THE NATURE AND 



[SECT. H. 



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 



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 



V -~. = the volume ; and does not at all agree with the experiments. 

 Pf 



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. 



