ON HYDltAUJLIC MACHINES,

335

wholly intermitted, is, however, by no means uniform in its velocity.(Plate XXIII. Fig. 314. . 317.)

The pipes, through which water is raised, by pumps of any kind, ought tobe as short and as straight as possible ; thus, if we had to raise water to aheight of 20 feet, and to carry it to a horizontal distance of 100 by meansof a forcing pump, it would be more advantageous to raise it first vertically into acistern 20 feet above the reservoir, and then to let it run along horizontally,orfind its level in a bent pipe, than to connect the pump immediately with asingle pipe carried to the place of its destination. And for the same reasona sucking pump should be placed as nearly over the well as possible, inorder to avoid a loss of force in working it. If very small pipes are used,they will much increase the resistance, by the friction which they occasion.

i'

Water has been sometimes raised by stuffed cushions, or by oval blocks ofwood, connected with an endless rope, and caused by means of two wheelsor drums, to rise in succession in the same barrel, carrying the water in acontinual stream before them; but the magnitude of the friction of thecushions appears to be an objection to this method. From the resemblanceof the apparatus to a string of beads, it has been called a bead pump, or apaternoster work. When flat boards are united by chains, and employedinstead of these cushions, the machine may be denominated a cellular pump;and in this case the barrel is usually square, and placed in an inclined posi-tion, but there is a considerable loss from the facility with which the waterruns back. The chain pump generally used in the navy is a pump of thiskind, with an upright barrel, through which leathers, strung on a chain, aredrawn in constant succession; these pumps are only employed, when alarge quantity of water is to be raised, and they must be worked with con-siderable velocity in order to produce any effect at all. Mr. Cole has im-proved the construction of the chain pump, so as materially to increase thequantity of water raised by it. (Plate XXIII. Fig. 318.)

It is frequently necessary to procure alternate motion in pumps by meansof wheelwork, and for this purpose the application of a crank is the mostusual and perhaps the best method. Provided that the bar by which it actsbe sufficiently long, very little will be lost by the obliquity of its situation, and