72 
MR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
From the above Table, it appears that the effect of each degree of difference between 
the temperature of the laboratory and that of the apparatus was 0°’ 13742. Hence 
2°'4l395+0°'0657+0°‘007654=2°'487304, will be the proximate value of the increase 
of temperature in the experiments. The further correction on account of the mean 
temperature of the apparatus in the friction experiments having been in reality 
0°'028484 higher than is indicated by the table, will be C°'003914, which, added to 
the proximate result, gives 2°’491218 as the true thermometrical effect of the friction 
of the mercury. 
In order to obtain the absolute quantity of heat evolved, it was requisite to ascer- 
tain the capacity for heat of the apparatus. I therefore caused it to be suspended by 
iron wire from a lever so contrived that the apparatus could be moved with rapidity 
and ease to any required position. The temperature of the apparatus having then 
been raised about 20°, it was placed in a warm air-bath, in order to keep its tempe- 
rature uniform for a quarter of an hour, during which time the thermometer C, im- 
mersed in the mercury, was from time to time observed. The apparatus was then 
rapidly immersed into a thin copper vessel containing 141826 grs. of distilled water, 
the temperature of which was repeatedly observed by thermometer A. During the 
experiment the water was repeatedly agitated by a copper stirrer; and every precau- 
tion was taken to keep the surrounding atmosphere in a uniform state, and also to 
prevent the disturbing effects of radiation from the person of the experimenter. In 
this way I obtained the following results : — 
Time of 
Temperature 
Temperature 
observation. 
of water. 
of apparatus. 
r ^ 
4^705 
70-518 
Apparatus in air-bath . . 
5 
47705 
70-492 
Lio 
47713 
70-5 1 8 
Instant of immersion . . . 
• 
11 
'I3i 
49-836 
57-673 
16 
50-493 
52-641 
Apparatus immersed in water 
21 
50-694 
50-941 
< 
26 
50-690 
50-778 
31 
50-667 
50-744 
.36 
50-636 
50-709 
By applying the correction to the temperature of the water due to its observed in- 
crease during the first ten minutes of the experiment, and the still smaller correction 
due to the rise of the water in the can covering 60 square inches of copper at the tem- 
perature of the atmosphere, 47°'714 was found to be the temperature of the water at 
the instant of immersion. To remove the apparatus from the warm air-bath, and 
to immerse it into the water, occupied only 10", during which it must (according 
to preliminary experiments) have cooled 0°‘027. The heating effect of the air-bath 
