THE SYPHON. 



219 



water, and then placed with the short- 

 er arm in the vessel of water, A, the 

 weight of the column of water in the 

 longer arm, which is outside, will over- 

 balance the weight of the other col- 

 umn, and will therefore run out in a 

 stream. This tends to cause a vacu- 

 um in the tube, which is instantly fill- 

 ed by the water rushing up the short- 

 er arm, being driven up by the pressure of the atmos- 

 phere, A stream will consequently continue running 

 through the syphon until the vessel is drained. 



The syphon may sometimes be very usefully em- 

 ployed in emptying pools or ponds of water on high 

 ground, without the trouble of cutting a ditch for this 

 purpose. For instance, let a {Fig. 182) represent a 



Fig. 182. 



body of water which it is desirable to drain off; by 

 placing the lead tube, b c, so that the arm, c, may be 

 lowest, and applying a pump at this arm to withdraw 

 the air and fill the syphon with water, it will com- 

 mence running, and continue till the water has all been 

 drawn off. Difficulties, however, sometimes occur. If 

 the tube is small and very long, and the descent is 

 trifhng, the friction of the water in the tube may pre- 

 vent success. Water usually gives out small quanti- 

 ties of air, which collects in the higher part of the sy- 



