THE SIPHON. 251 



is the surface of the water in a. The considerable 

 pressure exerted by the atmosphere upon the surface 

 jf the water is^transmitted downwards, and in the 

 interior of the liquid the pressure increases, so that 

 it the extremity of the immersed branch of the tube 

 the pressure is somewhat greater than at the surface. 

 The water is there pressed upwards into the tube. 

 Within the tube the pressure at first gradually de- 

 creases ; at a it is equal to the pressure outside, that 

 is, to the atmospheric pressure ; at b it is less, and from 

 ] ) to c it remains the same ; but from e downwards it 

 increases, and at d it is again equal to the external pres- 

 sure. The pressure of the water and of the air at d 

 are thus in equilibrium; no water will leave the tube, 

 nor will the air enter it. If the tube is somewhat 

 raised, so that d is higher than the surface of the 

 water, the pressure of the water at d becomes less than 

 that of the air, the latter overcomes the former and 

 air enters the tube, while the water is driven back; if, 

 on the contrary, the tube is lowered in the water so 

 that d is lower than a, the pressure of the water at d 

 is greater than that of the air, and water begins to flow 

 out of the tube until the surface of the water in the 

 ivessel is again at the same height as d\ the pressure of 

 the air upon the surface of the water continually drives 

 \v uter into the tube to replace that w r hich has run out 

 at d. 



A tube about 4 mm wide, and from 25 cm to 30 cm long, is bent in the 

 shown in fig. 171 ; the ends should be made smooth and 

 straight before bending the tube. It may be filled by immersing it 

 completely in a capacious vessel full of water, or the tube is held 

 *vith the open ends upwards, and water is poured into the longer 

 oranch until the shorter is filled ; the end of the shorter branch is 



