with the fame velocity as his hand moves round in this inftance, and that for ten hours in a day: here a machine may be applied, whereby one man em→ ploying his whole ftrength fhall do the work of fifty men in the fame time. In other cafes machines may afford great profit by rendering effectual the force or intensity of the power or powers employed as in the fire-engine, water-wheel, &c.

OF THE REGULATION OF MOTION BY FLY-WHEELS.

A uniform motion is continued by applying the heavy wheel, (or a cross bar loaded with equal weights called a fly) to the machine; this being made to revolve about it's axis, keeps up the force of the power, and diftributes it equally in all parts of it's revolution, by going on at the fame rate it makes the motion uniform, for on account of it's weight a fmall variation in force does not senfibly alter it's motion, while it's friction and the refiftance of the machine prevents it from accelerating. If the motion of the machine flackens, it helps it forward; if it tends to move too faft; it will keep it back.

Every regulating wheel fhould be fixed upon that axis where the motion is fwifteft, and fhould be heavy when the motion is defigned to be flow, and light where it is to be fwift. In all cafes, the center of motion fhould coincide with the center of gravity of the wheel. The axis may be either perpendicular or parallel to the horizon..

Moft mechanical operations confift either in commmunicating motion to quiefcent bodies, or in overcoming refiftance; in which operations the actions of the moving force, and it's effects, are not always cotemporary; but motion is frequently firft generated, and at fubfequent times, employed in producing

Y 4

producing the result. Thus great mechanical effects may be produced by means of fmall velocities, generated in ponderous bodies by inconfiderable moving forces. Suppofe an heavy cylinder, of iron or lead, moveable about it's axis, and in a vertical plane; a fmall force being applied to turn the cylinder, will, if long continued, generate fuch a force as will produce effects in raifing weights by no means obtainable by the moving force immediately applied.

Mr. Atwood has fhewn, that a force of twenty pounds, applied for thirty-feven feconds to the circumference of a cylinder of ten feet radius, and weighing 4713 pounds, would, at the distance of one foot from the center, give an impulfe to a mufquetball equivalent to what it receives from a full charge of gunpowder.

The fame effect would be produced in fix minutes ten feconds, by a man turning the cylinder with a winch one foot long, on which he conftantly exerted a force of twenty pounds.

This accumulation of mechanic force appears extraordinary at firft fight, and may fuggeft wrong notions concerning the fubject, unlefs fully confidered; when it will appear, that the cylinder has no principle of motion in itfelf, and cannot have any more than it receives.

An accumulation of motion in ponderous wheels, is, as I have juft obferved, of great fervice when applied to practical purpofes. Thus machines to lift weights, to grind corn, &c. as well as many others, having a large ponderous wheel (terned a fy) fixed to fome part of their machinery, fo that it fhall revolve round it's axis, have been found to produce much greater mechanical effels than could have been produced without fuch an additional mafs of matter.

Here, however, I muft beg you to observe,

that

that there is no increase of power communicated to any engine by flies, ballast-wheels, &c. they act upon quite another principle.

In all machines in which flies are used, either a confiderably greater force must be applied than what is neceffary to move the machine without it, or the fly must have been fet in motion fome time before it is applied to the machine. It is this fuperfluous power which is, as it were, collected by the fly; and ferves as a refervoir, by which the machine may be fupplied when the effort of the power is leffened.

Suppofe a wheel and axle (with a ponderous wheel affixed to it), moveable round an axis by means of an handle, which is impelled by mufcular force. Now here the ufe of a ponderous wheel is evident; for fuppofe a certain uniform motion generated therein, this will continue for fome time to raife the weight, although the moving force of the arm be difcontinued, which must in fome degree happen when the arm is afcending. Now if there were no motion in the ponderous wheel, to continue the afcent of the power, the refiftance would begin to preponderate as foon as the moving power was at all diminished; from which it is manifeft how much motion would be loft without the fly.

It is a refervoir to fupply the machine when the animal flackens his efforts; and this is always the cafe with animals, for none are able to exert a great power with abfolute conftancy. When he begins to move the machine, he is vigorous, and exerts a great power; which not only overcomes the refiflance of the machine, but communicates a confiderable degree of power to the fly. Hence the machine, when moving, yields for a time to a small impulfe; during which time the fly acts as a moving power, and the animal recovers his ftrength.

By

By degrees, however, the motion of the machine decreases, and the animal is then obliged to renew his efforts. In this cafe, if it were not for the fly, the velocity of the machine would be confiderably increased; but it now acts as a refifting power, and the greater part of the fuperfluous motion is lodged therein, fo that the increafe of velocity in the machine is fcarcely perceptible; and the animal again acquires time to reft himself, till the machine requires an increafe of impulse; and fo on alternately.

It is the fame with a machine moved with water, or by a weight; for though the ftrength of thefe does not exhauft itself like mufcular force, yet the yielding of the parts of the machine renders the impulfe much lefs after it begins to move: hence it's velocity is accelerated for fome time, till the impulfe becomes fo fmall, as to require an increase of power to keep up the neceffary motion; but when the impufe flackens, the fly communicates part of it's motion, fo as to render the motion equable. The truth of what has been advanced, will be rendered fenfible by confidering the inequality of motion in a clock, when the pendulum is off; and how very regularly it goes when regulated by the pendulum, which acts as a fly.

Flies are particularly ufeful in any kind of work which is done by alternate ftrokes; as the lifting of large peftles, pumping of water, &c. In this cafe the weight of the wheel employed is a principal object; and it is calculated by a comparifon with the weight to be raised.

Thus, fuppofe it to be required to raise a peftle thirty pounds weight to the height of one foot fixty times in a minute. Let the diameter of the fly be feven feet, and fuppofe the peftle to be raifed once at every revolution of the fly, we must then confider what weight, pafling through 22 feet

in a fecond, will be equivalent to 30 pounds, moving through one foot in a fecond. This will be 30 divided by 22, or 1 pounds. Were a fly of this kind to be applied, and the machine fet a going, the fly would be able to lift the peftle once after the moving power was withdrawn; but by increafing the weight of the fly to ten, twelve, or twenty pounds, the machine, when left to itself, would make a confiderable number of strokes, and be worked with much less labour than if no fly had been used. It is certain, however, that the fly does not communicate any abfolute increase of motion to the machine; for if a man, or any animal, is not able to set any mechanical engine in motion without a fly, he will not be able to do it though a fly be applied, nor will he be able to keep it in motion, though fet a going with a fly, by means of a greater power.

Encyclopedia Britannica, vol. x. part ii,

LECTURE