Mechanics.

the End or(XLI).

Intention of the Machine

In

(XLI) I. The Fly is the only Part I shall heretake Notice of in the Chimney-Jack, and shall explainits Nature and Use both there and in other MechanicalMachines. For it may be applied to any Sort of En-gines to very good Purposes, which have a quick andcircular Motion, and where the Power or Resistanceacts unequally in the different Parts of a Revolution.

2. The Use of the Fly is to facilitate the Motion ofEngines, by accumulating and retaining the Powercommunicated to it, and exerting it gradually andequally in each Revolution of the Machine ; whence itcomes to pass that the Motion of the Machine is ren-dered very nearly uniform and of an equal Tenour inall Parts of the Revolution, and therefore more easy,pleasant, and convenient to be acted by the impellingPower. For neither the Strength of Men, nor otherPowers or Resistances affecting the Engine, can or doact equally and uniformly in every Part of a Revolution.Thus Meat on a Spit gives always more Resistance onone Part than another, by which Means the Motionwould be so irregular and jolting, that it would soonbecome shaken and loosed from the Spit, and so not becarried round, were not this Irregularity prevented bythe Fly.

3. The Fly does not add any new Power to an En-gine, as some have imagined ; as is evident for the fol-lowing Reasons. (1.) The Fly has no Motion bqtwhat it receives at first from the Machine. (2.) A De-gree of Force is always necessary to maintain the Mo-tion of the Fly, which must be supplied from the Marchine. (3.) The Friction of the Pivot, Screw, ESk.of the Fly is a Resistance to the imprefs’d Force, andmust abate it. (4.) The Air likewise makes Resistanceto the Weights at the End of the Fly. Upon all whichAccounts it is easy to understand, that the Fly, insteadof adding, does very much decrease or lessen the Powerimpressed on the Machine.

4. Th?