LESSONS IN PHYSICS. 59 



a cord to the stone, pass the cord over a pulley and wind a por- 

 tion of it around a shaft connected with a sewing machine, coffee 

 mill, or other convenient machine. Suddenly withdraw the shelf 

 from beneath the stone, the stone falling communicates motion 

 to the machinery and you may sew, grind coffee, etc., with the 

 energy given to the machinery by the stone. The work done on 

 the stone in raising it was not lost. The stone gives it back 

 while descending to the earth. There is a very important differ- 

 ence between the stone lying on the ground and the stone lying 

 on the shelf. The former is powerless to do work, the latter can 

 do work. Both are alike motionless, and you can see no differ- 

 ence excepting the advantage which the latter has over the 

 former in having a position such that it can move. The work 

 done in raising it to its place on the shelf gave it this ad- 

 vantage. A body thus may possess energy due merely to the 

 advantage of its position derived always from work bestowed 

 upon it. Energy due to advantage of position is called potential 

 energy. We see, therefore, that energy may exist in either of two 

 widely different states. It may exist as actual motion in the 

 case of the bat, or it may exist in a stored-up condition, as in 

 the case of the stone lying on the shelf. The energy which the 

 stone on the shelf possesses is due to the fact that its position is 

 such that it can move, and that there is stress between it and the 

 earth which will cause it to move. The force of gravity is em- 

 ployed to do work as when mills are driven by falling water; but 

 the water must first be raised from the ocean bed to the hill side 

 by the work of the sun's heat. The elastic force of springs is 

 employed as a motive power; but this power is due to the ad- 

 vantage of position, which the molecules of the springs have 

 acquired by work done upon them. We are as much accustomed 

 to storing energy for future use as provisions for winter's con- 

 sumption. We store it when we wind up the spring or weight of 

 a clock to be doled out gradually in the movements of the ma- 

 chinery. We store it when we bend the bow, raise the hammer or 

 any body above the earth's surface. 

 The energy of a body is dependent wholly upon the work which 



