
MECHANICAL ACTION OF HEAT. 157 
by mechanical power employed in different ways, viz., by electric currents excited 
by the rotation of a magnet, by the forcing of water through narrow tubes, by the 
agitation of water and oil with a paddle, by the compression of air, and by the fric- 
tion of air rushing through a narrow orifice. The value of the depth of fall equiva- 
lent to a rise of one degree of Fanrenuett’s scale in the temperature of a mass of 
water, as determined by that gentleman, varies, in the different series of experi- 
ments, between the limits of 760 feet and 890 feet, the value in which Mr JouLe 
appears to place the greatest confidence being about 780 feet. 
Although the smallness of the differences of temperature measured in those 
experiments renders the numerical results somewhat uncertain, it appears to me 
that, as evidence of the convertibility of heat and mechanical power, they are 
unexceptionable. Nevertheless, there is reason to believe that the true mecha- 
nical equivalent of heat is considerably less than any of the values deduced from 
Mr Joue’s experiments; for in all of them there are causes of loss of power, 
the effect of which it is impossible to calculate. In all machinery, a portion of 
the power which disappears is carried off by waves of condensation and expansion, 
along the supports of the machine, and through the surrounding air: this portion 
cannot be estimated, and is, of course, not operative in producing heat within the 
machine. It is also impossible to calculate, where friction is employed to produce 
heat, what amount of it has been lost in the production of electricity, a power 
which is, no doubt, convertible into heat, but which, in such experiments, pro- 
bably escapes without undergoing that conversion. To make the determination 
of the mechanical equivalent of heat by electro-magnetic experiments correct, it 
is necessary that the whole of the mechanical power should be converted into 
magnetic power, the whole of the magnetic power into what are called electric 
currents, and the whole of the power of the electric currents into heat, not one of 
which conditions is likely to be exactly fulfilled. Even in producing heat by the 
compression of air, it must not be assumed that the whole of the mechanical power 
is expended in raising the temperature. 
(3.) The best means of determining the mechanical equivalent of heat are 
furnished by those experiments in which no machinery is employed. Of this 
kind are experiments on the velocity of sound in air and other gases, which, 
according to the received and well-known theory of Lapuacg, is accelerated by 
the heat developed by the compression of the medium. 
The accuracy of this theory has lately been called in question. There can 
be no doubt that it deviates from absolute exactness, in so far that the magnitude 
of the displacements of the particles of air is neglected in comparison with the 
length of a wave. It appears to me, however, that the Astronomer-Royal, in his 
remarks on the subject in the London and Edinburgh Philosophical Magazine for 
July 1849, has shewn, in a satisfactory manner, that although the effect of the 
appreciable magnitude of those displacements, as compared with the length of a 
