THE SIPHOX. 253 



joiiJucted by a siphon over an elevation less than 10 m , 

 is for example a dyke, or a wall, but not over a moun- 

 tain. In the same manner, a siphon used 

 for transferring mercury must be somewhat 

 less in height than the column of mercury 

 which is supported by the atmospheric 

 pressure, that is, less than 76 cm . 



A siphon may be kept filled with liquid, 

 if both branches are precisely of equal length, 

 for the pressures at both ends are in this case 

 equal and act in opposite directions. But if 

 the siphon be ever so slightly inclined, as 

 in fig. 173 A, the downward pressure of the 

 liquid at a becomes less than that at &, be- 



cause b is below a, and since the upward pres- 

 sure of the air at a is now greater than at b, 

 the air ascends at a while the liquid runs out 

 a FlG ; l . 72 at b. The air enters slowly at first, but 



[^ real size). * ^ ' 



gradually more rapidly, because the difference 

 :he pressures at both ends evidently becomes greater 

 !he more the water recedes in a, in consequence of its 

 running out at b. But if the siphon has the form 

 hown at B, and it be inclined, some liquid will run 



