229
Chap. 7.]
end is covered by a valve also opening upward. The pump rod is at-tached to the same end by a fork, as represented in the figure. By mo-ving the cylinder up and down, the air within it and the pipe is soon ex-pelled, and its place occupied by a portion of the water in which thelower end of the suction pipe is immersed. When the cylinder is thenraised the atmosphere forces up water into it, and when it is depressed,the water being prevented by the valve on the end of the pipe from de-scending into the well, escapes out of that on the top of the cylinder, pre-cisely as in the bellows pump. (p. 206.) By keeping water in the cistern,air is effectually prevented from entering between the pipes at the stuffingbox, even if it be not perfectly tight. A cup or dish formed on the upperend of the cylinder to contain a little water over the valve, would be anadvantage in this description of pumps, for any defects in it by whichair is admitted would be fatal, as a vacuum could not then beformed within the cylinder, and of course no water raised by it. Ourcommon pumps would be almost useless if water was not kept over thevalves; it is that which renders them air tight, and consequently efficient.
In the early part of the 18th Century, a new method of exciting the pres-sure of the atmosphere for the purpose of raising water was adopted. Itsdiscoverer burst the fetters with which long established fnodes of accom-plishing this object had embarrassed common minds. He left the old trackentirely, and the result of his researches was a philosophical machine thatbears no resemblance to those by which it was preceded.
^ Most people are practically acquainted with the principle of the Centri-fugal pump, viz. that by which a body revolving round a centre tendsto recede from it, and with a force proportioned to its velocity: thus mudis thrown from the rims of carriage wheels, when they move rapidly overwet roads ; a stone in a sling darts off the moment it is released ; a bücketof water may be whirled like a stone in a sling and the contents retainedeven when the bottom is upwards. A sailor on ship board, or a house-maid, dries a wet mop by whirling it tili the force communicated to thewatery particles overcomes their adhesion to the woolen fibres. Boyssometimes stick pellets of tough clay to the end of a switch or flexible rod,and then drawing it quickly through the air, the force imparted to the balls
sends them to their destination. If atube be substituted for the rod, andthe end that is held in the hand clo-sed, by a similar movement, ballsdropped or water poured into it,would be thrown forward in likemanner; and if by some arrange-ment the movement of the tube wasmade continuous, projected streamsof either balls or water might berendered constant: the centrifugalgun is a contrivance to accomplishthe one—the centrifugal pump theother.
This pump generally consists oftubes, United in the form of a crossor letter T, placed perpendicularlyin the water to be raised. (No. 95.)The lower end is supported on a pivot; perforations are made to admitthe water, and just above them a valve to retain it when the pump is notm motion. The ends of the transverse part are bent downwards to dis-