1182
THEORY AND PRACTICE OF ENGINEERING.
Book II.
anil the leather fixed between them by means of several iron bolts fitted with screws : thus thepiston consists of a kind of purse,which turns itself at every stroke ofthe piston. At the bottom of thispurse is a hole, L, covered with avalve, K, which, when raised, restsagainst the handle M1YM, to which arod, N, is attached, serving to raiseand lower the piston; for this rodthere is another hole in the bottom ofthe upper box, corresponding with thepipe in which the rod N passes; thishole is slished, to allow the plate totouch the top O Q when the pistonascends; the lower box has also a holecorresponding with the suction pipe,covered by a valve, I, as usual: whenthe piston is raised the water opensthe valve Rand passes into the vacuum formed to the height of 4 inches, which is theplay the piston ought to have so as not to weaken the leather: when the piston descendsthe valve I closes, the other, K, opens, and the water contained between the bottom, TV,and the leather, passes through the hole L, enters the space O P Y Z Q, and thence is forcedup the ascending pipe ; thus the piston, always floating between two waters, will have nofriction : when the leather is good, it may be worked continually for three or four monthswithout any repair, as has been proved in the mines of Bretagne .
The sole defect in this piston is, that, whatever be the size of the discharge-pipe, thepower is always loaded with the weight of a column of water having for its base the circle() Q, and for its height the elevation of the reservoir above its source : it is true thediameter of the pipe may be increased, and that of the piston diminished, in order that thesebeing equal, the power may not be loaded with a greater weight than would naturally beraised : the play of the piston being small, it will raise but little water at each stroke; thestrokes may, however, be more frequent. If the suction-pipe and the piston-rod are each25 feet long, the water may be raised 50 feet above the source at a very small expense.
On Valves -The different valves in use may be classed under four kinds, the cup valve,
conical valve, spherical valve, and clack valve; the three first are of copper.
Fig. 1805. shows a cup valve E, at the bottom of a pump-barrel ; the stalk, accompaniedby two leather roundles, is fixed between the flanges of the barrel and those of a cup I K,to which the suction-pipe, LM, made of lead, is soldered : the valve E fits the cup B C,and G II represents the support of the ring in which the rod F plays.
The upper fig. 1805. represents a section of the same valve, in order to show its defects,which are decidedly great: the superficies of the great circle E diminishes the passageof the water, as it can only escape through the space between the circumference of the circleB C and the surface of the suction-pipe, whereas the water forced by the piston should havea free passage equal to the circle of the barrel, in order that it may not pass more rapidlyinto one part than another; otherwise the power will be forced to make a greater effort thanif the water were not confined.
To give more facility to the water in ascending, it is only necessary to diminish the circleE; but this cannot be done without also diminishing its lower surface, or its equalG H, consequently, without confining the passage of the water through the cup B C : all thatcan be done is to regulate the upper and lower diameter of E, so that the water, in traversingthe cup and passing round the valve, should be as little confined as possible ; therefore thesuperficies of the circle G H must be equal to the ring causing the difference between thesuperficies of the upper and lower surfaces of E. Of whatever size the barrel is, to proportionthe valve, we must divide the radius of the barrel into five equal parts, and take three forthat of the small circle of E, and four for that of the great circle: due considerationhaving been given to the height which best suits the convex surface of the valve, it appearsthat its side should be one-fourth the diameter of the upper circle, and the breadth ofthe rest one-eighth ; the angle which it makes with the side will then be of 60°, because thesection of the valve will form a trapezium drawn by an equilateral triangle, having theupper diameter for its base.
The water forced from the barrel up the branch will be greatly confined in traversinga very narrow passage; it will in its course meet the under part of the cup, and thecircle ND which causes it to rebound, and it will only be raised by an extraordinaryforce, besides which it will be pressed in a direction obliquely to the surface of thepipe. The power must be twelve times greater than if the water mounted with a uniformvelocity. This valve is not applicable to forcing-pumps, if placed at the bottom of thebranch, but may be used at the bottom of the barrel : for if the greatest height of the
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