MECHANICS. 



piston upirards ; the connecting rod 

 B'D would thereby be pressed down- 

 wards ; but since the crank and rod are 

 now in the same vertical plane, the 

 pressure of the rod will be entirely spent 

 on the pivots, and, having no leverage, 

 cannot turn the crank. Thus, it ap- 

 pears that when a machine is con- 

 structed in this way, an uniform action 

 of the first mover will be modified, and 

 rendered desultory and variable, and at 

 certain moments totally destroyed by 

 the nature of the machinery by which it 

 is transmitted to the working point. 

 If, however, on the axis of the crank 

 a FLY-WHEEL (L M, fig. 87) be fixed 

 so as to turn with the crank, this incon- 

 venience will be completely removed. 

 The moving force of the fly-wheel will 

 extricate the machine from the dilem- 

 mas in which it is involved at those 

 moments at which the impelling force 

 loses its power over the crank, and it 

 will equalize the effects of the varying 

 leverage by which the first mover acts 

 on the crank. 



A very remarkable instance of the 

 use of a fly occurs in the engine con- 

 structed by Mr.Vauloue, for driving the 

 piles of Westminster bridge. In this 

 machine, a heavy mass is elevated by 

 horse-power acting upon it through the 

 intervention of a rope and wheel- work, 

 and when it has reached a considerable 

 height it is disengaged, and permitted 

 to fall upon the pile which is to be 

 driven. Now the moment this mass 

 is disengaged, the machine having no 

 resistance, and the horses being relieved 

 from the weight they before encounter- 

 ed, would immediately fall forward. 

 This is prevented by connecting the 

 wheel- work with a heavy fly, the iner- 

 tia of which opposes the strength of the 

 animals when they are suddenly relieved 

 from the weight of the elevated mass. 



The advantages of a fly-wheel are 

 sensibly perceptible when a man acts 

 upon a winch (Jig. 34). In this case the 

 action of the power is very unequal : its 

 effect is greatest when he pulls upwards 

 from the height of his knee, and least 

 when, the handle being in a vertical 

 position, he thrusts from him in an 

 horizontal direction. The force is in- 

 creased when, pressing the handle down- 

 wards, he is assisted by his own weight. 

 If a fly be placed upon the axis of the 

 winch, all these unequal effects run into 

 one another, and the force becomes uni- 

 form. 



Besides the use of a fly, in regulating 



the action of machinery, it is employed 

 for the purpose of accumulating or col- 

 lecting together successive exertions of 

 a power, so as to produce a much more 

 forcible effect by then* aggregation than 

 could possibly be done by the separate 

 and successive actions. In this respect 

 a fly-wheel serves the same purpose as 

 condensed air (Treatise I. 56). If a 

 small force be repeatedly applied in 

 giving rotation to a fly-wheel, and be 

 continued until the wheel has acquired 

 a very considerable velocity, such a 

 quantity of force will be at length ac- 

 cumulated in its circumference as to 

 overcome resistance and produce ef- 

 fects utterly disproportionate to the im- 

 mediate action of the original force. 

 Thus it would be very easy in a few 

 seconds, by the mere action of a man's 

 arm, to give to the circumference of a 

 fly-wheel a force which would give an 

 impulse to a musket ball equal to that 

 which it receives from a full charge of 

 powder. 



The same principle explains the force 

 with which a stone may be projected 

 from a sling. The thong is swung 

 several times round by the force of the 

 arm, until a considerable portion of 

 force is accumulated, and then it is 

 projected with all the collected force. 



If a heavy leaden ball be attached 

 to the end of a strong piece of cane or 

 whalebone, it may easily be driven 

 through a board : by taking the end of 

 the rod remote from the ball in the 

 hand, and striking the board a smart 

 blow with the end bearing the ball, such 

 a velocity may easily be given to the 

 ball as will drive it through the board. 



Much of the efficacy of a fly depends 

 on the position assigned to it in the 

 machinery. If it be used as a regulator 

 of force, it should be placed near the 

 prime mover ; but if, on the other hand, 

 it be used as a magazine of power, it 

 should be nearer to the working point. 

 No general rules can, however, be given 

 for its exact position. 



The accumulating power of the fly 

 has led some persons into the error of 

 supposing that it adds force to the ma- 

 chine, besides what is received from the 

 first mover. That this is not the case is 

 very plain, from considering the -perfect 

 inactivity of matter, and its incapability 

 of possessing any force that it has not 

 received from some effective agent. On 

 the contrary, the fly never retains all the 

 force communicated to it by the first 

 mover, for the resistance of the air and 



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