66 W.J. M. Rankine on the Mechanical Action of Heat. 
puting the efficiency of engines. This reduces the formula C to 
identity with A. 
The section to which I have referred was published in the 20th 
volume of the Transactions of the Royal Society of Edinburgh, 
and re-printed in the London, Edinburgh and Dublin Philosoph- 
ical Magazine for March, 1854. 
Professor Thomson afterwards pointed out, (in consequence of | 
a suggestion by Mr. Joule) that if Carndét’s Function be supposed 
to have the following value 
* 
f= = . = = - = (D.) 
the formula B is transformed into A. 
It is also obvious, that if Carnét’s Function have the value 
J 
= Paes > - - - (E.) 
B becomes identical with C. = 
Although the law expressed by the equation C, being partly 
founded on hypothetical principles, was at first to a certain extent 
conjectural, yet it has subsequently being so closely confirmed by 
the experiments of Messrs. Regnault, Joule, and Thomson, that 
it may be regarded as almost, if not altogether, demonstrated. 
It is still, however, uncertain, whether the constant x has an ap- 
preciable value. The values computed from the experiments 
absolute zero of a perfect gas thermometer, and «=0. ; 
It gives me much gratification to find, that the conclusion to 
which Mr. Joule, Professor Thomson, and myself have been le 
by our researches, as to the great economy of fuel to be expected 
from the Air-Engine when its practical difficulties have been over” 
come, is confirmed by the opinion of an investigator who has 8? 
carefully examined the subject as Professor Barnard. 
59 St. Vincent street, Glasgow, 14th April, 1854. 
# 
ed 
‘ 
