144 Dr. Hare on the Cause of Heat. 
These suggestions of Sir H. Davy’s are to me unsatis- 
factory. 
It is fully established in mechanics, that when a body 
in motion is blended with and thus made to communicate 
motion to another body, previously at rest, or moving slower, 
the velocity of the compound mass after the impact will be 
found, by multiplying the weight of each body, by its res- 
pective velocity, and dividing the sum of the products, by 
the aggregate weight of both bodies. Of course it will be 
more than a mean or less than a mean, accordingly as the 
quicker body was lighter or heavier than the other. Now 
according to Sir Humphrey Davy, the particles of sub- 
stances which are unequally heated are moving with une- 
qual degrees of velocity: of course when they are reduced 
by contact to a common temperature, the heat, or what is 
the same (in his view), the velocity of the movements of 
their particles, ought to be found by multiplying the heat 
of each by its weight and dividing the sum of the product 
by the aggregate weight. Hence if equal weights of matter 
be mixed, the temperature ought to be a mean ; and if equal 
bulks, it ought to be as much nearer the previous tempera- 
ture of the heavier substance as the weight of the latter is 
greater ; but the opposite is in most instances true. When 
equiponderant quantities of mercury and water are mixed at 
different temperatures, the result is such as might be ex- 
pected from the mixture of the water, were it three times 
heavier ; so much nearer to the previous heat of the water, 
is the consequent temperature. It may be said that this 
motion is not measurable upon mechanical principles. How 
then, I ask does it produce mechanical effects? These 
must be produced by the force of the vibrations, which are 
by the hypothesis mechanical : for whatever laws hold good 
in relation to moving matter in mass, must operate in re- 
gard to each particle of that matter; the effect of the form- 
er, can only be a multiple of that of the latter. Indeed, 
one of Sir Humphrey Davy’s reasons for thinking heat to 
consist of corpuscular motions is that mechanical attrition 
generates it. Surely then a motion, produced by mechan- 
ical means, aud which produces mechanical effects, may 
be estimated on mechanical principles. 
In the case cited above, the power of reciprocal commu- 
nication of heat in two fluids, is shown to be inconsistent 
