

MECHANICAL ACTION OF HEAT. 169 
Rocus and Brrarp made experiments as perfect gases, would lead to sensible 
errors. I have, therefore, confined my calculations for the present to oxygen, 
hydrogen, and atmospheric air. 
(18.) The heat produced by compressing so much of a perfect gas as would 
occupy unity of volume under the pressure unity, at the temperature 0° centigrade, 
. Te i . . . . : 
from its actual volume n MV, =p into a volume which is less in a given ratio s 
(when « is neglected as compared with 7), is expressed by the following motion :— 
1 rsV T s 
ear aniele: 1aV = ney 2 CTO EES) 
being, in fact, equal to the mechanical power used in the compression. When the 
temperature is maintained constant, this becomes 
y oh iy o a 
aMQ =Gloe,-5 - - - - @6) 
s 
which is obviously independent of the nature of the gas. 
Hence equal volumes of all substances in the state of perfect gas, at the same 
pressure and at equal and constant temperatures, being compressed by the same 
amount, disengage equal quantities of heat; a law already deduced from experi- 
ment by DuLone. 
(14.) The determination of the fraction N affords the means of calculating 
the mechanical or absolute value of specific heat, as defined by Equation 1, Sec- 
tion First. The data for atmospheric air being taken as follows :— 
N = 0-4, C = 274°-6 centigrade, 
= = height of an imaginary column of air of uniform density, at the tempera- 
ture 0° cent., whose pressure by weight on a given base is equal to its pressure 
by elasticity, - . . . =7990 métres, 
= 26214 feet :— 
the real specific heat of atmospheric air, or the depth of fall equivalent to one 
centigrade degree of temperature in that gas, is found to be 
K =72:'74 métres=238'66 feet . . . (27.) 
Belg tt 
~ CnxaMN 
The apparent specific heat of atmospheric air, under constant pressure, 
according to De ua Rocue and Berarp, is equal to that of liquid water at 0° 
centigrade x 0:2669. The ratio of its real specific heat to the apparent specific 
heat of water at 0° centigrade, is, therefore, 
10. 
2669 x = ="1906, 
And, consequently, the mechanical value of the apparent specific heat of liquid 
water, at the temperature of melting ice, is 
