OF HYDRAULICS. 



125 



A syphon, generally used for decanting liquor, is a 

 bended pipe, whose legs are of unequal lengths ; and the 

 shortest leg must always be put into the liquor intend- 

 ed to be decanted, that the perpendicular altitude of 

 the column of liquor in the other leg may be longer than 

 the column in the immersed leg, especially above the 

 surface of the water. For if both columns were equally 

 high in that respect, the atmosphere, which presses as 

 much upward as downward, and therefore acts as much 

 upward against the column in the leg that hangs without 

 the vessel as it acts downward upon the surface of the 

 liquor in the vessel, would hinder the running of the li- 

 quor through the syphon, even though it were brought 

 over the bended part by suction. So that there is no- 

 thing left to cause the motion of the liquor, but the su- 

 perior weight of the column, in the longer leg, on ac- 

 count of its the greater perpendicular height. 



Let D be a cup filled with water to C, 

 and A B C a syphon, whose shorter leg 

 B C F is immersed in the water from C to 

 F. If the end of the other leg were no 

 lower than the line A C, which is level with 

 the surface of the water, the syphon would 

 not run, even though the air should be 

 drawn out of it at the mouth A. For al- 

 though the suction would draw some water 

 at first, yet the water would sfop at the 

 moment the suction ceased ; because the air would act 

 as much upward against the water at A, as it acted 

 downward for it by pressing on the surface at C. But 

 if the leg A B comes down to G, and the air be drawn 

 out at G by suction, the water will immediately follow, 

 and continue to run, until the surface of the water in 

 the cup comes down to F; because, till then, the per- 

 pendicular height of the column BAG will be greater 

 than that of the column C B ; and consequently, its 

 weight will be greater, until the surface comes down 



LECT. 

 v. 



