Look in the tables under a 20 feet water-wheel, and opposite 808 gallons will be found 2 bolls of corn ground per hour.

EXAMPLE II.-If a stream of water, producing 808 gallons ale measure per minute, can be applied to an undershot waterwheel 20 feet diameter, what quantity of corn can it grind per hour?

It is found by the tables, that, if applied on an overshot water-wheel 20 feet diameter, the stream will grind 2 bolls per hour; and, from page 199, the power required by the undershot to that of the overshot water-wheel, to produce an equal effect, is as 2.4 to 1; therefore, as 2'4 : 1 :: 2·5: 1:04 bolls of corn ground per hour by means of the stream.

EXAMPLE III-If a stream of water, producing 808 gallons ale measure per minute, can be applied on a breast waterwheel 20 feet diameter, what quantity of corn can it grind per hour?

It is found by the tables, that, if applied on an overshot water-wheel of equal size, 21 bolls of corn will be ground per hour and, from page 199, the power of a breast water-wheel to that of an overshot water-wheel, to produce an equal effect, is as 1.75 to 1; therefore, as 1.75 : 1 :: 2·5 : 1′42 bolls of corn ground per hour by the stream.

EXAMPLE IV. Of what diameter must the cylinder of a common steam-engine be made, to grind 10 bolls of corn per hour?

By looking in the table, page 199, opposite 10 bolls ground per hour, the diameter of the steam cylinder will be found to be 86 inches.

FLY, is a name given to a certain appendage to many machines, either as a regulator of their motions, or as a collector of power. When used as a regulator, the fly is commonly a heavy disk or hoop balanced on its axis of motion, and at right angles to it: though sometimes a regulating fly consists of vanes or wings, which as they are whirled round meet with considerable resistance from the air, and thus soon prevent any acceleration in the motion: but this kind of regulator should rarely, if ever, be introduced in a working machine, as it wastes much of the moving force. When the fly is used as a collector of power, it is frequently seen in the form of heavy knobs at the opposite ends of a straight bar; as in the coining-press. In arts. 18..23. of the introductory part of this volume the reader will find several remarks on the nature and operation of the different kinds of flies used in machines.

FOOT-MILL, is a mill for grinding corn or any other sub

stance, moved by the pressure of the feet of men or animals. In some foot-mills, a horse or an ox is fixed to a stall upon a floor above a vertical wheel; and a hole is made in the floor in the place where the hind feet of the animal should stand, thus admitting those feet to press upon the rim of a wheel, and cause the wheel to turn upon its axle, and give motion to the whole mill. But in this kind of machine the animal will be obliged very unnaturally to move his hind feet while his fore feet will be at rest and further, the motive force being applied near the vertex of the wheel will act but with little advantage; and the work done will be comparatively trifling.

A much more judicious construction of a foot-mill is given in G. A. Bockler's Theatrum Machinarum, published at Nuremburgh, in 1661. This mill is represented in fig. 1. pl. XV. A is an inclined wheel, which is turned by the weight of a man, and the impulsive force of his feet while he supports himself, or occasionally pushes with his hands at the horizontal bar H. The face of this wheel has thin pieces of wood nailed upon it at proper distances, to keep the feet of the man from slipping while he pushes the wheel round; and the under side has projecting teeth or waves which catch into the cogs of the trundle B, and by that means turn the horizontal shaft G with its wheel c: this latter wheel turns the trundle D, the axle of which carries the upper millstone E. This kind of footmill will answer extremely well to grind malt, &c. when no very great power is required. The advantages and disadvantages of the inclined wheel have been stated under the article CRANE, when describing the contrivance of Mr. Whyte, which is the same as this of Bockler's with respect to the wheel.

FORCER, TEMPORARY, for a pump, is a contrivance to produce a constant stream. A very simple forcer of this kind has been devised by Mr. R. Trevithick: it consists in fixing a barrel with a solid piston along the side of the common pump, in such a manner, that the lower space of the additional barrel may communicate with the space between the two valves of the pump; and, lastly, by connecting the rods so that they may work together. This is shewn in fig. 1. plate IX.; and the effect is, that when the pistons are raised, the spaces beneath, A and B, become filled by the pressure of the atmosphere, at the same time that the upper column flows out at E. But again, when the pistons descend, the valve c shuts, and, consequently, the water driven by the piston in в must ascend through A, and continue to produce an equal discharge through E in the down stroke. (Nich. Journ. No. 7. N. S.)

FOUNTAIN, HERO'S. See HYDRAULIC machines, No. 6.

GIBBET, or JIB, of a crane, the projecting beam,upon the extremity of which is fixed a pulley for the rope to pass over that raises the weight: it is represented by DEF, in fig. 3. pl. IX. Jibs of the usual construction turn on two solid gudgeons. The rope by which the goods are raised passes over the upper gudgeon, and is confined between two small vertical rollers, in order that it may constantly lead fair with the pulley or sheave at the extremity of the jib. According to this construction, whenever the crane turns round its axis, the rope is bended so as to form an angle more or less acute, which causes a great increase of friction, and produces a continual effort to bring the arm of the jib into a parallel position to the inner part of the rope. These inconveniences may appear to be trifling on paper, but in actual practice they are of no small importance, for they necessarily imply a much greater exertion of power in raising goods, and the application of a constant force to keep the jib in the position that may be requisite; while the partial stress which is exerted on only a few strands of the rope, when bended into an acute angle, infallibly destroys it in a very short time. The simple construction proposed by Mr. J. Bramah obviates all these defects, and at the same time possesses the very desirable property of permitting the jib of what is termed a campshut or landing crane wholly to revolve round its axis, and to land goods at any point of the circle described by the arm of the jib.

It consists in perforating the axis or pillar of the crane, and in conducting the rope through this perforation by means of an additional pulley fixed on the top of the arm of the jib. The rope proceeds from the goods which are hoisted, through a pulley fixed as usual at the extremity of the jib; it then passes over another pulley fixed at the opposite extremity of the jib, and is by this pulley conducted through the perforated axis or pillar to a third pulley; whence it is immediately directed to the crane by which the weight is elevated.

It is almost unnecessary to state that the lower axis is usually fixed in an oil box, and that friction rollers are applied to the axis wherever the circumstances may render it necessary.

When great weights are to be raised, as large stones from a quarry, or pieces of ordnance from a ship to a quay, the crane is commonly a fixed one, and only the gibbet moveable, from which the weight hangs. Here, in the common way of working a crane, the rope of which runs between two vertical rollers, there is often much danger in turning the gibbet upon its axis. A small rope, called a guide-rope, is fastened to the weight, or to the upper part of the gibbet near its extremity, which a mau pulls to bring the weight over the place where it is to be lower

ed. Now, in performing this, the main rope not continuing parallel to the arm of the gibbet, gives the weight a tendency towards that side to which it deviates, and that sometimes so suddenly, that without care and much force applied, the load will swing with great violence, and do much mischief. To prevent this, Mr. Ralph Allen of Bath, about the year 1728, recommended the following method: Upon the shaft of the gibbet let there be fixed an iron wheel with several teeth or cogs, to be carried round by a pinion fixed upon a horizontal axis, such axis passing through the wall or frame-work behind the shaft of the gibbet, and having at its further extremity a vertical wheel with handles projecting from the rim in the plane of the wheel. A man standing at this wheel is out of the reach of danger from the load, and by applying a small portion of his strength at the handles of the wheel he can easily bring the gibbet and its load to any position required, and retain it as long as necessary in that position. A figure representing this contrivance is given in the Phil. Trans. No. 411. and in Ferguson's Select Lectures.

GIMBALS, a contrivance by means of which barometers, vessels of oil, mariner's compasses, &c. may be suspended so as to arrange their upper parts horizontally. The nature of this contrivance will be at once understood by shewing its application to a mariner's compass. It consists of a hoop or ring supported upon two pins diametrically opposite each other, and issuing from the external surface of the ring in such a direction that both lie in the same diametrical line. When the hoop is suspended on these pins it is at liberty to turn freely about the diameter of which they constitute the prolongation. The notches or holes of support are disposed horizontally. The compass-box itself is placed in a similar ring with two projecting pivots; and these pivots are inserted in holes made in the former ring at equal distances from each of its pivots. If therefore the whole be left at liberty, the compass-box may vibrate upon the diametral line of the outer ring, as well as upon a line formed by its own pivots, at right angles to that diametral line. The consequence of this arrangement is, that the centre of gravity of the compass-box will dispose itself immediately beneath the intersection of both lines on which it is at liberty to move: -that is to say, if the weight of the box and its component parts be properly disposed, the compass will assume a position in which the upper surface shall be horizontal.

GIN. See CRAB.

GLAZIER'S VICE, is an instrument for drawing window lead. See fig. 3. pl. XII. PG, QH, are two axles running in the frame KI., ML. C, D, two wheels of iron case-hardened, 13

inch broad, and of the thickness of a pane of glass; these wheels are fixed to the axles, and run very near one another, their distance not exceeding of an inch across their edges several nicks are cut, the better to draw the lead through. È, F, are two pinions each of twelve leaves, turning one another and going upon the ends of the axles, which are square, being kept fast there by the nuts P, Q, which are screwed fast with a key. A, B, are two cheeks of iron, case-hardened, and fixed on each side to the case with screws; these are cut with an opening where the two wheels meet, and set so near to the wheels as to leave a space equal to the thickness of the lead; so that between the wheels and the cheeks there is left a hole of the form represented at N, which is the shape of the lead when cut through. The frame KLML is held together by cross bars passing through the sides, and screwed on: and a cover is put over the machine to exclude the dust. The whole is screwed down fast to a bench by screw nails LL. When the vice is used, the lead to be drawn is first cast in moulds, into pieces a foot long, with a gutter on each side. One of these pieces is taken, and an end of it sharpened a little with a knife; then, being put into the hole between the wheels, by turning the handle 1 the lead is drawn through the vice, and receives the form designed.

GOVERNOR, a contrivance for the purpose of equalizing the motion of mills and other machinery.

When a part of the machinery of a mill is suddenly stopped, or suddenly set going, and the moving power remains the same, an alteration in the velocity of the mill will take place; it will move faster or slower. Every machine having a certain velocity at which it will work at greater advantage than at any other, the change of velocity arising from the above cause, is in all cases a disadvantage, and in delicate operations exceedingly hurtful. In the case of a cotton-mill, for instance, which is calculated to move the spindles at a certain rate, if from any cause the velocity is much increased, a loss of work immediately takes place, and an increase of waste from the breaking of the threads, &c.; on the other hand, there must be an evident loss from the machinery moving too slow.

In steam-engines this evil is remedied by a contrivance called a governor. (See fig. 1. pl. XL.)-"Two balls are fixed to the ends of rods, in continual revolution, and as soon as the motion becomes a little too rapid, the balls rise considerably," and, by the intervention of a lever, act upon a throttle-valve, which diminishes the quantity of steam admitted, and of course serves to make the motion less rapid.

1. The steam-engine governor.-IK, fig. 1. represents a spindle kept in motion by the engine; A, B, the centrifugal balls;