416 Hydrodynamics. 



521. The velocity of the water flowing from the sucking 

 pipe into the barrel should be equal to the velocity with which 

 the piston moves. For if it be greater, less work will be done 

 than the pump is competent to effect ; and if it be less, a vacuum 

 will be produced below the piston, which will therefore be moved 

 upwards with great difficulty. 



Of the Syphon. 



Fig. 250. 522. I F we introduce into a vessel of water or other liquid, 

 the shorter branch of a recurved tube DEF, called a syphon, and 

 exhaust the air from this tube by the mouth or otherwise, the 

 water will rise in the tube and flow out at F, until the surface of 

 the fluid in the vessel shall have descended to the opening I) of 

 the syphon. 



The reason of this phenomenon is, that when the contained 

 air is withdrawn, the pressure of the interior air upon the surface 

 JIE causes the fluid to rise into the syphon arid to flow through 

 the branch EF. And although when the flowing has commenced, 

 the air presses the fluid at the point F with a force very nearly 

 equal to that which is exerted upon the surface of the water in 

 the vessel, still a transverse lamina at F is urged downward by 

 the entire column of water 7F; this column must therefore de- 

 scend ; but in descending it tends to form a vacuum at /, which 

 cannot fail of being filled by the action, always exerted, of the 

 incumbent air upon the surface of the fluid in the vessel. 



It will hence be seen that during the discharge through the 

 syphon the air acts only with an effort proportional to the differ- 

 ence of level IF between F and the surface of the water in the 

 vessel ; so that the flowing will be so much the more rapid 

 according to the difference of the two branches of the syphon ; 

 thus if F and D were on a level, no motion of the fluid would take 

 place. We say generally, that the branch EF must be longer 

 than the branch ED ; but in using this language it must be under- 

 stood that the vertical height of E above F must be greater than 

 that of above D. The absolute height is not concerned, DE may 



