584 MR W. J. M. RANKINE ON THE 
Professor THomson and Mr Jour have expressed the opinion, which is un- 
doubtedly correct, that those experiments in which the largest quantities of air 
were used were the least liable to error from disturbing causes, such as the con- 
duction of heat. 
Now it may be observed in the preceding table, that the calculated values of 
k are generally greatest, and the discrepancies amongst them least, for the experi- 
ments in which most air was used. To illustrate this, the results of the last eight 
series are arranged below in the order of the quantities of air employed. 
Cubic inches per second, 1-4 2°8 56 56 6-4 8-4 11-2 11:2 
Values ofx, . . . . L683 1:°762 2:09 2:228 1:51 2:087 2:345 214 
It is further to be remarked, that the discrepancy between the highest and 
the lowest of the values of x is 
2°:345 — 1°08 = 1°-265 centigrade: 
a quantity which corresponds to a difference of less than one three-hundredth part 
in computing the proportion of heat converted into mechanical power by any or- 
dinary expansive engine, according to the formula (98), which has been deduced 
from the hypothesis of Molecular Vortices. 
The experiments, therefore, may be considered as tending to prove, that the 
formule deduced from this hypothesis are sufficiently correct for practical pur- 
poses; and also as affording a strong probability that the principles to which it 
leads are theoretically exact, and that the temperature of absolute privation of 
heat is a real fixed point on the scale, somewhat more than two centigrade de- 
grees above the absolute zero of a perfect gas-thermometer (which is, of course, an 
imaginary point); that is to say, about 272} centigrade degrees, or 4904 degrees 
of FanreNuEIT, below the temperature of melting ice. 
If these conclusions be correct, it follows, that when the temperatures T, and 
T,, between which an expansive engine works, are measured from the ordinary 
zero points of the centigrade and of FAHRENHEIT’s scales respectively, the follow- 
ing are the utmost proportions of the total heat expended which it can be made 
to convert into mechanical power :— 
i ype 
A | . a”). 
= 
For FAHRENHEIT’S scale, —~+—~. 
* 1 +4584 
For the centigrade scale, pean | 
In the fifth section of this paper, where a comparison is made between the 
actual duty of theCornish engine at Old Ford, as determined by Mr WicxstTEgp, 
and the greatest possible duty which could be obtained from a given quantity of 
heat by a theoretically perfect engine working between the same temperatures, 
the constant « is treated as being so small that it may be neglected in practice. 
