ON THE MECHANICAL EQUIVALENT OE HEAT. 
355 
tion, radiation, &c. These losses were made as nearly as possible equal by keeping 
the temperatures of the brake and its supports and surroundings at the same levels 
in the two trials. 
Then the quantity of work (U — u) should be exactly equivalent to the quantity 
of heat (H' - li), and by dividing the first of these by the second, a value of the 
constant required is obtained. 
The power available for the purposes of the investigation enabled me to deal with 
quantities approaching the following values in trials of one hour’s duration 
Revolutions, 18,000. 
Total work done, I-35,000,000 ft.-lbs. 
Total weight of water raised 180° Fahr, := 960 lbs. 
Total apparent heat generated = 170,000 B.T.U. 
In quantities so large as these some of the small errors inevitable to all physical 
experiments became quite or nearly negligible. 
Preliminary Apparatus and Trials. 
3. It will, perhaps, be sufficient to indicate the general arrangement of the 
apparatus as first set up. This is illustrated in the annexed sketch. The water was 
Eig. 1. 
supplied from the mains through the iron stand-pipe, A, and the regulating cock, B. 
Before it entered the brake its temperature was measured by means of the 
2 z 2 
