438 



THE STEAM-ENGINE. 



er, since while the motion was accelerated it was less, and while retarded 

 greater, than that power, yet as the whole moving power has been expended 

 upon the resistance, the mechanical effect which the moving power has pro- 

 duced under such circumstances will be equal to the actual amount of that 

 power. If in an engine of this kind the steam was not cut off till the conclu- 

 sion of the stroke, a part of the moving power would be lost upon those fixed 

 points in the machinery which would sustain the shock produced by the in- 

 stantaneous cessation of motion at the end of the stroke. 



Independently, therefore, of any consideration of the expansive principle, it 

 appears that, in an engine of this kind, the steam ought to be cut off before the 

 completion of the stroke. 



To render the expansive action of steam intelligible, let A B, fig. 12, repre- 



Fig. 12. . 



sent a cylinder whose area we will suppose, for the sake of illustration, to be 

 a square foot, and whose length, A B, shall also be a foot. If steam of a 

 pressure equal to the atmosphere be supplied to this cylinder, it will exert a 

 pressure of about one ton on the piston ; and if such steam be uniformly sup- 

 plied from the boiler, the piston will be moved from A to B with the force of 

 one ton, and that motion will be uniform if the piston be opposed throughout 

 the same space by a resistance equal to a ton. When the piston has arrived 

 at B, let us suppose that the further supply of steam from the boiler is stopped 

 by closing the upper steam-valve, and let us also suppose the cylinder to be 

 continued downward so that B C shall be equal to A B, and suppose that B C 

 has been previously in communication with the condenser, and is therefore a 

 vacuum. The piston at B will then be urged with a force of one ton down- 

 ward, and as it descends the steam above it will be diffused through an in- 

 creased volume, and will consequently acquire a diminished pressure. We 

 shall, for the present, assume that this diminution of pressure follows the law 

 of elastic fluids in general ; that it will be decreased in the same proportion 

 as the volume of the steam is augmented. While the piston, therefore, moves 

 from B downward, it will be urged by a continually-decreasing force. Let us 

 suppose, that, by some expedient, it is also subject to a continually-decreasing 

 resistance, and that this resistance decreases in the same proportion as the 

 force which urges the piston. In that case the motion of the piston would 

 continue uniform. When the piston would arrive at P', the middle of the 

 second cylinder, then the spaces occupied by the steam being increased in the 

 proportion of 2 to 3, the pressure on the piston would be diminished in the 



