THE STEAM-ENGINE. 461 



revolves, a corresponding variation of speed would necessarily be produced in 

 the motion imparted to the crank. The speed at the dead points would be 

 least, being due altogether to the momentum already imparted to the revolving 

 mass of the crank and axle ; and it would gradually increase and be greatest at 

 the points where the effect of the crank on the connecting-rod is greatest. Al- 

 though this change of speed would not affect the actual mechanical efficacy of 

 the machine, and although the same quantity of steam would perform the same 

 work at the varying velocity as it would do if the velocity were regulated, yet ! ' 

 this variation of speed would be incompatible with the purposes to which it 

 was now proposed that the steam-engine should be applied in manufactures. 

 In these a regular uniform motion should be imparted to the main axle. 



One of the expedients which Watt proposed for the attainment of this end 

 was, by placing two cranks on the same axle, in different positions, to be 

 worked by different cylinders, so that while one crank should be at its dead 

 points, the other should be in the attitude most favorable for its action. This 

 expedient has since, as we shall see, been carried into effect in steam-vessels ; 

 but one more simple and efficient presented itself in the use of a fly-wheel. 



On the main axle driven by the crank Watt placed a large wheel of metal, 

 called a fly-wheel. This wheel being well constructed, and nicely balanced 

 on its axle, was subject to very little resistance from friction ; any moving 

 force which it would receive it would therefore retain, and would be ready to 

 impart such moving force to the main axle whenever that axle ceased 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, the effect 

 of the moving power upon the axle and upon the crank is generally enfeebled, 

 and at these points vanishes altogether. The momentum which has been im- 

 parted 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 favorable 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 magni- 

 tude to the fly-wheel. 



By such arrangements the motion imparted to the main axle K would be 

 uniform, provided that the moving power of the engine be always proportionate 

 to the load which it drives. But in the general application of the steam-engine 

 to manufactures it was evident that the amount of the resistance to which any 

 given machine would be subject must be liable to variation. If, for example, 

 the engine drive a cotton-mill, it will have to impart motion to all the spinning 

 frames in that mill. The operation of one or more of these may from time to 

 time be suspended, and the moving power would be relieved from a cor- 

 responding amount of resistance. If, under such circumstances, the energy 

 of the moving power remained the same, the velocity with which the machines 

 would be driven would be subject to variation, being increased whenever the 

 operation of any portion of the machines usually driven by it is suspended ; 

 and, on the other hand, diminished when any increased number of machines 

 are brought into operation. In fine, the speed would vary nearly in the inverse 

 \ proportion of the load driven, increasing as the load is diminished, and vice versa. 



