282 Mr. J. J. Waterston on the Deviation from 
or absorption of heat in consequence of the molecules of gases 
changing their mutual distance, the following is the demonstra- 
Fig. 3. 
tion of the above process for com- 
puting the deviation by means of 
the thermal effectof Messrs. Thom- 
son and Joule’s experiments. 
§ 6. Let the two upright cylin- 
ders GH and EF, fig. 3, of un- 
equal diameters, be connected at 
PP by a plug of compressed cot- 
ton, as in Messrs. Thomson and 
Joule’s experiments. Let the 
weight of the piston H W be first 
considered as equal to FV, and 
above them a perfect vacuum, so 
that the elastic force of the air in 
each cylinder may be maintained 
by the superincumbent weight of 
the pistons only. We have also 
to assume that the motion of each 
piston has attained uniformity, 7. e. 
FV descends, and H W ascends 
with uniform velocity. The con- 
version of heat into motion, and of 
motion into heat, that took place 
through thewhole space 8 W, fig. 2, 
now takes place insensibly within 
the pores of P. The space in which the alternate conversion and 
reconversion take place, instead of being, as in S W, one and of 
very sensible magnitude, now consists of an infinite number of in- 
definitely minute receptacles, in each of which the same kind of 
reciprocal action may be supposed to be going forward. If the 
law of Mariotte were perfectly maintained, the obvious result 
would be a rise of W equal to the descent of the same weight V, 
and the air changed from the higher density to the lower without 
either the performance of work or change of temperature. 
If, in condensing air at a constant temperature, its elastic 
force increases in somewhat a less proportion than its density 
(which we can infer to be the case from M. Regnault’s experi- 
ments, although not the absolute extent of the divergence between 
any two given densities), the weight of the piston V must be 
taken as somewhat less than that of W, to maintain the densities 
in the inverse ratio of the square diameter of the cylinders. In 
consequence of this diminution (say of ;35dth) of the weight of 
V, the work performed by it in descending (F E) a unit of height 
is less than the work required to raise W through (GH) a unit 
