Dr. Hare on the Cause of Heat. 145 
with the views of this ingenious theorist. If we compare 
the same power in solids, the result will be equally objec- 
tionable. ‘Thus the heating power of glass being 443, that 
of an equal bulk of lead will be 487, though so many times 
heavier ; and if equal weights be compared, the effect of 
the glass, will be four times greater than that of the lead. If 
it be said, that the movements of the denser matter are made 
in less space, and therefore require less motion, I answer, 
that if they be made with equal velocity, they must go 
through equal space in the same time, their alternations be- 
ing more frequent. And if they be not made with the same 
velocity, they could not communicate to matter of a lighter 
kind, a heat equally great; since, agreeably to experience, 
no superiority of weight will enable a body, acting directly 
onanother, to produce in it a motion quicker than its own. 
Consistently with this doctrine, the particles of an aeriform 
fluid, when they oppose a mechanical resistance, do it by 
aid of a certain movement, which causes them effectively 
to occupy a greater space than when atrest. It is true, a 
body, by moving backwards and forwards, may keep off 
other bodies from the space in which it moves. ‘Thus leta 
weight be partially counterbalanced by means of a scale 
beam, so thatif left to itself it would descend gently. Place 
exactly under it another equally solid mass, on which the 
weight would fall unobstructed. If between the two bodies 
thus situated, a third be caused to undergo an alternate mo-= 
tion, it may keep the upper weight from descending, pro- 
vided the force with which the latter descends, be no great- 
er than that of the movement in the interposed mass, and 
the latter acts with such celerity, that between each stroke 
the time be too small for the weight to move any sensible 
distance. Here then we have a case analagous to that sup- 
posed, in which the alternate movements or vibrations of 
matter enable it to preserve to itself a greater space in oppo- 
sition to a force impressed; and it must be evident that 
lengthening or shortening the extent of the vibrations of the 
interposed body, provided they are made in the same time, 
will increase or diminish the space apparently occupied by 
it, as the volume of substances is affected by an increase or 
reduction of heat. It ought however to be recollected that 
in the case we have imagined, there is a constant expendi- 
ture of momentum to compensate for that generated in the 
Vou. IV.....No. 1. 19 
