or STEAM-ENGESTES WITH DET SATIJEATED STEAM. 
183 
In using equation 6, the unit of volume and the unit of pressure to be employed 
depend on each other. The following are examples 
Unit of Pressure. 
Pound on the square foot. 
Pound on the square inch. 
Unit of Volume. 
Cubic foot. 
Prism one foot long by one inch square, 
or cubic foot. 
In either of the above cases, quantities of energy and of heat are expressed in foot- 
pounds. 
The latent heat of so much steam as occupies a unit of volume more in the gaseous 
state than it did in the liquid state is obviously 

(hyp. log 10=2-3026 nearly). 
In every case of the working of steam which occurs in practice, the volume of the 
liquid water is so small a fraction of the volume of the steam, that it may be neglected 
without sensible error. AVhen this is done, the indicator-diagram of a steam-engine 
working perfectly, and without transmission of heat to or from the steam in the cylindei, 
may be represented in the following manner. 
In fig. 2, let OB=j?, represent the “pressure of admission” at which the steam is 
admitted into the cylinder; t^ the corresponding boiling-point: — 
BC='yi the volume of one pound of steam when admitted : — 
0 A=P 2 the final pressure of the steam in the cylinder at the end of the expansion ; 
#2 the corresponding boiling-point : — 
0F=_P3 the “ pressure of exhaustion” at which the steam is expelled from the cylin- 
der ; C the corresponding boiling-point. 
Let AID and FJE be parallel to OV and BC. Draw the adiabatic curves CIM, BL, 
PR. Then Cl will be the curve of expansion of the steam, and AI=52 will represent 
the volume occupied by one pound at the end of the expansion. The work of one 
pound of steam on the piston will be represented by the area FABCIJF, consisting ot 
the parts 
ABCIA=r sdp-, and AIJF= 52(^2— Pa) (10.) 
The symbol s is used to denote the volume occupied by one pound of the mixture of 
steam and liquid water which the cylinder contains at any given time during the expan- 
sion, $2 being the final value of that volume. 
Let CDG be a curve whose ordinates parallel to OV represent the volumes of one 
pound of dry saturated steam at the pressures represented by its ordinates parallel to 
OP. Then AD='?;2 is the volume which one pound of steam would occupy at the end 
