66 REGULATION OP MOTION BY FLY WHEELS. 



pestle to be lifted once at every revolution of the 

 fly ; we must then consider what weight, passing 

 through 22 feet in a second, will be equivalent to 

 30 pounds moving through one foot in a second. 

 This will be 30 divided by 22, or 1 TT pounds. 

 Were a fly of this kind to be applied, therefore, 

 and the machine set a-going, the fly would just be 

 able to lift the pestle once, after the moving power 

 was withdrawn : but by increasing the weight of 

 the fly to 10, 12, or 20 pounds, the machine, when 

 left to itself, would make a considerable number of 

 strokes, and be worked with much less labour than 

 if no fly had been used, though, no doubt, at the 

 first, it would be found a considerable incumbrance 

 to the motion. 



This is equally applicable to the action of 

 pumps j but the weight which can be most advan- 

 tageously given to a fly, has never yet been deter- 

 mined by mechanics. It is certain, however, that 

 the fly does not communicate any absolute increase 

 of power to the machine ; for if a man, or other 

 animal, is not able to set any engine in motion 

 without a fly, he will not be able to do it though a 

 fly be applied, nor will be able to keep it in motion, 

 though set a-going with a fly, by means of a greater 

 power. 



This may seem to be contradicted by the ex- 

 ample of a common clock ; for if the pendulum be 

 once stopped, the weight is not able to set it in 

 motion again, though it will keep it going when 

 once put in motion by an external power. This, 

 however, depends not upon the insufficiency of 

 the weight, but on the particular mechanism of 

 the pallet-wheel, which is such, that when once 

 the pendulum is stopped, it would require a much 

 greater weight than that commonly applied to set 



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