THE NATURE OF ELECTRICITY. 25 



well established, and universally recognized in all 

 practical work. 



Another illustration may render the subject more 

 clear. A pound weight raised to a height of 20 feet 

 has acquired 20 foot-pounds of energy ; and, in de- 

 scending to its former level, can accomplish 20 foot- 

 pounds of work; as in raising to the same height 

 another weight of nearly equal mass. But if stopped 

 in its descent at a level of 10 feet, it has expended 

 only 10 foot-pounds of energy, and has still a reserve of 

 10 more. 



It is evident, that to raise the weight in the first place 

 required the expenditure of 20 foot-pounds of energy ; 

 and that though this energy was consumed in the pro- 

 cess, and had disappeared, it was not lost, but merely 

 stored up, ready to be expended, either at once, by 

 the weight descending the entire distance, or in detail, 

 as when stopped half way or at any other point. If it 

 had descended but one foot, it would still have a re- 

 serve of 19 foot-pounds of energy. 



It will be noticed that the second weight, raised by 

 the descent of the first, is required to be of less 

 magnitude, since part of the energy must be expended 

 in overcoming friction and inertia. For if it were of 

 equal magnitude, the force expended would exceed the 

 force stored up; since it must perform not only the 

 same work, but the added amount for friction and 

 inertia; in which case it would be possible to create 

 force, and the doctrine of the conservation of energy 

 would cease to be true. 



It is immaterial whether the descent of the one pound 

 raises a small weight to the height of 20 feet, or a 

 laree weight to the height of one foot ; which it can be 



