206 Relation between Heat and Mechanical Power. iCI kO 



weights, pulleys, &c., exactly in the manner described inji^ 

 previous paper*. ■' 



The paddle moved with great resistance in the can of water, 

 so thai the weights (each of four pounds) descended at the 

 slow rate of about one foot per second. The height of the 

 pulleys from the ground was twelve yards, and consequently, 

 when the weights had descended through that distance, they 

 had to be wound up again in order to renew the motion of the 

 paddle. After this operation had been repeated sixteen times, 

 the increase of the temperature of the water was ascertained 

 by means of a very sensible and accurate thermometer. 



A series of nine experiments was performed in the above 

 manner, and nine experiments were made in order to elimi- 

 nate the cooling or heating effects of the atmosphere. After 

 reducing the result to the capacity for heat of a pound (rf 

 water, it appeared that for each degree of heat evolved by the 

 friction of water, a mechanical power equal to that which can 

 raise a weight of 890 lbs. to the height of one foot, had been 

 expended. 



The equivalents I. have already obtained are,--<lst, 823 lbs., 

 derived from magneto-electrical experimentsf; 2nd, 795 lbs., 

 deduced from the cold produced by the rarefaction of air J; and 

 Srd, 774 lbs. from experiments (hitherto unpublished) on the 

 motion of water through narrow tubes. This last class of expe-* 

 riments being similar to that with the paddle-wheel, we may 

 take the mean of 774? and 890, or 832 lbs., as the equivalent 

 derived from the friction of water. In such delicate experi- 

 ments, where one hardly ever collects more than half a degree 

 of heat, greater accordance of the results with one another 

 than that above exhibited could hardly have been expected. 

 I may therefore conclude that the existence of an equivalent 

 relation between heat and the ordinary forms of mechanical 

 power is proved ; and assume 817 lbs., the mean of the results 

 of three distinct classes of experiments, as the equivalent, until 

 still more accurate experiments shall have been made. 



Any of your readers who are so fortunate as to reside amid 

 the rom.antic scenery of Wales or Scotland, could, I doubt 

 not, confirm my experiments by trying the temperature of the 

 water at the top and at the bottom of a cascade. If my views 

 be correct, a fall of 817 feet will of course generate one d^ 



* Phil. Mag. vol. xxiii. p. 436, The paddle- wheel used by Reiinieifl 

 his experiments on the friction of water (Phil. Trans. 1831, plate xi. fig, 1) 

 was somewhat similar to mine. I employed, however, a greater number of 

 "floats," and also a corresponding number of stationary floats, in order to 

 prevent the rotatory motion of the water in the can. 



t Phil. Mag. vol. xxiii. {)p. 263, 347. J Phil. Mag. May 1845, p. 369. 



