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EXPERIMENTAL GENERAL SCIENCE 



this is only sufficient to force water up to a height of about 

 34 feet. The pumps which raise water to much greater heights 

 are called force pumps. In such pumps, the upward stroke of 

 the piston causes more water to rise in the pipe, and the down- 

 ward stroke closes the valve and forces the water to flow 

 through another pipe to higher levels. Even in this type of 

 pump, the valves and plunger must be located near enough 

 to the surface of the liquid to take advantage of the air 

 pressure. A siphon is a device 

 for lifting liquids against gravity 

 with the aid of air pressure. 

 It is simply a bent tube with 

 one end in the liquid to be 

 siphoned and the other extend- 



FIG. 44. Siphon. (Duff.) 



FIG. 45. Water does not run out 

 when tumbler is inverted. (Touvr, 

 Smith and Turton.) 



ing outside to a lower level. When the air is removed from the 

 tube, either by pumping it out or by filling the tube with the 

 liquid, the liquid in the vessel will run out, though it may have 

 to rise a considerable distance to do it. The air pressure on 

 each end of the tube is, of course, practically the same, but 

 since the outer end of the siphon is always longer and lower, 

 it contains a greater weight of water which, beginning to run 

 out, tends to form a vacuum at the bend, but this is immedi- 

 ately filled by more water which is pushed up by the air 

 pressure over the liquid in the vessel, and so the siphon con- 



