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XIV. On the Thermal Effects of Fluids in Motion. By William Thomson, M.A., 
F.R.S., F.R.S.E., 8^c., Professor of Natural Philosophy in the University of 
Glasgow, For. Mem. of the Royal Swedish Academy of Sciences ; and J. P. Joule, 
F.R.S., F.C.S., Corr. Mem. R.A. Turin, Vice-President of the Literary and 
Philosophical Society of Manchester, ^c. 
Received June 15, — Read June 16, 1853. 
In a paper communicated to the Royal Society, June 20, 1844, “ On the Changes of 
Temperature produced by the Rarefaction and Condensation of Air*,” Mr. Joule 
pointed out the dynamical cause of the principal phenomena, and described the 
experiments upon which his conclusions were founded. Subsequently Professor 
Thomson pointed out that the accordance discovered in that investigation between 
the work spent and the mechanical equivalent of the heat evolved in the compression 
of air may be only approximate, and in a paper communicated to the Royal Society 
of Edinburgh in April 1851, “On a Method of discovering experimentally the 
relation between the Mechanical Work spent, and the Heat produced by the com- 
pression of a Gaseous Fluid-f',” proposed the method of experimenting adopted in 
the present investigation, by means of which we have already arrived at partial 
results:}:. This method consists in forcing the compressed elastic fluid through a 
mass of porous non-conducting material, and observing the consequent change of 
temperature in the elastic fluid. The porous plug was adopted instead of a single 
orifice, in order that the work done by the expanding fluid may be immediately spent 
in friction, without any appreciable portion of it being even temporarily employed to 
generate ordinary vis viva, or being devoted to produce sound. The non-conducting 
material was chosen to diminish as much as possible all loss of thermal effect by 
conduction, either from the air on one side to the air on the other side of the plug, 
or between the plug and the surrounding matter. 
A principal object of the researches is to determine the value of p, Carnot’s 
function. If the gas fulfilled perfectly the laws of compression and expansion 
ordinarily assumed, we should have§ 
l_ 
I KS 
J Epo^ologP’ 
where J is the mechanical equivalent of the thermal unit ; poU^ the product of the 
* Philosophical Magazine, S. 3, vol. xxvi. p. 369. 
t Transactions of the Royal Society, Edinburgh, vol. xx. Part II. 
X Philosophical Magazine, S. 4, vol. iv. p. 481. 
§ Dynamical Theory of Heat, equation(7), § 80, Transactions of the Royal Society of Edinburgh, vol. xx p.297. 
