
pr: iy ae 

MECHANICAL ACTION OF HEAT. 175 
The coefficient 305 K,,=382 feet per centigrade degree is the apparent specific 
heat of steam at constant pressure; that is to say, for steam,— 
\ 
= 882 feet per centigrade degree, 

1 
a Oa 
1 
CaM 
Therefore the real specific heat of steam is 5 
1 : es : 
CaN 229 feet per centigrade degree, 
=127-4 feet per deg. of Fahrenheit, 
=K, x 183 
153. 2 
Die) Brey 
but =158 feet. 

k= 

and N= 
The quantity — We a dP . v has been neglected, as already explained, in these 
A 
calculations, on account of its smallness. When 7,=C, or the fixed point is 0° 
centigrade, this integral is nearly equal to 
se PR see yee Sl ee 
oP ay GMT HEN yen + BD) 
which, for steam, is equal to 

— Ky x 122 v. 7. 
For a pressure of eight atmospheres, 
Grif. iat 
V, 252 
consequently, —v P,=—Ky x 0°22 cent. 
nearly, 7,=445°5 (T=170°9 cent.) 
a quantity much less than the limit of errors of observation in experiments on 
latent heat. 
This shews that in practice we are justified in overlooking the influence of 
the volume of the liquid water on the heat of evaporation. 
Section 1V.—OF tHE MucHanican ACTION OF STEAM, TREATED AS A PERFECT GAS, 
AND THE PoWER OF THE STEAM-ENGINE. y 
(21.) In the present limited state of our experimental knowledge of the den- 
sity of steam at pressures differing much from that of the atmosphere, it is desir- 
able to ascertain whether any material error is likely to arise from treating it as 
a perfect gas. For this purpose the ratio of the volume of steam at 100° centi- 
grade, under the pressure of one atmosphere, to that of the water which produces 
it at 4°:1 centigrade, as calculated theoretically on the supposition of steam being 
a perfect gas, is to be compared with the actual ratio. 
VOL. XX. PART I. 3A 
