44

MECHANICS.

[Chap. 2,

and the lever he fifteen inches radius, and the power ap-plied he 10 lbs. then the power will describe a circle of94 inches, while the weight rises half an inch; then, ashalf an inch is to 94 inches, so is 10 lbs. to 1880 lbs. theweight the engine would raise with 10 lbs. power. Hutthis is supposing the screw to have no friction, of whichit has a great deal.

article 29.

OF THE FLY WHEEL, AND ITS USE.

Before I dismiss the subject of mechanical powers, Ishall take some notice of the fly-wheel, the use of whichis to regulate the motion of engines; it is best made ofcast iron, and should be of a circular form, that it maynot meet with much resistance from the air.

Many have supposed this wheel to be an increaser ofpower, whereas it is, in reality, a considerable destroyerof it: which appears evident, when we consider that ithas no motion of its own, but receives all its motion fromthe first mover; and as the friction of the gudgeons, andthe resistance of the air are to be overcome, this cannot bedone without the loss of some power : yet this wheel is ofgreat use in many cases; namely:

1st. For regulating the power, where it is irregular-ly applied, such as the treadle and crank moved by thefoot or hand; as in spinning-wheels, turning-lathes, flax-mills, or where steam is applied, by a crank, to producea circular motion.

2d. Where the resistance is irregular, or by jerks,as in saw-mills, forges, slitting-mills, powder-mills, &c.the fly-wheel, by its inertia, regulates the motion; be-cause, if it be very heavy, it will require a great manylittle shocks or impulses of power to give it a considera-ble velocity; and it will, of course, require as manyequal shocks to resist or destroy the velocity it has ac-quired.

While a rolling or slitting-mill is running empty, the