62 
MR. JOULE ON THE MECHANICAL EQUIVALENT OF HEAT. 
little attention has hitherto been paid, is that in which he makes an estimate of the 
quantity of mechanical force required to produce a certain amount of heat. Refer- 
ring to his third experiment, he remarks that the total quantity of ice-cold water 
which, with the heat actually generated by friction, and accumulated in 2'* 30™, might 
have been heated 180°, or made to boil,=26'58 lbs.” * In the next page he states that 
“ the machinery used in the experiment could easily be carried round by the force of 
one horse (though, to render the work lighter, two horses were actually employed in 
doing it).” Now the power of a horse is estimated by Watt at 33,000 foot-pounds 
per minute, and therefore if continued for two hours and a half will amount to 4,950,000 
foot-pounds, which, according to Count Rumford’s experiment, will be equivalent to 
26'58 lbs. of water raised 180°. Hence the heat required to raise a lb. of water 1° will 
be equivalent to the force represented by 1034 foot-pounds. This result is not very 
widely different from that which I have deduced from my own experiments related 
in this paper, viz. 7/2 foot-pounds; and it must be observed that the excess of Count 
Rumford’s equivalent is just such as might have been anticipated from the circum- 
stance, which he himself mentions, that “ no estimate was made of the heat accu- 
mulated in the wooden box, nor of that dispersed during the experiment.” 
About the end of the last century Sir Humphry Davy communicated a paper to 
Dr. Beddoes’ West Country Contributions, entitled, “ Researches on Heat, Light 
and Respiration,” in which he gave ample confirmation to the views of Count Rum- 
ford. By rubbing two pieces of ice against one another in the vacuum of an air- 
pump, part of them was melted, although the temperature of the receiver was kept 
below the freezing-point. This experiment was the more decisively in favour of the 
doctrine of the immateriality of heat, inasmuch as the capacity of ice for heat is much 
less than that of water. It was therefore with good reason that Davy drew the in- 
ference that ‘‘ the immediate cause of the phenomena of heat is motion, and the laws 
of its communication are precisely the same as the laws of the communication of 
motion-^.” 
The researches of Dulong on the specific heat of elastie fluids were rewarded by 
the discovery of the remarkable fact that equal volumes of all the elastic fluids, 
taken at the same temperature, and under the same pressure, being compressed or 
dilated suddenly to the same fraction of their volume, disengage or absorb the same 
absolute quantity of heat\r This law is of the utmost importance in the development 
of the theory of heat, inasmuch as it proves that the calorific effect is, under certain 
conditions, proportional to the force expended. 
In 1834 Dr. Far.vday demonstrated the Identity of the Chemical and Electrical 
Forces.” This law, along with others subsequently discovered by that great man, 
showing the relations which subsist between magnetism, electricity and light, have 
* “ An Inquiry concerning the Source of the Heat whieh is exeited by Friction.” Phil. Trans. Abridged, 
vol. xviii. p. 283. 
t Elements of Chemical Philosophy, p. 94. X Memoires de I’Academie des Sciences, t. x. p. 188. 
