THE SIPHON. 



265 



Fig. 71 



Let A and B be two vessels containing 

 water, B being on the lower level, and 

 ACB a bent tube. Suppose this tube to 

 be filled with water from the vessel A, 

 and to have its extremities immersed*in 

 the water in the two vessels. The water 

 will then flow from the vessel A to B, as 

 long as the level B is below A, and the 

 end of the shorter branch of the siphon is 

 btlow the surface of the water in the 

 vessel A. 



The atmospheric pressures upon the surfaces A and B 

 tend to force the water up the two branches of the tube. 

 When the siphon is filled with water, each of these pressures 

 is counteracted in part by the pressure of the water in the 

 branch of the siphon that is immersed in the water upon 

 which the pressure is exerted. The atmospheric pressures 

 are very nearly the same for a difference of level of several 

 feet, owing to the slight density of air. The pressures of 

 the suspended columns of water, however, will for the same 

 difference of level differ considerably, in consequence of the 

 greater density of water. The atmospheric pressure opposed 

 t to the weight of the longer column will therefore be more 

 resisted than that opposed to the weight of the shorter, 

 thereby leaving an excess of pressure at the end of the 

 shorter branch, which will produce the motion. Thus, 

 draw the vertical line DEO, let h denote the height of the 

 water barometer, k the area of a section of the tube, and w 

 the weight of a unit of volume ; then the water at the point 

 C is urged from left to right by a force 



= wlcli wlc x EC ; 

 and it is urged from right to left by a force 

 = wkh wk x DC. 



