[ 61 ] 
III. On the Mechanical Equivalent of Heat. By James Prescott Joule, F.C.S., 
Sec. Lit. and Phil. Society, Manchester, Cor. Mem. B.A., Turin, 8^c. Commu- 
nicated hy Michael Faraday, D.C.L., F.R.S., Foreign Associate of the Academy 
of Sciences, Paris, Sfc. S^c. 8fc. 
Received June 6, — Read June 21, 1849. 
“ Heat is a very brisk agitation of the insensible parts of the object, which produces in us that sensation 
from whence we denominate the object hot; so what in our sensation is heat, in the object is nothing hut 
motion ." — Locke, 
“ The force of a moving body is proportional to the square of its velocity, or to the height to which it would 
rise against gravity.” — Leibnitz. 
In accordance with the pledge I gave the Royal Society some years ago, I have now 
the honour to present it with the results of the experiments I have made in order to 
determine the mechanical equivalent of heat with exactness. I will commence with 
a slight sketch of the progress of the mechanical doctrine, endeavouring to confine 
myself, for the sake of conciseness, to the notice of such researches as are imme- 
diately connected with the subject. I shall not therefore be able to review the valu- 
able labours of Mr. Forbes and other illustrious men, whose researches on radiant 
heat and other subjects do not come exactly within the scope of the present memoir. 
For a long time it had been a favourite hypothesis that heat consists of “ a force or 
power belonging to bodies*,” but it was reserved for Count Rumford to make the first 
experiments decidedly in favour of that view. That justly celebrated natural philoso- 
pher demonstrated by his ingenious experiments that the very great quantity of heat 
excited by the boring of cannon could not be ascribed to a change taking place in the 
calorific capacity of the metal ; and he therefore concluded that the motion of the 
borer was communicated to the particles of metal, thus producing* the phenomena of 
heat: — “ It appears to me,” he remarks, “extremely difficult, if not quite impossible, 
to form any distinct idea of anything, capable of being excited and communicated, 
in the manner the heat was excited and communicated in these experiments, except 
it be motion-f'.” 
One of the most important parts of Count Rumford’s paper, though one to which 
* Crawford on Animal Heat, p. 15. 
j" “ An Inquiry concerning the Source of the Heat which is excited by Friction.” Phil. Trans. Abridged, 
vol. xviii. p. 286. 
