FLY-WHEEL. 



G, it is evident that, by the adjustment of the eccentric upon the 

 shaft, the valves may be opened and closed at any required position 

 of the crank, and therefore at any required position of the piston 

 in the cylinder. 



Such is the contrivance by which the valves, whatever form may 

 be given to them, are now almost universal!^ worked in double- 

 acting steam engines. 



46. Notwithstanding the regulating influence of the governor, 

 the motion of the engine would still be subject to a certain 

 inequality, owing to the varying action of the connecting rod, o 

 (fig. 26), on the crank. It will be quite evident that this action 

 is most efficient when o is placed at right angles to the crank, 

 which it is twice in every revolution, but that the more oblique it 

 is to the crank the less efficient will be its action upon it. 



Now this inequality is effaced very nearly, if not altogether, by 

 means of a large and massive wheel of cast iron, called the FLY- 

 WHEEL, which is keyed upon the axle of the crank so as to revolve 

 with it, as shown in fig. 26. This wheel being well constructed, 

 and nicely balanced on its axle, is subject to very little resistance 

 from friction; any moving force which it receives it therefore 

 retains, and is ready to impart such moving force to the main 

 axle whenever that axle ceases to be driven by the power. "When 

 the crank, therefore, is in those positions in which the action of 

 the power upon it is most efficient, a portion of the energy of the 

 power is expended in increasing the velocity of the mass of matter 

 composing the fly-wheel. As the crank approaches the dead 

 points, that is the points where it is in the same straight line with 

 the connecting rod, the effect of the moving power upon the axle 

 and upon the crank is gradually enfeebled, and at these points 

 vanishes altogether. The momentum which has been imparted to 

 the fly-wheel then comes into play, and carries forward the axle 

 and crank out of the dead points with a velocity very little less 

 than that which it had when the crank was in the most favourable 

 position for receiving the action of the moving power. 



By this expedient, the motion of revolution received by the 

 axle from the steam piston is subject to no other variation than 

 just the amount of change of momentum in the great mass of the 

 fly-wheel which is sufficient to extricate the crank twice in every 

 revolution from the mechanical dilemma to which its peculiar form 

 exposes it ; and this change of velocity may be reduced to as small 

 an amount as can be requisite by giving the necessary weight and 

 magnitude to the fly-wheel. 



47. The- combination of jointed rods represented at cdgb, in 

 fig. 26, called the parallel motion, constitutes one of the many 

 inventions of Watt, which has always excited the greatest admi- 



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