the Primary Laws of Elastic Fluids. 285 
Appendix. 
(1) § 7 (Phil. Mag. vol. iv. p. 432, § 77, Prof. Thomson on the 
Dynamical Theory of Heat) : the following extracts prove this state- 
ment :— ‘ 
“From equation III. it follows, that if Mayer's hypothesis be true, 
there is neither emission nor absorption of heat, on the whole, required 
to reduce the temperature of the air after passing through the orifice 
to its primitive value....... Hence the simplest conceivable test of 
Mayer’s hypothesis would be, to try whether the temperature of the 
air is exactly the same on the two sides of the orifice. 
P. 433, § 78. ‘* Should the differential method of experimenting 
just described indicate any difference of temperature whatever on the 
two sides of the orifice, Mayer’s hypothesis would be shown to be not 
exactly fulfilled, and, according as the air leaving the orifice is found 
to be warmer or colder than the entering air, we should infer that 
the heat absorbed, when air expands at a constant temperature, is less 
than or greater than the equivalent of the mechanical effect pro- 
duced by the expansion.” 
(2) Note A referred to in § 5. In Phil. Mag. vol. iv. p. 429, § 71, 
Prof. Thomson thus refers to those experiments of Mr. Joule :— 
“In Joule’s actual experiments, the test is simply this :—the total 
external thermal effect is determined when air is allowed to expand, 
through a small orifice, from one vessel to another previously ex- 
hausted by an air-pump. Here the first mechanical effect produced 
by the expanding gas is vis viva generated in the rushing of the air. 
By the time equilibrium is established, all this mechanical effect 
has been lost in fluid friction; and no truth in physical science can 
be more certain, than that by the time thermal as well as mechanical 
equilibrium is established at the primitive temperature, the contents 
of the two vessels must have parted with just as much more heat than 
they would have parted with had the air in expanding pushed out, 
the piston against an external resisting force, as is equivalent to the 
mechanical effect thus produced externally.” How are we to recon- 
cile this with the fact, that the contents of the two vessels were 
found not to have parted with any heat? Perhaps the word more is 
a misprint, and should have been less. Prof. Thomson frequently 
makes use of the word friction. His view appears to be, that the 
motion of the air rushing e. g. from S to W, fig. 2, is lost in “ fluid 
friction ;”” and then the “ heat of friction” restores the temperature 
to nearly what it was before, but not quite, the difference being what 
is shown in the experiments with plugs. Thus in the second paper 
on Thermal Effects (Phil. Trans. 1854, p. 339), he expresses himself 
as follows :—The thermal effect ‘ shows precisely how much the heat 
of friction in the plug falls short of compensating the cold of expan- 
sion. But the heat of friction is the thermal equivalent of all the 
work done actually in the narrow passages by the air expanding as it 
flows through.” ‘Thus the plug does not allow the air to pass from 
a higher to a lower density without performing work. Again, a 
little further on, Prof. Thomson thus writes:—< Jn any case, w deno- 
