
MECHANICAL ACTION OF HEAT. 173 
First, Unity of weight of liquid being raised from the temperature r, to the 
temperature 7,, absorbs the heat, 
Ky (7) —7)) 
and produces the expansive power, 
v) 
dv.P 
% 
Secondly, It is evaporated at the temperature 7,, absorbing the heat 
L 
1 
and producing the expansive power, 
P, (V,-%) 
Thirdly, The vapour expands, at saturation, until it is restored to the origi- 
nal temperature 7. In this process it absorbs the heat, 
Ty 
~f at . K, 
T) 
and produces the expansive power, 
Vg 
pe GVLE 
Wir 
Fourthly, Vt is liquefied at the original temperature, giving out the heat 
L 
and consuming the compressive power, 
P, (Vo—%)- 
The equation between the heat which has disappeared, and the expansive 
power which has been produced, is as follows :— 
0 
- 
L,-L,+K, (4,—7))—["'dt - K 
1a 
: be (31.) 
VU. 
=P, (V,—»,)—P,(V.-%)+ f/ ‘dv. P+f/ °av-P 
% is v, 
If the vapour be such that it can be regarded as a perfect gas without sen- 
T 
sible error, the substitution of k+ pay for K,, and of =k Nv for PV, trans- 

CxraM 
forms the above to 
L,—L, + {K,—& (1+ N)} (7,—7) | 
° P BUT D Gogg 
aOR tbat fide. Pa fp i aP.v | 
In almost all cases which occur in practice, v is so small as compared with V, 
that —faP .» may be considered as sensibly = 0; and therefore (sensibly) 
L,+K,(7,—7)=L, + #(+N)(1—-%)- - - (83) 
