MR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
69 
From the various experiments in the above Table in which the effect of radiation 
was observed, it may be readily gathered that the effect of the temperature of the sur- 
rounding air upon the apparatus was, for each degree of difference between the mean 
temperature of the air and that of the apparatus, 0°*04654. Therefore, since the ex- 
cess of the temperature of the atmosphere over that of the apparatus was 0°‘32295 in 
the mean of the radiation experiments, but only 0°’305075 in the mean of the friction 
experiments, it follows that 0°'000832 must be added to the difference between 0°’57525 
and 0°'012975, and the result, 0°’563]07, will be the proximate heating effect of the 
friction. But to this quantity a small correction must be applied on account of the 
mean of the temperatures of the apparatus at the commencement and termination of 
each friction experiment having been taken for the true mean temperature, which 
was not strictly the case, owing to the somewhat less rapid increase of temperature 
towards the termination of the experiment when the water had become warmer. The 
mean temperature of the apparatus in the friction experiments ought therefore to be 
estimated 0°'002]84 higher, which will diminish the heating effect of the atmo- 
sphere by 0°'000102. This, added to 0°'563107, gives 0°‘563209 as the true mean 
increase of temperature due to the friction of water*. 
In order to ascertain the absolute quantity of heat evolved, it was necessary to 
find the capacity for heat of the copper vessel and brass paddle-wheel. That of the 
former was easily deduced from the specific heat of copper according to M. Regnault. 
Thus, capacity of 25.541 grs.-l- of copper X 0*095 15= capacity of 2430*2 grs. of water. 
A series of seven very careful experiments with the brass paddle-wheel gave me 
1783 grs. of water as its capacity, after making all the requisite corrections for the 
heat occasioned by the contact of the water with the surface of the metal, &c. But 
on account of the magnitude of these corrections, amounting to one-thirtieth of the 
whole capacity, I prefer to avail myself of M. Regnault’s law, viz. that the capa- 
city in metallic alloys is equal to the sum of the capacities of their constituent metals 
Analysis of a part of the wheel proved it to consist of a very pure brass containing 
3933 grs. of zinc, to 14968 grs. of copper. Hence 
Cap. 14968 grs. copper X 0*095 15= cap. 1424*2 grs. water. 
Cap. 3933 grs. zinc X 0*09555= cap. 375*8 grs. water. 
Total cap. brass wheel = cap. 1800 gi*s. water. 
* This increase of temperature was, it is necessary to observe, a mixed quantity, depending partly upon the 
friction of the water, and partly upon the friction of the vertical axis of the apparatus upon its pivot and bear- 
ing, cc, fig. 1. The latter source of heat was however only equal to about -g-*oth of the former. Similarly also, 
in the experiments on the friction of solids hereafter detailed, the cast-iron discs revolving in mercury, rendered 
it impossible to avoid a very small degree of friction among the particles of that fluid. But since it was found 
that the quantity of heat evolved was the same, for the same quantity of force expended, in both cases, i. e. whe- 
ther a minute quantity of heat arising from friction of solids was mixed with the heat arising from the friction 
of a fluid, or whether, on the other hand, a minute quantity of heat arising from the friction of a- fluid was 
mingled with the heat developed by the friction of solids, I thought there could be no impropriety in con- 
sidering the heat as if developed from a simple source, — in the one case entirely from the friction of a fluid, 
and in the other entirely from the friction of a solid body. 
t The washer, weighing only 38 grs., was reckoned as copper in this estimate. 
I Ann. de Ch. 1841, t. i. 
