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sliding valve has a pin projecting from each side, the pins being included between clifts made at the end of a forked lever i, moveable upon an axis which passes through the sides of the chest, and has a long lever (shown by dotted lines behind the cylinder) fastened to it. By moving the upper end of this lever towards the engine, the sliding valve will be raised up, and by moving it in a contrary direction, the valve will be gushed down. is a sinall iron rod jointed to the end of the long lever by one of its ends, and suspended by a hook from a spindle turning upon pivots. This spindle has several levers upon it: mno is a three arm ed lever, the arm m has a weight at the end, and is called the tumbling bob: n and o are two other arms made in the same piece with m: these two arms strike against a pin fixed across in the end of the rod k; p and 7 are two crooked levers by which the spindle is moved: these levers are struck by pins fixed in a wooden rod r, which is jointed to the beam and works up and down with it. r is a piece of wood fixed to the upright beam of the frame having pins projecting from it, which catch the tumbling bob m, and prevent its moving too far. S is a stop-cock in the main pipe, which regulates the quantity of water coming to the engine, and consequently the velocity with which the engine will work.

To describe the operation of the engine, we will suppose every thing to be in the position represented in the figure, except that the pipes, cylinder, and pump are full of water. The engine is then at the top of its stroke, which is determined by the top of the arch, on the end of the beam, meeting a bolt put across the frame. The whole column of water 34 feet always presses upon the top of the piston; and in the present position of the sliding valve, the water contained in the lower part of the cylinder can get into the eduction pipe H, and thence away from the engine. This pressure added to the 12 feet fall below the engine causes the piston to descend, bringing down the beam and pump-rod d with it. The valve in the bottom of the pump now shut, and the water in the pump barrel being pressed by the piston, opens the other valve at X, and goes up the pipe Y to the reservoir, overcoming a column of water of 80 feet. When the engine gets to the middle of its stroke, a pin in the other side of the wooden rod r takes the lever q and forces it down, raising the tumbling bob m at the same time. By the time the piston arrives at the bottom of the cylinder, the tumbling bob is brought past the vertical position, and suddenly oversets by its own weight. The lever a now runs against the pin across the end of the rod k, and shoves it from the engine moving the long lever of the sliding valve, and the short lever i down, just in the contrary manner to what it is in the drawing. This closes the four lower holes in the fixed cylinder, and prevents the water going down the pipe H, and at the same instant opens the four upper holes, forming a communication between the top and bottom of the cylinder. The pressure of 66 feet, which,

caused the piston to descend, is now removed, and the column of water of 54 feet coming down the pipe A, forces open the lower valve of the pump (the valve at a closing and taking the bearing of the column of 80 feet), presses the underside of the pump bucket and raises it up, moving the beam and piston with it. There being now an equal pressure both above and below the piston, it will be moved up easily. When the piston arrives at the middle of its stroke, the pia y in the rod r takes the lever p, and raises it with it, until it arrives at the top of its stroke, when it passes the vertical position, and instantly falls over into the position repre sented in the figure. The lever o taking the end of the rod, and putting it toward the engine, raises the sliding valve, opens the passage to the pipe H. The whole column of 66 feet now presses upon the piston and forces it down, overcoming a column of 80 feet upon the pump, though the diameter of the pump is larger than that of the cylinder: this happens from the chain of the piston acting upon a much longer. lever than the pump. K and L are two air vessels upon the pipes A and Y.

ENGINES AT LONDON BRIDGE, for supplying the metropolis with water. These, by reason of their great magnitude and importance, deserve particular notice They are worked by the water of the river Thames, which at that part is so contracted in the narrow passages between the sterlings or wooden piers upon which the stone piers of the bridge are built, as to occasion a considerable fall at all times except at high or low water. The first five arches of the London end of the bridge are devoted to the water engines which supply the city with water. The largest of these machines is shown in Plate 62 and 63. Plate 62 is an elevation of the engine taken from the sixth pier of the bridge, and Plate 63 is an elevation taken from the upper side of the bridge; the same letters of reference are used in both plates. AA, Plate 62, represents the sterling of the fifth pier of the bridge composed of a vast body of piles driven into the bed of the river, and the interstices filled up with chalk and gravel; upon the heads of these a set of horizontal beams are laid in the manner of joists, and all is made level by chalk and gravel. The fourth pier (BB, Plate 63) is made in the same manner. The water wheel EEFF is made of such a length as to fill the space between the two sterlings as exactly as possible without touching, and the bearings for the pivots of its axis are supported upon headstocks DD resting upon the sterlings. The water wheel has four rings EEEE, each having six arms morticed into the axis: each ring has 24 starts ee morticed into it, to which are nailed boards, upon which the water acts when turning the wheel round. FF is the main axis, upon the ends of which are fixed two large wooden wheels GG, round which cast-iron rings of cogs are fixed in segments. The wheels turn two trundles, which give motion to the pumps, of which there are six, three on each side of the water wheel. Only one of the sets of pumps are

shown in Plate 63; but as the other is exactly the same, one is sufficient. The axis on which the trundle His fixed is of cast-iron; it has three cranks, al, Plate 62, and another head behind the frame; fgh are cast-iron rods jointed to the cranks at their lower ends, and to the ends of the great levers or regulators IKL at the upper ends. The regulators have arches ikl at the other ends, struck from the centres of the beams, upon which chains are laid to give motion to the rods of the pumps MNO. By the motion of the water, the water-wheel is made to revolve on its axis, and the large cogwheel G with it: by its cogs it turns the trundle H and cranks ab, which being arranged round the axis at equal intervals, successively elevate and depress the crank-rods fgh and regulators IKL, and give to the pump-rods a vertical motion.

The joints of the crank-rods fgh are made to screw together round the crank neck with brass between; by which means they work very pleasantly, and when worn can be screwed up tight again that they may have no shake. The crank-rods have a flaunch m in the middle of them, and are held together by four screws, so as they may be taken apart occasionally without difficulty when the pump buckets are to be drawn out of the barrels to new leather them. The joints at the end of the beam are made with brasses and screws to adjust them.

The beams or regulators are admirably well designed to be strong, with but little timber: they are formed of two pieces of timber, between which the cast-iron axes on which they turn are placed, and then the ends are bent to touch, and kept together by hoops and screw bolts. At the ends square pieces of wood n are let into both timbers; and thus when they are firmly bound together and held from sliding endwise upon each other, they form an excellent trussbeam, which cannot bend without stretching one timber and compressing the other. The pump-rods are attached to the arches at the ends of the beams by four iron chains each, as is shown in Plate 63. The rod has a cross piece o fixed on the top of it, to which the two outside chains are screwed, and the lower ends of the same chains are fastened to the lower end of the arch. These chains act to push down the piston rods: the other two chains which raise the rods are fastened to the top of the arch and to the rods at lower ends, as shown in the beam. The pumps are forcing-pumps. pis a square iron pipe screwed down upon the groundsills of the engine frame; it has a flaunch at each end, on one of which a lid is screwed, and the other joins it to the section pipe Q, which brings the water on the top of the pipe. The three barrels MNO are screwed, having a valve in the joint, which allows water to enter the barrels, but prevents its return. From the bottom of each barrel proceeds a crooked pipe P, which communicates with another square pipe R, having valves at the joint to prevent any water getting back into the barrels. On the top. of the pipe over each valve a lid is screwed, which can be removed to clean the valves when

necessary (similar lids are screwed on the pipe P at the back towards the cranks). At the ends of the pipe R are flaunches, one of which receives a lid like the lower pipe P, and the other flaunch the pipe r, which conveys the water away from the pumps. The pistons or buckets of the pumps are solid, that is, without valves in them; and their action is as follows: When the pistons are drawn up they make a vacuum in the barrels, and the pressure of the atmosphere on the surface of the water from which the pipe Q draws raises the valves in the bottom of the barrels, and fills them. At the descent of the buckets the lower valves shut, and the water contained in the barrels can find no passage but through the valves in the pipe R; and when the pistons are drawn up again, these valves close, and the lower ones open to give a fresh supply of water to the barrels. By the position of the cranks it always happens that one or other of the barrels is forcing the water into the force pipe; and as the strokes of the other set of pumps at the other end of the water-wheel are contrived to be intermediate or alternating to these, a constant succession is kept up. The pipe r is continued to the shore to convey the water into the streets, &c. A wooden cistern S is placed over the pumps to hold water, and keep a constant supply of it above the pistons. The whole engine is sur rounded by a strong timber fence, which guards it from the injuries it might receive from vessels striking it at high water, when the liquid rises above the level of the sterlings nearly to the axis of the water-wheel. On the tops of these piles a large stage is built to serve as a read from the shore to the engine, and the underside of it supports the pipes r, Plate 63, which convey the water ashore. There are also other stages in different parts of the machme to sup port workmen when repairing it; these prevent the whole engine from being seen from the bridge at one view, and for this reason they are omitted in the drawing. The original engine was contrived by Mr. Beighton; but its present improvements are by Mr. Smeaton.

ENGINEER. 2. s. (engingnier, Fr.) One who manages engines; one who directs the artillery of an army.

For 'tis the sport to have the engineer Hoist with his own petard.

SHAKSP. HAMLET. An engineer, in the military art, should be possessed of a perfect knowledge in mathematies, so as to delineate upon paper, or mark upon the ground, all sorts of forts, and other works proper for offence and defence. He should understand the art of fortification, so as not only to be able to discover the defects of a place, but to find a remedy proper for them, as also how to make an attack upon, as well as to defend, the place. Engineers should therefore be brave as well as ingenious. When st a siege they have narrowly surveyed the place, they are to make their report to the general, by acquainting him which part they judge the weakest, and where approaches may be made with most success. Their business is also to

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