64 
MR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
770 foot-pounds. Subsequently, in 1845* and 1847'f', I employed a paddle-wheel to 
produce the fluid friction, and obtained the equivalents 78 To, 782' 1 and 787‘6, re- 
spectively, from the agitation of water, sperm-oil and mercury. Results so closely 
coinciding with one another, and with those previously derived from experiments 
with elastic fluids and the electro-magnetic machine, left no doubt on my mind as to 
the existence of an equivalent relation between force and heat ; but still it appeared 
of the highest importance to obtain that relation with still greater accuracy. This I 
have attempted in the present paper. 
Description of Apparatus . — The thermometers employed had their tubes calibrated 
and graduated according to the method first indicated by M. Regnault. Two of them, 
which I shall designate by A and B, were constructed by Mr. Dancer of Manchester ; 
the third, designated by C, was made by M. Fastre of Paris. The graduation of 
these instruments was so correct, that when compared together their indications co- 
incided to about of a degree Fahr. I also possessed another exact instrument 
made by Mr. Dancer, the scale of which embraced both the freezing and boiling- 
points. The latter point in this standard thermometer was obtained, in the usual 
manner, by immersing the bulb and stem in the steam arising from a considerable 
quantity of pure water in rapid ebullition. During the trial the barometer stood at 
29‘94 inches, and the temperature of the air was 50° ; so that the observed point re- 
quired very little correction to reduce it to 0 760 metre and 0° C., the pressure used 
in France, and I believe the Continent generally, for determining the boiling-point, 
and which has been employed by me on account of the number of accurate thermo- 
metrical researches which have been constructed on that basis;}:. The values of the 
scales of thermometers A and B were ascertained by plunging them along with the 
standard in large volumes of water kept constantly at various temperatures. The 
value of the scale of thermometer C was determined by comparison with A. It was 
thus found that the number of divisions corresponding to 1° Fahr. in the thermo- 
meters A, B and C, were 12'951, 9*829 and 11*647, respectively. And since constant 
practice had enabled me to read off with the naked eye to ^th of a division, it fol- 
lowed that 2 -^th of a degree Fahr. was an appreciable temperature. 
Plate VII. fig. 1 represents a vertical, and fig. 2 a horizontal plan of the apparatus em- 
ployed for producing the friction of water, consisting of a brass paddle-wheel furnished 
with eight sets of revolving arms,«,a,&c., working between four sets of stationary vanes, 
* Phil. Mag., vol. xxvii. p. 205. f I14d. vol. xxxi. p. 173, and Comptes Rendus, tome xxv. p. 309. 
X A barometrical pressure of 30 inches of mercury at 60° is very generally employed in this country, and 
fortunately agrees almost exactly with the continental standard. In the “ Report of the Committee appointed 
by the Royal Society to consider the best method of adjusting the Fixed Points of Thermometers,” Philosophical 
Transactions, Abridged, xiv. p. 258, the barometrical pressure 29'8 is recommended, but the temperature is 
not named, — a remarkable omission in a work so exact in other respects. 
