70 
MR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
The capacity of a brass stopper which was placed in the neck h, fig-. 3, for the pur- 
pose of preventing' the contact of air with the water as much as possible, was equal 
to that of 10'3 grs. of water: the capacity of the thermometer had not to be esti- 
mated, because it was always brought to the expected temperature before immersion. 
The entire capacity of the apparatus was therefore as follows : — 
Water 932297 
Copper as water. . . 2430'2 
Brass as water . . . 1810'3 
Total . . 97470-2 
So that the total quantity of heat evolved was 0°-563209 in 97470-2 grs. of water, or, 
in other words, 1° Fahr. in 7’842299 lbs. of water. 
The estimate of the force applied in generating this heat may be made as follows : 
— The weights amounted to 406152 grs., from which must be subtracted the friction 
arising from the pulleys and the rigidity of the string; which was found by con- 
necting the two pulleys with twine passing round a roller of equal diameter to that 
employed in the experiments. Under these circumstances, the weight required to be 
added to one of the leaden weights in order to maintain them in equable motion was 
found to be 2955 grs. The same result, in the opposite direction, was obtained by 
adding 3055 grs. to the other leaden weight. Deducting 168 grs., the friction of the 
roller on its pivots, from 3005, the mean of the above numbers, we have 2837 grs. 
as the amount of friction in the experiments, which, subtracted from the leaden 
weights, leaves 403315 grs. as the actual pressure applied. 
The velocity with which the leaden weights came to the ground, viz. 2-42 inches 
per second, is equivalent to an altitude of 0-0076 inch. This, multiplied by 20, the 
number of times the weights were wound up in each experiment, produces 0-152 inch, 
which, subtracted from T260-248, leaves 1260-096 as the corrected mean height from 
which the weights fell. 
This fall, accompanied by the above-mentioned pressure, represents a force equiva- 
lent to 6050-186 lbs. through one foot ; and 0-8464 X 20= 16-928 foot-lhs. added to 
it, for the force developed by the elasticity of the string after the weights had touched 
the ground, gives 6067-114 foot-pounds as the mean corrected force. 
Hence 773-64 foot-pounds, will be the force which, according to the 
above experiments on the friction of water, is equivalent to 1 °Fahr. in a lb. of water. 
2nd Series of Experiments . — Friction of Mercury. Weight of the leaden weights 
and string, 203026 grs. and 203073 grs. Velocity of the weights in descending, 2-43 
inches 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 air, B. Weight of cast iron apparatus, 68446 grs. Weight 
of mercury contained by it, 428292 grs. 
