Chap. XVII.

ON PISTONS.

1179

is for each man 87*85 cube metres of water raised 1 metre: another pump 2-14 millimetres,(.or 96 inches English, ) worked by the same number of men for the same time, raised 470*04cube metres of water 3*573 metres in height, which gives for the useful effect furnished byeach man in 24 hours 79*97 cube metres of water raised a metre. 'Hie product of pumpsvaries much in consequence of the manner in which the piston is connected with theair-tube, but they are now brought to great perfection by the manufacturer, and it isfound that those of the greatest diameter are the most economical for raising water fromfoundations.

Engine for raisingWater from T)ykes iscontrived like a fire-engine with a doublehandle, working twopistons in barrels at-tached to an uprightpipe: after the wateris thrown up in thetrough above, it canbe poured out in anydirection required; themechanism and barrelsresemble some ofthose already treatedon.

Pistons and Valves.

— The pistons ingeneral use are oftwo kinds, solid orplungers, and piercedor buckets; both arecommonly of wood:the chief inconve-nience of woodenpistons is that whenpierced with holesthey are greatlyweakened, especiallyif sufficiently large toallow the water a free

passage, without which there is considerable resistance; and if a stroke of 6 feet be made intwo seconds, as in the machine at Frene near Conde, it is essential that the whole machineryshould work with the most perfect ease, otherwise the effort of the moving power tends tothe destruction of the machine. To avoid this, care must be taken that when the bucketdescends, its own weight should suffice to compel the water at the bottom of the barrel topass through the hole in the time necessary for its descent; and as this time is determinedby the velocity of the machine relatively to that of the moving power, it will be seenthat it depends on the quantity of w*ater sucked by the bucket at each stroke, and the sizeof the passage to be traversed.

Suppose, fig. 1776., a pump-barrel, AB, 8 inches in diameter inside ; that the stroke ofthe piston is 6 feet, and that it is made in two seconds; it will raise at each stroke about74 pints of water, which should pass through the hole Z in the time of the piston’s descent;with what weight ought it to be loaded in order to force the water, so that it may in twoseconds pass through the hole, which we suppose 3 inches in diameter, the greatest whichcan be given to it without weakening the bucket too much? The quantity of water whichwill pass through the piston in a given time will depend on the size of the hole and thevelocity communicated to it by the weight with which it will be loaded: the problem istherefore to ascertain what height of water must be given to a reservoir pierced at thebottom by a hole 3 inches in diameter, so that 74 pints may issue in two seconds.

If the piston weighed less than the column of water, the hole must be increased to makeup for the small velocity of the water arising from a want of sufficient pressure and thesuperficies of the two holes, and the velocity of the water which must pass must formfour terms reciprocally proportional. But as the weight spoken of may be expressed bycolumns of water whose base is the circle of the piston, and the square roots of the heightof these columns expresses the velocity of the water, we may substitute the square roots ofthe weight with which the piston is loaded without fear of error.

As wooden pistons are not very convenient, since we cannot pierce them with a hole ofsufficient size without endangering their solidity, one of the description of the fig. repre-

Fig. 1795.

ENGINE FOR RAISING WATER PROM DYKES.