244 



MACHINERY IN CONNECTION T\'ITH AIR. 



and down hill, the barometer might become of much 

 practical value. 



THE SYPHON. 



The syphon operates on a principle quite similar to that 

 of the pump ; but, instead of pumping out the air of the 

 tube through which the water rises, a vacuum is created 

 Fig. 26C. by the weight of a column of water, in 



the following way : Fig. 26G represents a 

 syphon, which is nothing more than a 

 tube bent in the form of a letter U in- 

 verted. Nov/, if this be filled through- 

 out with water, and then placed with the 

 shorter 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 column, 

 and will therefore run out in a stream. This tends to cause 

 a vacuum in the tube, which is instantly filled by the water 

 rushing up the shorter arm, being driven up by the press- 

 ure of the atmosphere. A stream will consequently con- 

 tinue running through the syphon until the vessel is 

 drained. 



The syphon may sometimes be very usefully employed in 

 emptying pools or 

 ponds of water on 

 high ground, with- 

 out the trouble of 

 cutting a ditch for 

 this purpose. For '"^^^^.x-.v 

 instance, let a (fig. 267) represent a body of water which 

 it is desirable to drain ofi*; by placing the lead tube, h c, 

 so that the arm, c, may be lowest, and apj^lying a pump 

 at this ai-m to withdraw the air and fill the syphon with 

 water, it will commence running, and continue until the 



Fig. 2G7 



