In the centrifugal or rotatory wimp, the centrifugal force produced by rotation is the inimediate agent in rais ing the water, combined with the pressure of the atmo sphere. This machine is represented in plate CCCCLXX. Fig. 15, where AB is a: vertical tube, moving round gudgeons at A and B, the lowermost of which is in the well from which the water is to be raised. To this vertical tube is connected one or more horizontal arms CD, with an aperture at one or both ends, by which the water is dis charged into a circular trough EF, from which it is taken out by the spout at F. The rotatory motion is communi cated to the vertical and horizontal pipe, by a handle, or by a band or string passing round the pulley P. The ma chine is first filled with water, and after it has acquired a sufficient velocity, the water is thrown out by the centrifu gal force, and its place supplied by the pressure of the at mosphere necessary to balance the effect of the centrifugal force. When the pump is filled, or is at rest, the valves shown at B shut, and pevent the water from descending, but when the machine is in motion, these valves are of course open. In 1816, an improvement was made upon the centrifu gal machine by M. Jorge. It consisted in making the ver tical tube AB immovable, and in limiting the rotatory motion to the horizontal pipe or pipes. The advantages of this construction are, that the quantity of matter put in motion is diminished, and that the vertical tube may have any form, and any position which local circumstances may require. See Recueil des Machines de l'4cademie, 1732; and Hachette's Traite Elementaire des Machines, p. 136.
12. Description of Smeaton's Pump keeping up a constant head of water.
This ingenious pump forms a part of the apparatus by which Mr. Smeaton performed his experiments on the affects of overshot and undershot water wheels ; and the object of it was to furnish the wheel with the same quanti ty of water at each stroke. It is represented in plate CCCCLXX. Fig. 16. where ABCD is the reservoir, in which the water is to be kept at a constant height m n, so as to flow out uniformly at an aperture in any of its sides. The piston rod a b carries a cylinder a h, of such a size that the surface 02 n is pressed as much up by the descent of the portion a f into the water, as it would descend by following the piston b in its descent towards EF. The diameter of the cylinder a f must be a little less than that of the pipe CDEF, as a portion of the latter is occupied by the forked rod h b, which carries the piston. In con sequence of the surface in n remaining always at the same height, the efflux of the water at any aperture in the sides of the reservoir ABCD will be uniform.
IS. Description of a Pump with a Double Piston.
Pumps with two pistons have been used principally at sea and in fire-engines, as it is an object in both these cases to apply the power of as many men as possible placed near each other. An excellent pump of this kind is re presented complete in Fig. 17, where MN is the standard to which the machinery is fixed, AB the body of the pump, CD the rising pipe from which the water is received, and EF the main through which it is forced. A lever handle H a', moving round a fixed fulcrum f, carries the two pis ton rods'a' v b', a w b, to the ends of which are attached the pistons b', b. The rods v b', w b, are made to work equably and vertically by means of the wheels v, w moving between vertical guides, so that though the rods a w, a' v vary their inclination to a vertical line, the piston rods v b', so b work vertically in the main barrel AB.
When the handle H is at its lowest position, the piston b' is near the top of the barrel, and the piston b near the bottom. There are valves in both the pistons, and its
operation is obviously similar to that of a sucking pu-np, the power of it being doubled by the use of two pistons.
In a pump of this kind, described by M. Hachette, the piston rod of the lower piston passes through a collar of leathers in the upper piston, the piston rod being in both placed a little to one side of the centre of the piston. See Traite Elementaire des Machines, p.• 153. This con• trivance is the same with that used by M. Noble, in the hand pump he has made for the navy.
14. Description of a Three-Barrelled Pump.
The object of a three-barrelled pump is to keep up a continued current of water by the action of three pistons, one of which is at the bottom of its working barrel, while the second is in the middle of its barrel, and the third, at the top of its barrel, as shown in Fig. 18. In each of the three pumps, which are similar to that shown in Fig 5, AB is the working barrel, a b the piston rod at the valves, and c the circular orifice or section of the pipe (above D, w Fig. 5) through which the water is forced. The rising pipe from which the water is raised is shown at EF, and n is the suction chamber common to all the three valves.
Pumps of this kind were used by Mr. Smeaton, in the numerous water engines which he erected at London Bridge, Sheffield, and other places which required to be supplied with When the pumps are small, the barrels are generally made of brass, but when they are made on•a large scale, cast iron is used. Opposite to the aperture c of each barrel, there is a projecting neck or short pipe, covered at the end by a door, through which a work man can get access, for the purpose of repairing the valves. The valves used by Mr. Smeaton were of iron, and shut down upon hinges like a door, being covered with leather on the lower side. The centre pin of the hinge was placed back from the hole which the valve covers ; and it was also raised above the surface of the under side of the valve, so that the valve opens in some degree on that side where the hinge is, as well as on the other side. Hence obstruc lions arc less liable to be detained in the valve, and have less power to break its hinge. The hinge is fastened to the body of the pump by a screw passing through the me tal into the end or the hinge. The piston or forcer b of each barrel consists of three metallic plates, secured to the rod a b. The middle plate, which is turned as true as pos sible, is accurately fitted to the barrel, and the upper and lower plates are somewhat smaller. Two round pieces of leather, larger than the barrel, arc placed above and below the middle plate, and are held fast between it and the up per and lower plates. When these pieces of leather are forced into the barrels, they bend the one up and the other down, round the upper and under plates, so as to form two leather cups, which fit the barrel in the nicest manner, and will not permit any water to pass between them. When the piston b of the first barrel is raised, a vacuum is pro duced below it, and the pressure of the atmosphere forces the water up through the valve d. The descent of the same piston forces open the valve in the pipe at c, and drives the water up that pipe. While this barrel is forcing up the water through c, the next barrel is sucking it up during the ascent of its piston, while the third keeps up the action in the interval when the change of motion takes place between the two. If the pistons are properly work ed, by means of well-adjusted cranks, they tvill furnish a very constant stream of water.