82 
MR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
total force applied. The corrections for vibration and sound (deduced from the data 
obtained in the last series, on the hypothesis that they were proportional to the fric- 
tion by which they were produced) will be 3‘47 and 16‘9 foot-lbs. These quantities, 
subtracted from the previous result, leave 2057'336 foot-lbs. as the quantity of force 
converted into heat in the apparatus. 
205 7*336 
*^•65504 — /74’88, will therefore be the equivalent as derived from this last series 
of experiments. 
The following- Table contains a summary of the equivalents derived from the expe- 
riments above detailed. In its fourth column I have supplied the results with the 
correction necessary to reduce them to a vacuum. 
Table IX. 
No. of 
series. 
Material 
employed. 
Equivalent 
in air. 
Equivalent 
in vacuo. 
Mean. 
1 
Water 
773*640 
772*692 
772*692 
2 
Mercury 
773*762 
772*814 
> 774*083 
3 
Mercury 
776*303 
773*352 
/ 
4 
Cast iron 
776*997 
776*045 
> 774*987 
5 
Cast iron 
774*880 
773*930 
J 
It is highly probable that the equivalent from cast iron was somewhat increased 
by the abrasion of particles of the metal during friction, which could not occur 
without the absorption of a certain quantity of force in overcoming the attraction of 
cohesion. But since the quantity abraded was not considerable enough to be weighed 
after the experiments were completed, the error from this source cannot be of much 
moment. I consider that 772'692, the equivalent derived from the friction of water, 
is the most correct, both on account of the number of experiments tried, and the 
great capacity of the apparatus for heat. And since, even in the friction of fluids, it 
was impossible entirely to avoid vibration and the production of a slight sound, it is 
probable that the above number is slightly in excess. I will therefore conclude by 
considering it as demonstrated by the experiments contained in this paper, — 
1st. J'hat the quantity of lieat produced by the friction of bodies, whether solid or 
liquid, is always proportional to the quantity of force expended. And, 
2nd. That the quantity of heat capable of increasing the temperature of a pound of 
water {weighed in vacuo, and taken at between 55° and 60°) by 1° Fahr., requires 
for its evolution the expenditure of a mechanical force represented by the fall of lbs. 
through the space of one foot. 
Oak Field, near Manchester, 
June Ath, 1849. 
