EFrECTS OP PLY-WHEELS. 175 



" If the moving power is a man acting with a handle or winch, 

 it is subject to great inequahties. The greatest force is exerted 

 when the man pulls the handle upwards from the height of his knee, 

 and he acts with the least force when the handle, being in a verti- 

 cal position, is thrust from him in a horizonal direction. The 

 force is again increased when the handle is pushed downwards by 

 the man's weight, and it is diminished when the handle, being at 

 its low3st point, is pulled towards him horizontally. But when a 

 fly is properly connected with the machinery, these irregular ex- 

 ertions are equalized, the velocity becomt^s uniform, and the load 

 is raised with an equable and steady motion. 



" In many cases, where the impelling force is alternately aug- 

 mented or diminished, the performance of the machine may be in- 

 creased by rendering the resistance unequal, and accommodating 

 it to the inequalities of the moving power. Dr. i^obison observes, 

 that ' there are some beautiful specimens of this kind of adjustment 

 in the mechanism of animal bodies.' 



" Besides the utility of liy-wheels as regulators of machinery, 

 they have been employed for accumulating or collecting power, 

 if motion is communicated to a tiy-whcel by means of a small force, 

 and if this force is continued till the wheel has acquired a great 

 velocity, such a quantity of motion will be accumulated in its cir- 

 cumference as to overcome resistances, and produce effects which 

 could never have been accomplished by tlie original force. Sq 

 great is this accumulation of power, that a force equivalent to 20 

 pounds, applied for the space of 37 seconds to the circumference 

 of a cylinder, 20 feet diameter, which weighs 4713 pounds, would, 

 at the distance of one foot from the centre, give an impulse to a 

 musket-ball equal to what it receives from a full charge of gun- 

 powder. In the space of six minutes and ten seconds, the same 

 effect would be produced, if the cylinder was driven by a man who 

 constantly exerted a force of 20 pounds at a winch one foot long. 



"This accumulation of power is finely exemplified in the sling. 

 When the thong which contains the stone is swung round the head 

 of the slinger, the force of the hand is continually accumulating in 

 the revolving stone, till it is discharged with a degree of rapidity 

 which it could never have received fiom the force of the hand 

 alone. When a stone is projected from the hand itself, there is 

 even then a certain degree of force accumulated, though the stone 

 only moves through the arch of a circle. If we fix the stone in an 

 opening at the extremity of a piece of wood two feet long, and dis* 

 charge it in the usual way, there will be more force accumulated 

 than with the hand alone, for the stone describes a larger arch in 

 the same time, and must therefore be projected with greater force. 



" When coins or medals are struck, a very considerable accumu- 

 lation of power is necessary, and this is effected by means of a fly. 

 The force is first accumulated in weights fixed in the end of the 

 fly : this force is communicated to two levers, by which it is farther 

 condensed ; and from these levers it is transmitted to a screw, by 

 which it suffers a second condensation. The stamp is then im- 



