fabrication of pendulums adapted to obviate the expansion of metals by increase of temperature, if the works themselves still remain constructed of such expansible materials. A correct time-piece, therefore, will be that of which not only the works and pendulum are constructed of the least expansible materials, but the pendulum itself shall vibrate in a medium of unalterable density; a desideratum only to be obtained by causing the vibrations to be performed in vacuo, or by a self-correcting pendulum, as above alluded to. Mr. G.J. Wright, of Kennington, who has some observations on this subject in Tilloch's Philosophical Magazine, No. 57. says the best substance to compose the works of a correct time-keeper is ivory, or the horn of the narwhal or sea-unicorn (which is almost entirely composed of enamel); especially if any means were known of increasing their hardness so as to vie with the metals.

The most general reinedy against the chief inconveniences of pendulums, is to make them long, to vibrate in small arcs, and to have the bobs as ponderous as is consistent with the structure of the machine. In those cases where it is wished to increase the time of vibration without increasing the length of the pendulum, recourse may be had to the angular pendulum, the theory of which has been given in art. 311, vol. I.; or to the pendulum formed of a slender cylinder with a ball at each end, and a contrivance for a moveable centre of motion, as already explained in the same article of the first volume. Akin to this is the contrivance described by M. Prony, at p. 229, Connaissance des Tems pour l'an 1817.

Hydraulic PENDULUM, a simple contrivance by which the rectilinear motion of flowing motion is made to communicate an alternating motion.

The hydraulic pendulum of Perrault (pl. XXXVIII. fig. 4A and supp. B) is a chest or cask ABCD moveable on two pivots m, and divided into two parts by the partition CD. When the bottom AB of this vessel is horizontal, the water from the source м falls on c the middle of the partition. Immediately it becomes inclined, the water falls into the part such as B, elevated above the horizon; this part of the vessel increases in weight in proportion as it fills with water; when it is full the entire vessel turns on its axis, and the water from the source falls in the part AC of the vessel, which in consequence filling up, the weight causes it to oscillate: this oscillatory motion will be common to a pendulum bar, or to any other body attached to the double vessel.

In the hydraulic pendulum of M. Boitias (pl. XXXVIII. fig. 4 E), there is placed at the inferior extremity of a pendulum a very large float-board, moveable on pivots which turn on the

parallel branches of a frame fixed to the pendulum. This floatboard will assume alternately the vertical and the horizontal position. In the first it plunges into the stream and receives the impulsion of the water; the float-board moves with the pendulum, and having reached the lowest point of its oscillation, a counterweight causes it to turn on its pivot and gives it the horizontal position: then the pendulum, the weight of which is no longer counterbalanced by the action of the water, takes as well as the float-board the horizontal position, and commences a new oscillation. Instead of employing a counterweight to open and shut the float-board, there may be employed two cords attached to fixed points and to the float-board; these threads stretched by the pendulum will draw the float-board at the beginning and end of the oscillation, and cause it to assume successively its horizontal and vertical positions. The hydraulic pendulum does not, like the common pendulum, oscillate on both sides of the vertical, but simply on one. It moves on towards that side to which it is impelled by the motion of the water, till, reaching the highest point of its oscillation, the floatboard opens, and the whole is brought back by its own weight to the vertical position; in that position the float-board again closes the frame, and the impulsion of the water upon it causes it to oscillate again. And so on.

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PENSTOCK is a sluice or floodgate, serving to retain or go at pleasure the water of a mill-pond. The following is a description of a pentrough and stock for equalising the water falling on water-wheels, by George Quayle, esq.

To insure a regular supply of water on the wheel, and to obviate the inconveniences arising from the usual mode of delivering it from the bottom of the pentrough, this method is devised of regulating the quantity delivered by a float, and taking the whole of the water from the surface.

Section of the pentrough. (Plate XVI. fig. 4.) A, the entrance of the water. B, the float, having a circular aperture in the centre in which is suspended c, a cylinder, running down in the case E below the bottom of the pentrough. This is made water tight at the bottom of the pentrough at F, by a leather collar placed between two plates, and screwed down to the bottom.

The cylinder is secured to the float so as to follow its rise and fall; and the water is admitted into it through the opening in its sides, and there, passing through the box or case E, rises and issues at G on the wheel. By this means, a uniform quantity of water is obtained at G; which quantity can be increased or diminished by the assistance of a small rack and pinion attached to the cylinder, which will raise or depress the cylinder above or under

the water line of the float; and, by raising it up to the top, it stops the water entirely, and answers the purpose of the common shuttle. This pinion is turned by the handle H, similar to a winch-handle; and is secured from running down by a ratchetwheel at the opposite end of the pinion axis.

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K and I are two upright rods to preserve the perpendicular rise and sinking of the float, running through the float, and secured at the top by brackets from the sides.

м, a board let down across the pentrough nearly to the bottom, to prevent the horizontal impulse of the water from disturbing the float.

Fig. 5. A transverse section, shewing the mode of fixing the rack and pinion, and their supports, on the float. The rack is inserted into a piece of metal running across the cylinder near the top. That the water may pass more freely when nearly exhausted, the bottom of the cylinder is not a plane, but is cut away so as to leave two feet, as at c, fig. 4. The float is also kept from lying on the pentrough bottom by four small feet; so that the water gets under it regularly from the first.

Fig. 6. An enlarged view of the cylinder, shewing the rack and ratchet-wheel, with the click, and one of the openings on the side of the cylinder: the winch or handle being on the opposite side, and the pinion, by which the rack is raised, inclosed in a box between them. (Transactions of the Society of Arts, vol. XI. A.D. 1793.)

PERSIAN-WHEEL. See HYDRAULIC Engines. No. 4. PILE-ENGINE, a machine by which piles are driven into the earth for the foundations of piers and other structures.

In pile-engines the contrivance consists in drawing up a great weight, called a ram or hammer, to a moderate height, and then letting it fall freely with a considerable momentum upon the head of the pile. In the most simple pile engines the ram is drawn up by men pulling at a cord running over a fixed pulley, and suffering the cord to slip from their hands when the weight is sufficiently elevated. Among more complex engines, the best we have seen are those invented by Mr. Vauloue and by Mr. S. Bunce.

Description of Vauloue's pile-engine. (See pl. XXII.) A is a great upright shaft or axle, on which are the great wheel B and the drum c, turned by horses joined to the bars s, s. The wheel B turns the trundle x, on the top of whose axis is the fly o, which serves to regulate the motion, as well as to act against the horses, and to keep them from falling when the heavy ram o is discharged to drive the pile P down into the mud in the bottom of the river. The drum c is loose upon the shaft A, but is locked to the wheel в by the bolt Y.

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this drum the great rope HH is wound; one end of the rope being fixed to the drum and the other to the follower G, to which it is conveyed by the pulleys i and'. In the follower G is contained the tongs F, that take hold of the ram o by the staple R for drawing it up. D is a spiral or fusee fixed to the drum, on which is wound the small rope r that goes over the pulley u, under the pulley v, and is fastened to the top of the frame at 7. To the pulley-block v is hung the counterpoise w, which hinders the follower G from accelerating as it goes down to take hold of the ram; for as the follower tends to acquire velocity in its descent, the line T winds downwards upon the fusee on a larger and larger radius, by which means the counterpoise w acts stronger and stronger against it; and so allows it to come down with only a moderate and uniform velocity. The bolt y locks the drum to the great wheel, being pushed upward by the small lever 2, which goes through a mortise in the shaft A, turns upon a pin in the bar 3, fixed to the great wheel B, and has a weight 4, which always tends to push up the bolt Y through the wheel into the drum. L is the great lever turning on the axis m, and resting upon the forcing bar 5, 5, which goes through a hollow in the shaft A, and bears up the little lever 2.

By the horses going round, the great rope н is wound about the drum c, and the ram o is drawn up by the tongs F in the follower G, until the tongs come between the inclined planes E; which by shutting the tongs at the top, opens it at the foot, and discharges the ram, which falls down between the guides bb upon the pile, P, and drives it by a few strokes as far into the mud as it will go; after which, the top part is sawed off close to the mud by an engine for that purpose. Immediately after the ram is discharged, the piece 6 upon the follower G takes hold of the ropes aa, which raise the end of the lever L, and cause its end N to descend and press down the forcing bar 5 upon the little lever 2, which by pulling down the bolt y, unlocks the drum c from the great wheel B; and then the follower being at liberty comes down by its own weight to the ram; and the lower ends of the tongs slip over the staple R, and the weight of their heads causes them to fall outward and shut upon it. Then the weight 4 pushes up the bolt y into the drum, which locks it to the great wheel, and so the ram is drawn up as before.

As the follower comes down, it causes the drum to turn backward, and unwinds the rope from it, whilst the horses, great wheel, trundle, and fly, go on with an uninterrupted motion; and as the drum is turned backward, the counterpoise w is drawn up, and its rope T wound upon the spiral fusee D.

There are several holes in the under side of the drum, and the bolt y always takes the first of them that it finds, when the drum stops by the falling of the follower upon the ram; until which stoppage the bolt has not time to slip into any of the holes.

The peculiar advantages of this engine are, that the weight called the ram, or hammer, may be raised with the least force; that, when it is raised to a proper height, it readily disengages itself and falls with the utmost freedom; that the forceps or tongs are lowered down speedily, and instantly of themselves again lay hold of the ram and lift it up.

This engine was placed upon a barge on the water, and so was easily conveyed to any place desired. The ram was a ton weight; and the guides bb, by which it was let fall, were 30 feet high.

Description of Bunce's Pile-engine.

Fig. 1 and 2. plate XXII. represent a side and front section of the machine. The chief parts are, ▲, fig. 1. which are two endless ropes or chains, connected by cross pieces of iron, B (fig. 2.), corresponding with two cross grooves cut diametrically opposite in the wheel c (fig. 1.), into which they are received; and by which means the rope or chain A is carried round. FHK is a side-view of a strong wooden frame moveable on the axis H. D is a wheel, over which the chain passes and turns within at the top of the frame. It moves occasionally from F to Gupon the centre H, and is kept in the position F by the weight fixed to the end K. In fig. 3. L is the iron ram, which is connected with the cross pieces by the hook M. N is a cylindrical piece of wood suspended at the hook at o, which, by sliding freely up the bar that connects the hook to the ram, always brings the hook upright upon the chain when at the bottom of the machine, in the position of GP. See fig. 1.

When the man at s turns the usual crane-work, the ram being connected to the chain and passing between the guides, is drawn up in a perpendicular direction; and when it is near the top of the machine, the projecting bar o of the hook strikes against a cross piece of wood at R (fig. 1.), and consequently discharges the ram; while the weight I of the moveable frame instantly draws the upper wheel into the position shewn at F, and keeps the chain free of the ram in its descent. The hook, while descending, is prevented from catching the chain by the wooden piece N for that piece being specifically lighter than the iron weight below, and moving with a less degree of velocity, cannot come into contact with the iron till it is at the bottom and the ram stops. It then falls, and again connects the hook with the chain, which draws up the ram as before.