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.