136 WRITINGS OF JOSEPH HENRY. [1855- 



out in the waste-pipe would be just equal to that received 

 into the boiler, provided there were no loss from conduction 

 and radiation ; but in the engine drawing up water, for ex- 

 ample, a quantity of heat is actually annihilated in doing 

 the work. The vibrations of the atoms which constitute 

 heat are stopped in giving motion to the piston-rod. Con- 

 versely, if the water which has been pumped up to an eleva- 

 tion were made in its descent to produce heat by means of 

 revolving disks, the amount generated would be just equal 

 to that which disappeared in the other case. 



For practical purposes it is therefore of great importance 

 that the ratio of equivalents of heat and mechanical power 

 should be accurately determined, and for this purpose James 

 P. Joule, of Manchester, has made a series of most delicate and 

 beautiful experiments on the heat evolved by the revolution 

 of paddle-wheels in baths of water, mercury, or oil. Motion 

 was given to the paddle-wheels by a known weight descend- 

 ing from a given height; the amount of heat was found to 

 be precisely the same with a given expenditure of mechan- 

 ical power, whether the wheel revolved in water, mercury, 

 or oil, proper allowance being made for the different densi- 

 ties and the different capacities of these bodies for heat. In 

 this way, he found that the fall of a weight of one pound 

 through 772 feet, or what would be the equivalent, the fall 

 of a weight of 772 pounds through one foot, is just suf- 

 ficient to raise the temperature of one pound of water one 

 degree of Fahrenheit's scale. Seven hundred and seventy- 

 two pounds falling through one foot is therefore considered 

 as the unit of the working power of heat; and in honor of 

 the investigator who has thus enriched modern science with 

 one of its most valuable means of calculation, applicable to 

 every part of physical research, it is denominated "Joule's 

 unit." By it we are enabled to express in terms of the des- 

 cent of a weight the equivalency of all the forces of Nature, 

 and thus to reduce the mechanical conception of their rela- 

 tions to its greatest simplicity, and to apply mathematical 

 reasoning to a variety of problems heretofore excluded from 

 the province of this great logical instrument, so essential in 



