118 G
THEORY AND PRACTICE OF ENGINEERING.
Book II
edge O P of its frame* and the other, B M, upwards against Q R : when the pistonforces, the water presses the valve upwards, but with greater force, against the seg-ment HA, fig. 1822., than against HB: the valve then assumes a vertical position,and the water passes freely on each side the valve. On the other hand, the instantthe piston begins to descend, the valve ceasing to be supported by the ascending column,closes by its own weight, and the column above pressing with much greater forceon the segment HA than on II B, it is impossible that it can open : in order to givegreater superficies to the inner circle of the ring or plug than to the piston, the summit ofeach pump-barrel is enlarged to make way for the valve when open, and not to opposeany obstacle to the water; on which account the fork of the old pumps was suppressed,and their place supplied by a large receiver, as in fig. 1818., cast with the barrels ; thusthe water forced by the pumps unites in the receiver, and passes in a mass up the brand).
Manner of setting out the head of the Pump-barrel. — Divide the diameter A B into eightequal parts or modules; on the centre raise theperpendicular C D three modules high ; throughthe point D draw the line H G parallel to thediameter A B, and from the point D as a centre,with the radius 1) A or D B, describe the arcsA E and B F. The rebates E II and F G mustbe one module wider than the thickness of thepump-barrel: to trace the profile of the head,draw the rectangle IZLK, whose base IKis eleven modules, and whose height IZ istwo; on the line I K draw another rectangleM T X N, whose base M N is equal to thediameter A B of the pump-barrel, and whoseheight INI T is six modules. Divide the line Z Linto three equal parts, as at QR, and fromthese points as a centre describe the arcs ZTand L X; prolong the perpendiculars M T, N Xtwo and a half modules to the height T V, X Y.
Figs. 1827. 1828. 1829. represent the piston of this pump : its construction is based uponfour principles ; first, that it should have an aperture sufficiently large to allow water enoughto p;iss to fill the barrel at each stroke ; secondly, that the valve in the piston should openand allow the water to pass freely, and yet itself close firmly ; thirdly, that the axis of thepiston should always be vertical, notwithstanding the obliquity given to the rod by themotion of the crank or balance beams, so as to avoid any straining, and to prevent thepistons wearing on one side more than on another ; fourthly, that the leather which causesthe adhesion of the piston to the inner surface of the barrel should be so disposed as to lasta long time without requiring reparation.
This piston consists of a cast-iron box, I C DK, figs. 1827. 1828. 1829., serving as a nucleusto a number of leather rings, G, H, pressed against one another, having a flange E F fortheir base: at the summit D is a screw furnished with a nut, A B, to bring the leatherrings together. To the piston a swing valveis attached, similar to that previously de-scribed. The base of the box has two ears,
I, K, traversed by a bolt, LM, which securesthem to the fork of the piston-rod.
The suction piston is entirely on the sameprinciple as the preceding, figs. 1830. 1831.
1833., only the flange AB must be at theupper part as well as the ears C, D, whichattach the piston to the rod The ring of thevalve is let into a rebate, and attached tothe flange with four screws.
The various methods of raising water bymeans of pumps may all be classed underthree heads. The first, when water is raisedfrom a depth to the surface of the earth,which may be performed by employing asmany suction pumps as may be necessary.
The second, when water is to be raised to agreat height above the surface, either verti-cally or on an inclined plane ; in this caseforcing-pumps are employed. The third,when the water is to he raised from a depth