80 SCIENCE IX THE LAST HALF CENTURY. 



stoue is solicited to move, but can not so loug- as tlie muscular strength 

 of the holder prevents the solicitation from taking effect. The stone, 

 therefore, has potential energy, which becomes kinetic if it is let go, 

 and the amount of that kinetic energy which will be developed before 

 it strikes the earth depends upon its position, — on the fact that it is, say, 

 G feet off the earth, neither more nor less. Moreover, it can be proved 

 that the raiser of the stone had to exert as much energy in order to 

 place it in its position as it will develop in falling. Hence the en- 

 ergy which was exerted, and apparently exhausted, in raising the stone 

 is potentially in the stoue in its raised position, and will manifest it- 

 self when the stone is set free. Thus the energy, withdrawn from the 

 general stock to raise the stone, is returned when it falls, and there is 

 no change in the total amount. Energy, as a whole, is conserved. 



Taking this as a very broad and general statement of the essential 

 facts of the case, the raising of the stone is intelligible enough, as a 

 case of the conununicatiou of motion from one body to another. But 

 the potential energy of the raised stone is not so easily intelligible. 

 To all appearance, there is nothing either pushing or pulling it toward 

 the earth, or the earth toward it; and yet it is quite certain that the 

 stone tends to move toward the earth, and the earth toward the stone, 

 in the way defined by the law of gravitation. 



In the currently accepted language of science, the cause of motion, 

 in all such cases as this, when bodies tend to move toward or away from 

 one or another, without any discernible impact of other bodies, is termed 

 a " force," which is called " attractive " in the one case, and " repul- 

 sive" in the other. And such attractive or repulsive forces are often 

 spoken of as if they were real things, capable of exerting a pull, or a 

 push, upon the particles of matter concerned. Thus the potential 

 energy of the stone is commonly said to be due to the "force " of gravity 

 which is continually operating upon it. 



Another illustration may make the case plainer. The bob of a pend- 

 ulum swings first to one side and then to the other of the center of the 

 arc which it describes. Suppose it to have just reached the summit of 

 its right-hand half-swing. It is said that the "attractive forces" of the 

 bob for the earth, and of the earth for the bob, set the former in motion ; 

 and as these " forces" are continually in operation, they confer an ac- 

 celerated velocity on the bob; until, when it reaches the center of its 

 swing, it is, so to speak, fully charged with kinetic energy. If, at this 

 moment, the whole material universe, except the bob, were abolished, 

 it would move forever in the direction of a tangent to the middle of the 

 arc described. As a matter of fact, it is compelled to travel through 

 its left-hand half-swing, and thus virtually to go up hill. Consequently 

 the " attractive forces" of the bob and the earth are now acting against 

 it, and constitute a resistance which the charge of kinetic energy has to 

 overcome. But as this charge represents the operation of the attractive 

 forces, during the passage of the bob through the right-hand half-swing 



