SECT. XI.] 
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