170 MR W. J. M. RANKINE ON THE 
& (at. air) _ 
[1906 aan 

381°64 métres=1252 feet per centigrade degree, (28.) 
or 695°6 feet per degree of Fahrenheit’s scale, 
This quantity we shall denote by K,. It is the mechanical equivalent of the 
ordinary thermal unit. 
I have already pointed out (in Article 2. of the First Section) the causes 
which tend to make the apparent value of the mechanical equivalent of heat, in 
Mr Jouxr’s experiments, greater than the true value. The differences between 
the result I have just stated, and those at which he has arrived, do not seem 
ereater than those causes are capable of producing, when combined with the un- 
certainty of experiments, like those of Mr JouLx, on extremely small variations 
of temperature. 
(15.) Besides the conditions of constant volume and constant pressure, there 
is a third condition in which it is of importance to know the apparent specific 
heat of an elastic fluid, namely, the condition of vapour at saturation, or in con- 
tact with its liquid. 
The apparent specific heat of a vapour at saturation, is the quantity of heat 
which unity of weight of that vapour receives or gives out, while its temperature 
is increased by one degree, its volume being at the same time compressed so as to 
bring it to the maximum pressure corresponding to the increased temperature. 
It has been usually taken for granted, that this quantity is the same with the 
variation for one degree of temperature, of what is called the total heat of evapor- 
ation. Such is, indeed, the case according to the theory of Carnor; but I shall 
shew that, according to the mechanical theory of heat, these two quantities are 
not only distinct, but in general of contrary signs. 
I shall, for the present, consider such vapours only as may be treated in prac- 
tice as perfect gases, so as to make the first of the Equations (20.) applicable. 
It has been shewn that the logarithm of the maximum elasticity of a vapour 
in contact with its liquid may be represented by the expression 
ee ee ee 
log Pea- a 
The coefficients a, 8, y, being those adapted for calculating the common loga- 
rithm of the pressure, I shall use the accented letters a’, (6, 7, to denote those 
suited to calculate the hyperbolic logarithm, being equal respectively to the for- 
mer coefficients x 2°3025851.° 
Then for vapour at saturation, 
Making this substitution in the general Equation (21.), we find the following value 
for the apparent specific heat of perfectly gaseous vapour at saturation :— 
eh Ce oe 
