80 
MR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
the ground with anything like a uniform and moderate velocity. Hence, although 
the mean velocity was 3' 12 inches per second, the force with which the weights 
struck the ground could not be correctly estimated by that velocity as in the case of 
fluid friction. However, it was found that the noise produced by the impact was on 
the average equal to that produced by letting the weights fall from the height of 
one-eighth of an inch. It generally happened also that in endeavouring to regulate 
the motion, the weights would stop suddenly before arriving at the ground. This 
would generally happen once, sometimes twice, during the descent of the weights, 
and I estimate the force thereby lost as equal to that lost by impact with the ground. 
Taking therefore the total loss at one-fourth of an inch in each fall, we have twenty 
times that quantity, or 5 inches, as the entire loss, which, subtracted from 1260'027, 
leave 1255*027 inches as the corrected height through which the weight of 403242 
grs. operated. These numbers are equivalent to 6024*757 foot-lbs., and adding 16*464 
foot-lbs. for the effect of the elasticity of the string, we have 6041*221 foot-lbs. as the 
force employed in the experiments. 
The above force was not however entirely employed in generating heat in the 
apparatus. It will be readily conceived that the friction of a solid body like cast 
iron must have produced a considerable vibration of the framework upon which the 
apparatus was placed, as well as a loud sound. The value of the force absorbed by 
the former was estimated by experiment at 10*266 foot-lbs. The force required to 
vibrate the string of a violoncello, so as to produce a sound which could be heard at 
the same distance as that arising from the friction, was estimated by me, with the 
concurrence of another observer, at 50 foot-lbs. These numbers, subtracted from 
the previous result, leave 5980*955 foot-lbs. as the force actually converted into heat. 
5980*955 
7*69753 =776*997, will therefore be the equivalent derived from the above experi- 
ments on the friction of cast iron. The next series of experiments was made with 
the same apparatus, using lighter weights. 
bth Series of Experiments . — Friction of Cast Iron. Weight of leaden weights, 
68442 grs. and 68884 grs. Average velocity of fall, 1*9 inch per second. Time 
occupied by each experiment, 30 minutes. Thermometer for ascertaining the 
temperature of the mercury, C. Thermometer for registering the temperature of the 
laboratory, A. 
