LESSONS IN PHYSICS. 61 



hour. Thus, light substances, as the air, exhibit great energy 

 when their velocity is great. As a body is raised higher and 

 higher the work accumulates in the form of potential energy. 

 The accumulated work, or the potential energy, does not always 

 equal the work performed. Some of the work done is used in 

 overcoming friction, and is wasted. The work done by the saw- 

 yer and the bullet imparted no energy to the bodies upon which 

 they did work; it was entirely consumed in overcoming resist- 

 ance, disappearing as energy. Of the vast amount of work done 

 in propelling steam cars or vessels none accumulates, all is 

 wasted, and cannot be recovered or made available for doing 

 work. The available energy that a body possesses is the work 

 done upon the body, less the lost or wasted work. We may 

 calculate in foot-pounds the work performed on a body, and from 

 this deduct the number of foot-pounds wasted, and the remain- 

 der is the number of foot-pounds of energy that is imparted to 

 the body. 



In estimating the total amount of work done, the time con- 

 sumed is not considered. The work done by a hod-carrier in 

 carrying 1,000 bricks -to the top of a building is the same 

 whether he does it in one day or a week. But in estimating the 

 power of any agent to do work, or the rate at which it is capable 

 of doing work, time is an important element. The unit in which 

 power or rate of doing work is estimated, is called a horse 

 power. A horse power represents the power to perform 33,000 

 foot-pounds of work in a minute. 



Energy, like matter, is indestructible. It may be transmitted, 

 it may be changed into other forms, but not destroyed. The 

 quantity of energy cannot be increased or diminished. Whenever 

 a body in motion meets an obstacle energy is transmitted to it as 

 in the case of the bat and the ball. 



Let a ball be shot vertically upwards. On starting, its energy 

 is kinetic, and it rises more and more slowly until it stops, and 

 then falls to the ground. At the moment it stops at the top of 

 its path its energy is potential. During its flight upwards kinetic 

 energy has been changing into potential energy, and again, dur- 



