86 
THE NATURE AND 
[sect. II. 
increased in volume by the formation of vapour, and increase of temperature, as 
shown in the second column of the table ; while the third is the force of vapour at 
these temperatures by our rule, page 59 : the fourth is computed by the rule in the 
preceding article. 1 
Temperature. 
Volume of air 
and vapour by 
experiment. 
Force of vapour, 
by Rule 
page 59. 
Volume of air and 
vapour by formula, 
art. 124. 
0° 
1000-00 
0-032 
1000 
32 
1071-29 
0-172 
1076 
52 
112305 
0-401 
1132 
72 
1182-50 
0-842 
1190 
92 
1255-14 
1-629 
1260 
112 
1353-75 
295 
1360 
132 
1491-06 
5-07 
1500 
152 
1688-96 
8-33 
1680 
172 
1929-78 
13-17 
1930 
192 
2287-44 
20-16 
2300 
212 
2671-94 
3000 
2850 
The agreement with experiment is in this case very near, and it affords a further 
confirmation of the accuracy of our rule for the force of steam, below the boiling 
point. 
126. In the condenser of a steam engine the vapour will be of the elastic force 
corresponding to its temperature, and that temperature is determined by that of the 
fluids which condense it. 
It will also always become, after a few strokes of the engine, mixed with as much 
air as it will saturate at the given temperature and pressure ; and by the preceding 
inquiry it appears, that this saturation will take place when there is an equal mix 
ture of air and vapour in the condenser; consequently, only half the quantity 
drawn out by the air pump at one stroke will be air, the rest will be uncondensed 
vapour ; and the quantity of air drawn out at each stroke must be at least equal to 
all the air which enters both from the boiler, from the injection water, and from 
leakage at the joints in the time between stroke and stroke : a slight variation on 
either side, however, will not, it may easily be proved, have much effect in 
retarding the engine. 
As the volume the air and vapour occupy determines the air pump to be of a 
large size, and consequently expensive both in construction and power, in order to 
1 An erroneous formula for this purpose has been copied into several works : it is 
l ~ die volume ; and does not at all agree with the experiments. 
I gave an analysis of the correct rule in my work on Warming and Ventilating, p. 291. It has 
also been investigated by M. Poisson, whose mode of illustration is followed in the above.