STEAM. 



It appears also, that under a pressure of 15 Ib. per square inch, 

 water swells into 1669 times its bulk when it is converted into 

 steam. Since a cubic foot is 1728 cubic inches, and since the 

 mean atmospheric pressure is a little under 15 Ib., it may be 

 stated with sufficient precision for all practical purposes, that a 

 cubic inch of water, evaporated under the mean atmospheric 

 pressure, will produce a cubic foot of steam. 



14. II. FORCE DEVELOPED BY EXPANSION. 



Steam, in common with all vapours and gases, exerts a certain 

 mechanical force by its property of expansibility. 



To render this source of mechanical power intelligible, let us 

 suppose the piston p loaded at first with 60 Ib. for example, and 

 under this pressure let the water be evaporated, and the piston 

 raised to the height of 35 feet. The power thus developed will be 

 that due to evaporation alone. But after the evaporation has 

 ceased, and when the piston, with its load of 60 Ib., is suspended 

 at the height of 35 feet, let 15 Ib. be taken from it, so as to leave 

 a load of only 45 Ib. The pressure below the piston being then 

 greater than its load, it will be elevated, and as it is elevated, the 

 steam below it increasing in volume, will be diminished in pressure 

 in the same proportion, until the piston is raised to a height equal 

 to one-third part of 140 feet, when the pressure below it will be 

 equal to the load upon it, and it will remain suspended. During 

 this expansive action of the steam, therefore, 45 Ib. have been 

 raised through a height equal to a difference between 1 and |, that 

 is, through -^ of 140 feet. 



At this point let 15 Ib. more be supposed to be removed from 

 the piston, so that its load shall be reduced to 30 Ib. The pressure 

 below it being, as before, greater than its load, the piston will be 

 raised, and will continue to rise, until it rise to a height equal to 

 half of 140 feet, when the pressure, reduced by expansion, will 

 become equal to the load, and the piston will again become 

 suspended. 



In this interval 30 Ib. have therefore been raised by the 

 expansive action of the steam, through the difference between | 

 and ^ that is, through of 140 feet. 



Finally, suppose 15 Ib. more to be removed, and the piston will 

 rise with the remaining 15 Ib. to the height of 140 feet, so that, 

 in this last expansive action, 15 Ib. are raised through a height 

 equal to the half of 140 feet. 



It is evident that the result of the expansive action may be 

 indefinitely varied by varying the extent of its play. 



Meanwhile, whatever may be its amount, it is clearly quite 

 202 



