112 THE REALITIES OF MODERN SCIENCE 



What, however, is the effect of increasing the kinetic 

 energy of the molecules of a substance? An experi- 

 ment which was performed by the Englishman, Joule, 

 in about 1843 not only answered this question but is 

 the basis of the reasoning as to energy which we are 

 able to follow to-day. He arranged a falling weight 

 so that it turned a paddle wheel in a vessel of water 

 as shown in Fig. 8. The motion of the falling weight 



is retarded in part by 

 the friction of the pad- 

 dle wheels and water 

 and in part by a second 

 weight which it lifts. 

 This second weight was 

 increased until its re- 

 tarding effect plus that 

 of the paddles was just 

 sufficient to insure that 

 the first weight fell so gradually as practically to have 

 no kinetic energy. The first weight, being greater 

 than the second, lost potential energy while the latter 

 gained. The total loss in potential energy of the system 

 formed by the earth and the two weights was the same 

 as if a single weight, equal to the difference of the 

 two, had fallen through the distance through which the 

 first weight fell. 



Joule found that the rapidly revolving paddle wheels 

 heated the water, and he measured the rise in its 

 temperature with a thermometer. He made several 

 trials and he found that for every 42,700 g. cm. of 

 potential energy which the weights lost there was an 

 equivalent rise of temperature of one degree Centi- 



FIG. 8. 



