440 THE STEAM-ENGINE. j 



ence being due to the expansive action through each of the ten divisions, which 

 was omitted in the above computation. 



It is evident that the expansive principle, as here explained, involves the 

 condition of a variation in the intensity of the moving power. Thus, if the 

 steam act with a uniform energy on the piston so long as its supply from the 

 boiler continues, the moment that supply is stopped, by closing the steam-valve, 

 the steam contained in the cylinder will fill a gradually-increasing volume by 

 the motion of the piston, and therefore will act above the piston with a gradually- 

 decreasing energy. If the resistance to the moving power produced by the 

 load, friction, &c., be not subject to a variation corresponding precisely to such 

 variation in the moving power, then the consequence must be that the motion 

 imparted to the load will cease to be uniform. If the energy of the moving 

 power at any part of the stroke be greater than the resistance, the motion pro- 

 duced will be accelerated ; if it be less, the motion will be retarded ; and if 

 it be at one time greater, and another time less, as will probably happen, then 

 the motion will be alternately accelerated and retarded. This variation in the 

 speed of the body moved will not, however, affect the mechanical effect pro- 

 duced by the power, provided that the momentum imparted to the moving mass 

 be allowed to expend itself at the end of the stroke, so that the piston may be 

 brought to rest as nearly as possible by the resistance of the load, and not by 

 any shock on any fixed points in the machine. This is an object which, con- 

 sequently, should be aimed at with a view to the economy of power, independ- 

 ently of other considerations connected with the wear and tear of the machine- 

 ry. So long as the engine is only applied to the operation of pumping water, 

 great regularity of motion is not essential, and, therefore, the variation of speed 

 which appears to be an almost inevitable consequence of any extensive applica- 

 tion of the expansive principle, is of little importance. In the patent which Watt 

 took out for the application of the expansive principle, he specified several 

 methods of producing a uniform effect upon a uniform resistance, notwithstand- 

 ing the variation of the energy of the power which necessarily attended the 

 expansion of the steam. This he proposed to accomplish by various mechani- 

 cal means, some of which had been previously applied to the equalization of a 

 varying power. One consisted in causing the piston to act on a lever, which 

 should have an arm of variable length, the length increasing in the same pro- 

 portion as the energy of the moving power diminished. This was an expedient 

 which had been already applied in mechanics for the purpose of equalizing a 

 varying power. A well-known example of it is presented in the mainspring 

 and fuzee of a watch. According as the watch goes down, the mainspring 

 becomes relaxed, and its force is diminished ; but, at the same time, the 

 chain by which it drives the fuzee acts upon a wheel or circle, having a 

 diameter increased in the same proportion as the energy of the spring is 

 diminished. 



Another expedient consisted in causing the moving power, when acting with 

 greatest energy, to lift a weight which should be allowed to descend again, 

 assisting the piston when the energy of the moving force was diminished. 



Another method consisted in causing the moving force, when acting with 

 greatest energy, to impart momentum to a mass of inert matter, which should 

 be made to restore the same force when the moving power was more enfeebled. 

 We shall not more than allud here to these contrivances proposed by Watt, 

 since their application has never been found advantageous in cases where the 

 expansive principle is used. 



The application of the expansive principle in the engines constructed by 

 Boulton and Watt, was always very limited, by reason of their confining them- 

 selves to the use of steam having a pressure not much exceeding that of the 



