258 OF THE REGULATION AND [SECT. vm. 



the rim is considerable, and still more when both are so, it may acquire a great 

 momentum with but little increase of angular velocity, or lose a considerable 

 momentum with little diminution of that velocity. It thus becomes a receptacle 

 for the surplus energy of the power, when it acts with most intensity, or when the 

 resistance is least, and preserves it for future demand. 



By either a diminution of resistance, or an increase of power, the machine 

 would otherwise be considerably accelerated ; the excess of motive force is how- 

 ever, in a great measure, expended upon the fly, in which it generates a pro- 

 portional momentum with little increase of velocity : again, when the resistance 

 is increased, or the moving power diminished, the machinery would be very 

 sensibly retarded if the momentum accumulated in the fly did not continue the 

 motion with little diminution of its own velocity; and other things being the 

 same, the shorter the interval of reciprocation or of unequal resistance, the less 

 will be the change of velocity. 



The greater the angular velocity of the axis of the fly is, the greater will be 

 its dominion or equalizing power, all other things being equal ; for the variation 

 of velocity is inversely as the velocity of the rim. 



Every part of a machine which has either a continuous or pendulous motion, 

 particularly when it is massive, will obviously act as a fly in equalizing the motion 

 of the machine. 



The greater part of these remarks have been made in a less general form by 

 Dr. Robison 1 and Dr. Jackson ; 2 but they also state that when a more perfect 

 equalizer is wanted, we should increase the power of the fly wheel by enlarging 

 the diameter rather than the mass, because we thus produce the same effect with 

 less weight, consequently with less transverse strain upon the axle and supports, 

 and less friction. This must however be carried only to small extent, for a 

 mass of matter with an immense velocity, sustained by arms which must be com- 

 pletely incapable of resisting its impulse, becomes a very dangerous appendage 

 to a machine. Arms of cast iron could not resist a sudden check, with a rim 

 moving at the velocity of eighteen feet per second, and equal to the weight of the 

 arms, 3 consequently such wheels should be of limited diameter. 



538. When it is necessary to exceed a velocity of twelve feet per second at the 

 rim, malleable iron arms should always be used, and a velocity of thirty-three feet 

 per second at the rim is about the extreme limit for a fly, even where the ring is 

 of malleable iron. For cast iron rims, with arms of malleable iron, I should not 

 think a velocity exceeding eighteen feet per second to be safe. 



i Mechanical Philosophy, vol. ii. p. 250. - Theoretical Mechanics, p. 227. 



3 Essay on the Strength of Cast Iron, art. 261. 



