286 Mr. J. J. Waterston on the Deviation from 
ting the whole work of expansion,” &c. The value given to w is 
found at p. 341, viz. w=PV log st the hyperbolic area of expan- 
sion at constant temperature. Prof. Thomson assumes that the work 
represented by this area must inevitably be done when air passes from 
one state of density to another at constant temperature. If not by 
“* pushing out a piston against external resisting force,” then by ge- 
nerating “ heat of friction,” ‘‘ stirring its own mass.” P. 338. 
(3) The dynamical theory of heat rests on the idea that heat is 
the motion of the elementary parts of bodies,— intestine vis viva, 
accompanied with apparent quiescence, because the motion exists in 
every direction alike, e. g. through every part of the air contained in 
S (fig. 2). By the removal of the partition k, the intestine action 
in the direction SW has now scope to make its appearance ; and when 
the particles of air have filled the whole of S W, the apparent motion 
in one direction relapses into intestine motion in all directions alike. 
This theory of heat involves no necessity for introducing the idea of 
friction. So far from fixing our ideas or facilitating our reasoning, 
its manifest tendency is to obscure and confuse. As an illustration, 
we may take the case of arigid vessel impervious to heat, filled with 
air and moving at a high velocity in a straight line. Suppose it to 
be stopped instantaneously, and without fracture or any change in 
the vessel. At this instant the motion of translation of the air would 
necessarily be converted into heat, and there would be a rise of its 
temperature. Now it is impossible in such a case to introduce the 
idea of friction, but it is easy to conceive the apparent motion of the 
particles in the one direction changed into motion in all directions 
alike by simple impact with the sides of the vessel and with each 
other. In the arrangements for making the experiments with plugs, 
all work by the expansion of the air in passing through the plug 
seems to have been carefully prevented. Ifthe air acted disruptively 
on the fibres of the cotton, then work would be performed. Also 
the extra velocity of the current of air in passing out of the plug 
would cause a slight effect equivalent at a maximum to about ;4,dth 
of a degree. Even this might be prevented by having the plug 
shaped like the frustum of a cone, and thus equalize the velocity of 
the current on both sides. It may also be remarked, that as the 
cooling effect is a quantity of the differential kind, the differential 
thermometer seems to be the proper instrument to measure it by. 
(4) There appears to have been considerabie difficulty in equali- 
zing the pressure of the air entering the plug; might not this be 
attained with greater facility by interposing between the condensing 
pump and plug a cylinder with a heavy piston, with its weight ad- 
justed to the pressure required? ‘The action would be similar in 
principle to the bellows of an organ. ‘The induction and eduction 
pipes might be at the bottom of the cylinder, and the motion of the 
piston would probably be confined to so narrow limits as to enable 
it to act sufficiently as a regulator. 
(5) In the second paper on Thermal Effects, at p. 336 a small 
Table of results appears, the last column of which is headed ‘‘ Theo- 
