such an inference is untrue. It shows that the untrue, 

 universe has not existed from an eternal past. It shows 

 that cosmic changes do not come back to the point where 

 they began : that they do not move in closed curves, but in 

 open spirals ; though in the historical periods these spirals 

 are not distinguishable from closed curves. 



The reason of this is the destruction of motive power Eeason 

 which is always going on. Energy is indestructible, but ° 

 motive power is constantly diminishing in quantity. 

 Motive power is defined, for the purposes of the mechanic, of^mot^iw 

 as useful energy — energy that he can transform as he power. 



