160 



WELLS'S NATURAL PHILOSOPHY. 



Fig. 151. 



^adually roll forward to c. 

 When W.1S the 



tube until it was turned half round, as in the 

 position, tig. 2, the marble would be at a'; now, 

 if at liberty to move, it would roll down to b'. 

 but this effect, which we have supposed accom 

 plished all at once, is really, gradually performed 

 and a rolls down toward 6' by the gradual turn 

 ing of the tube, and reaches 6' as soon as the 

 screw comes into the position marked in fig. 2 

 another half turn of the screw would bring it 

 into its first position, and the marble would 



359. The common suction-pump is a later discovery than the 

 screw of Archimedes, and is supposed to have been invented 

 by Ctesibius, an Athenian engineer who lived at Alexandria, 

 in Egypt, about the middle of the second century before the Christian era.* 



common pump 

 invented ? 



Pescribe the 

 constniotion of 

 the chain-pump 



Fig. 152. 



SCO. The chaiu-pump 

 consists of a tube, or cyl- 

 inder, the lower part of 

 which is immersed in a well or reser- 

 voir, and the upper part enters the bot- 

 tom of a cistern into which the water is 

 to be raised. An endless chain is car- 

 ried round a wheel at the top, and is 

 furnished at equal distances with flat 

 discs, or plates, which fit tightly in the 

 tube. As the wheel revolves, they suc- 

 cessively enter the tube, and carry the 

 water up before them, which is dis- 

 charged into the cistern at the top of the 

 tube. The machine may be set in mo- 

 tion by a crank attached to the upper 

 wheel. 



Fig. 152 represents the construction 

 and arrangement of the chain-pump. 



, . ^ ., The chain-pump will 



In wn.at sitna- '■ ^ 



lions is this act with its greatest ef 



Chnin-pump f . , ^^ cyWudCT 



generally used ? ' 



in which the plates and 



chain move, can be placed in an inclined 

 position, instead of vertically. It is used 

 generally on board of ships and in sit- 

 uations where the height through which 

 the water is to be elevated is not very great, as iu cases where the founda- 

 tions of docks, etc., are to be drained. 



* The Buction-pump, and other machines for raising -n-nter which depend upon the 

 pressure of the atmosphere, are described under the head of Pacumatics. 



\ i>S^~\^ N\^$$J;^S^iilMW 



