OF STEAM-ENGINES WITH DEY SATUEATED STEAM. 
181 
Let TiTi, T 2 T 2 , and the dotted lines between them, be isothermal lines^ each of which 
by its coordinates represents the relation between the pressure and volume of the body 
for a particular uniform temperature. Then the scale of Absolute Temperatuies is such, 
that a series of isothermal lines, corresponding to a series of equal divisions upon that 
scale, divides the band between any pair of adiabatic curves into equal areas. 
A cycle of changes is represented by a closed figure, such as C ; and the area of 
that figure represents the heat transformed into mechanical energy, or the mechanical 
energy transformed into heat, according as the cycle of changes takes place in the 
dhection represented by the arrow, or in the contrary direction. That area is the quan- 
tity expressed by equation 4. 
The following are the results of applying the general equation of thermo-dynamics to 
fluids which are in the act of changing from the liquid to the gaseous state, or nascent 
vapours. 
In fig. 2, let the fine BC paraUel to OV represent the increase of volume which a 
given fluid mass undergoes in changing from the liquid to the gaseous state under a 
pressure represented by the ordinate OB=j9. Through B and C diaw a pair of inde- 
finitely extended adiabatic curves, BL, CIM ; then the area LBCM represents the latent 
heat of evajgoration of the fluid mass under the pressure^. 
To find the algebraical expression for that latent heat, it is to be considered, that in 
applying to this case the formula ll—td(p, t is the absolute temperature of the hoiling- 
point corresponding to the pressure p, and is constant ; so that 
p, and (pi being the values of the thermo-dynamic functions for the curves CM and BL 
respectively. It is next to be considered, that because t is constant, the teim of equa- 
tion 2 which depends on t alone, is the same in ^bi so that it disappears fiom 
