THE STEAM-ENGINE. 



437 



are in equilibrium. By the laws of motion the state of a body which is not 

 under the operation of any external force must be either in a state of rest or 

 of uniform motion. Whichever be its state, it will suffer no change if the 

 body be brought under the operation of two or more forces which are in equi- 

 librium ; for to suppose such forces to produce any change in the state of the 

 body, whether from rest to motion, or vice versa, or in the velocity of the mo- 

 tion which the body may have previously had, would be equivalent to a sup- 

 position that the forces applied to the body being in equilibrium were capable 

 of producing a dynamical effect, which would be a contradiction in terms. 

 This, though not always clearly understood by mere practical men, or by per- 

 sons superficially informed, is, in fact, among the fundamental principles of 

 mechanical science. 



When the piston is at the top of the cylinder, and about to commence its 

 motion downward, the steam acting upon it will have not only to overcome 

 the resistance arising from the friction of the various parts of the engine, but^ 

 will also have to put in motion the whole mass of matter of the piston pump- 

 rods, pump-pistons, and the column of water in the pump-barrels. Besides 

 imparting to this mass the momentum corresponding to the velocity with which 

 it will be moved, it will also have to encounter the resistance due to the pre- 

 ponderance of the weight of the water and pump-rods over that of the steam- 

 piston. The pressure of steam, therefore, upon the piston at the commence- 

 ment of the stroke must, in accordance with the mechanical principles just 

 explained, have a greater force than is equal to all the resistances which it 

 would have to overcome, supposing the mass to be moving at a uniform ve- 

 locity. The moving force, therefore, being greater than the resistance, the 

 mass, when put in motion, will necessarily move with a gradually-augmented 

 speed, and the piston of the engine which has been already described would 

 necessarily move from the top to the bottom of the cylinder with an accelera- 

 ted motion, having at the moment of its arrival at the bottom a greater velocity 

 than at any other part of the stroke. As the piston and all the matter which 

 it has put in motion must at this point come to rest, the momentum of the mo- 

 ving mass must necessarily expend itself on some part of the machinery, and 

 would be so much mechanical force lost. It is evident, therefore, indepen- 

 dently of any consideration of the expansive principle, to which we shall pres- 

 ently refer, that the action of the moving power in the descent of the piston 

 ought to be suspended before the arrival of the piston at the bottom of the cyl- 

 inder, in order to allow the momentum of the mass which is in motion to ex- 

 pend itself, and to allow the piston to come gradually to rest at the termination 

 of the stroke. 



Thus, if we were to suppose that after the piston had descended through 

 three fourths of the whole length of the cylinder, and had acquired a certain 

 velocity, the steam above it were suddenly condensed, so as to leave a vacu- 

 um both above and below it, the piston, being then subject to no impelling 

 force, would still move downward, in virtue of the momentum it had acquired, 

 until the resistance would deprive it of that momentum, and bring it to rest ; and 

 if the remaining fourth part of the cylinder were necessary for the accomplish- 

 ment of this, then it is evident that that part of the stroke would be accom- 

 plished without further expenditure of the moving power. 



In fact, this part of the stroke would be made by the expenditure of that ex- ! 

 cess of moving power, which, at the commencement of the stroke, had been ' 

 employed in putting the machinery and its load in motion, and in subsequently j 

 accelerating that motion. 



Although under such circumstances the resistance, during the operation of < 

 the moving power, shall not have been at any time equal to the moving pow- j 



