3$6 NINETEENTH CENTURY. PT. in. 



Now observe how this measured the work done and the 

 heat produced. Every time the weight fell, it turned the 

 paddle, and so, by agitating the water, added to its heat 

 The scale, D, told him exactly how far the weight fell, while 

 the thermometer, /, in the box told him how much hotter 

 the water grew. At the end of an hour, therefore, he had 

 only to see how many feet his pound-weight had fallen, and 

 how many degrees of Fahr. the temperature of his water had 

 risen ; and after allowing for the friction of his machinery 

 and for the heat lost in the cooling of his vessel, both of 

 which he ascertained by careful experiments, he could tell 

 how much energy had been expended in producing the 

 rise of temperature. In this way he found that a weight of 

 i Ib. would have to fall 772 feet in order to raise the tern- 

 perature of i Ib. of water by i Fahr. 



He next tried trie same experiment with oil and with mer- 

 cury instead of water, and also measured the heat produced 

 by rubbing together two plates of iron ; and in every case he 

 found that a certain amount of work gave a certain amount 

 of heat and no more. For example, if the weight in Fig. 

 62 fell double the distance, the temperature of the water was 

 raised two degrees instead of one, while if it fell only half the 

 distance, or 386 feet, the water was only raised half a degree. 



In this way Dr. Joule established what is called the 

 mechanical equivalent of heat, namely that the fall of a pound 

 weight through 772 feet sets free enough mechanical energy 

 to raise the temperature of a pound of water i Fahr. Arid 

 now you must try to form a clear idea what this means. 

 Looking at the diagram, try to picture to yourself what 

 would be taking place if the weight were able to fall the 

 whole 772 feet without stopping. First, a man must wind 

 up the weight, and in doing this he uses working power or 

 energy to overcome the force of gravitation which is pulling 



