The fl: may be applied to several sorts of engines, whether moved by men, hor ses, wind, or water, or any other animate or inanimate power ; and is of great use in those parts of an engine which have a quick circular motion, and where the power of the resistance acts unequally in the different parts of a revolution. This has made some people imagine, that the fly adds a new power ; but though it may be truly said to facilitate the motion, by making it more uniform, yet upon the whole it causes a loss of power, and not an increase : for as the fly has no mo tion of its own it certainly requires a constant force to keep it in motion ; not to mention the friction of the pivots of the axis, and the resistance of the air.
The reason, therefore, why the fly be comes useful in many engines, is not that it adds a new force to them, but because, in cases where the power acts unequally, it serves as a moderator, to make the mo tion of revolution almost every where equal : for as the fly has accumulated in itself a great degree of power, which it equally and gradually exerts, and as equally and gradually receives, it makes the motion in all parts of the revolution pretty nearly equal and uniform. The consequence of this is, that the engine becomes more easy and convenient to be acted on and moved by the impelling force; and this is the only benefit obtain ed by the fly.
The best form for a fly, is that of a heavy wheel or circle, of a fit size, as this will not only meet with less resistance from the air, but being continuous, and the weight every where equally distribut ed through the perimeter of•the wheel, the motion will be more easy, uniform, and regular. In this form, the fly is most aptly applied to the perpendicular drill, which it likewise serves to keep upright by its centrifugal force : also to a wind lass or common winch, where the motion is quick ; for in pulling upwards from the lower part, a person can exercise more power than in thrusting forward in the upper quarter, where, of course, part of his force would be lost. were it not accu
mulated and conserved in the equable motion of the fly. !fence, by this means, a man may work all day in drawing up a weight of 401b. whereas 301b. would create him more labour in a day without the fly.
In order to calculate the force of the fly, joined to the screw for stamping the image upon coins, let us suppose the two arms of the fly to be each fifteen inches long, measuring from the centre of the weight to the axis of motion, the weights to be 50 pounds each, and the diinneter of the axis pressing upon the dye to be one inch. if every stroke be made in half a second, and the weights describe an half circumference, which, in this case, will be four feet, the velocity will at the instant of the stroke be at the rate of eight feet in a second, so that the momentum of it will be 800; but the arms of the fly being as levers, each fifteen inches long, whilst the semi-axis is only half an inch, we must increase this force 30 times, which will give 24,000; an immense force, equal to 1001b. falling 120 feet, or near seconds in time; or to a body of 7501b. falling 161. feet, or one second in time. Some engines, for coining crown-pieces, used to have the arms of the fly five times as long, and the weights twice as heavy, so that the effect is ten times greater. See Comm*.
FLY, in the sea language, that part of the mariner's compass on which the se veral winds or points are drawn. "Let fly the sheet," is a word of command to let loose the sheet, in case of a gust of wind, lest the ship should overset, or spend her top-sails and masts ; which is prevented by letting the sheet go-amain, that it may hold no wind.
FLY boat, a large vessel with a double prow, carrying from four to six hundred tons.