SECT . .] PROPERTIES OF STEAM. 87 



lessen its bulk, a second injection might be made within the air pump ; but the 

 utmost that could be gained by this method would be very little more than the 

 difference of volume due to temperature, not perhaps one-tenth of the volume of 

 the pump in any case. 



It is important to remark, that in steam from salt water, the same quantity of air 

 will occupy more space, on account of the steam being of less elastic force at the 

 same temperature ; but perhaps this is more than compensated for, by salt water 

 containing less air. 



OF THE MOTION OF ELASTIC FLUIDS AND VAPOURS. 



127. A knowledge of the principles and circumstances which affect the motion 

 of elastic fluids, is of considerable importance, in assigning the relative proportion of 

 the parts of a steam engine. It is a subject that has been very little studied in 

 discussing the theory of this invaluable machine, and therefore it is one which will 

 engage a considerable share of our attention in this work. Steam is in motion 

 during its action ; it must move through passages to perform its office, and be 

 forced through others as it retires ; and the effect of disproportion it is difficult to 

 determine from practice alone, because the result depends on so many contingent 

 circumstances. 



The best method, therefore, must be to separate the effects, and study each 

 independently : there is then reason to hope that they may be united into a perfect 

 system ; and at least it shall be our endeavour to forward this desirable end to the 

 extent of our power. 



128. The condition of free elastic fluids has been shown to be regulated by 

 the pressure and temperature of the atmosphere ; and, when an elastic fluid is con- 

 fined in a close vessel, its condition as to temperature and pressure must be similar 

 to that it would be in, if in an atmosphere of the same fluid capable of producing 

 the same pressure upon it. 



129. The most convenient method of investigating the motion of an elastic 

 fluid, is, to find the height of a homogeneous column of the same fluid, capable of 

 producing the same pressure as that to which the fluid is subjected ; for then the 

 fluid would rush into a perfect vacuum with the velocity a heavy body would 

 acquire by falling through the height of the homogeneous column, when a proper 

 reduction is made for the contraction of the aperture. 



130. If a pipe of communication be opened between two vessels containing 

 elastic fluids of different elastic forces, the velocity of the efflux through the pipe at 

 the first instant will be that which a heavy body would acquire by falling through 

 the difference between the heights of homogeneous columns, of the fluid of greatest 



