V>1 REGULATION OF MOTION BY FLY WHEELS. 



Hence the use of flies, ballast-wheels, &c. which 

 are commonly supposed to increase the power of a 

 machine, though in reality they take something 

 from it, and act upon a different principle. 



In all machines in which flies are used, a consi- 

 derably greater force must at first be applied than 

 what is necessary to move the machine without it, 

 or the fly must have been set in motion some time 

 before it is applied to the machine. This super- 

 fluous power is collected by the fly, which serves 

 as a kind of reservoir from whence the machine 

 may be supplied when the motion slackens. 



This, we must observe, will always be the case 

 with machines worked by animals, for none are 

 able to exert a great power with absolute con- 

 stancy : some intervals of rest, even though almost 

 imperceptible, are requisite, otherwise the crea- 

 ture's strength would in a short time be exhausted. 

 When he begins to move in a machine, he is vigor- 

 ous, and exerts a great power ; in consequence of 

 which he overcomes not only the resistance of the 

 machine itself, but communicates a considerable 

 degree of power to the fly. The machine, when 

 moving, yields for a time to a smaller impulse ; 

 during which time the fly itself acts as a moving 

 power, and the animal recovers the strength he has 

 lost. By degrees, however, the motion of the 

 machine decreases, and the animal is obliged to 

 renew his efforts. The velocity of the machine 

 would now be considerably increased, were it not 

 that the fly now acts as a resisting power, and the 

 greatest part of the superfluous motion is lodged 

 in it, so that the increase of velocity is scarcely 

 perceptible. Thus the animal has time to rest 

 himself, until the machine again requires an in- 

 creased impulse, and so on alternately. 



