or further development of this is the chain pump or bucket-purnp, in which the inclined trough is replaced by a ver tical or nearly vertical pipe of circular cross-section, and in which the place of the slats is taken more perfectly by discs or by balls, forming in the latter case what is known as a " paternoster " pump," from the resemblance of the chain or rope and the balls to a rosary with its beads. In the disc of the chain-pump we find outlined the piston of many of on r modern reciprocating--piston pumps.
The Archimcdcan Screw (p1. r r2, fig. 4) consists of a spiral conduit formed either of a pipe wound about a central drum, or of a cylinder having a spiral partition. The drum or cylinder being inclined and hav ing one end dipping in a source of water, continuous rotation in the direc tion in which the spiral runs around the axis in coining from above down ward drives the water around and upward, so that it is discharged at the upper end of the spiral conduit. In this case the piston which forces the water along in the spiral passage is the water itself. The screw is a con tinuous scoop.
the devices above described suffice for raising liquids only comparatively short distances vertically, and for transporting them only short distances laterally by no other means than gravity after they have been raised. Where it is desired to transport liquids very long distances by other means than gravity, there are generally employed machines com posed of a pipe-conduit and a mechanism which, while it may or may not raise the liquid from a distance not exceeding a theoretical maximum of about 33 feet, forces it under pressure through the conduit. Such mech anisms for transporting liquids are called " pumps," and consist either of a reciprocating piston working with alternating lengthwise motion in a cylindrical pump-barrel, or of one or more rotating pistons working in a case, these being combined with a pipe system through which the fluid is drawn into and forced out of the pump.
A pump may lift its fluid from a lower level, discharging it at its own level, or may take it from its own level and raise it to a higher, or may do both. Instead of raising it to a higher level, it may do what is the same thing as regards the work done and the method of doing it—namely, it may force it•to a considerable lateral distance against the resistance offered by friction, or it may compress it into a vessel against a considerable pressure.
Sin; Suction ordinary suction or lifting piston-pump (p1. IT 3, fig. r) has a vertical cylinder or barrel in which there plays air-tight but easily a disc or piston in which is a valve that permits the fluid to pass upward, but not downward. When this bar
rel is placed in air-tight communication with a source of liquid directly below it at a distance of only a few feet, raising the piston from the bottom of the barrel to the top will form a vacuum, which the liquid below rises to fill. If there be at the bottom of the barrel a valve similar in action to that in the piston, this will permit the liquid to rise in the barrel, but not to flow back. Depression of the piston will cause the liquid to pass through the valve in the piston without rising in the barrel; but on the next up stroke, when the valve in the piston is closed, the piston will raise the water in the barrel and force it out of the top. The pipe through which the water is raised is called the "suction-pipe;" the orifice through which it is discharged is called the " discharge opening," and is sometimes arranged in spout-like form. The lower valve through which the water is admitted to the pump-barrel below the piston when the piston is raised is called the ''suction-valve," and the valve in the piston through which the water passes on the down-stroke is called the " discharge-valve." It is best to have at the bottom of the suction-pipe a strainer to keep out matters which would be apt to clog the valves or destroy the pump; and it is well also to have at or near the foot of the suction-pipe a valve opening in the same direction as the suction-valve of the pump. This is called the " foot-valve " or " check-valve." and may be developed into a single-acting suction- and force-pump by closing in the top of the barrel, providing a stuffing-box for the piston-rod, and making the discharge through a pipe connected at the discharging orifice. Instead of having a valve in the piston, either the lifting- or the forcing-pump may have at its base, besides the suction-valve, another which opens away from the barrel, so that on the up-stroke the water rushes into the barrel to fill the vacuum, and on the down-stroke it passes through the outward opening valve into the open air or into the pipe system. The single-acting forcing-pump which has a valve in its piston will, if the resistance in the pipe-system is greater than the work of lifting the fluid, be harder to work on the up-stroke of the piston than on the down-stroke. Where the pis ton is solid there will be more work, tinder the same circumstances, on the down-stroke than on the up-stroke of the piston.