106 PNEUMATIC COCK. 



perforations, ABC. DEF. which are opened or shut by 

 turning the key, in the same manner as the single perfora- 

 tion of the common cock. When this newly invented in- 

 strument, which for the sake of distinction may be termed 

 a pneumatic cock, is inserted into an air-tight cask containing 

 a fluid, and the key properly adjusted, the air enters at the 

 upper perforation, and the fluid passes out at the lower one, 

 with a velocity proportionate to the depth of the delivering 

 orifice of the cock at F, below the point C, at which the 

 air enters the cask. It may be worthy of observation, that 

 as long as there is a sufficient quantity of fluid in the cask, to 

 cover the orifices C, D, of the pneumatic cock, the velocity 

 of its efflux will be always equable, not being dependent 

 on the height of the fluid in the cask, but invariably propor- 

 tionate to the depth of the point F, at which the fluid is emit- 

 ted from the cock, below the point C, at which the air enters 

 the cask. Hence if it be desired to augment the velocity of 

 efflux, it may be effected by encreasing the length of the 

 nozzle F. — For if a line be supposed to be drawn from the ori- 

 fice C of the upper perforation, to the surface G of the fluid in 

 the cask, and another be imagined to be let fall from the sur- 

 face of the fluid, to D the orifice of the lower perforation, and 

 from thence to be extended through the lower perforation to 

 the nozzle of the cock at F, these lines may be considered as 

 marking out the courses of two unequal columns of the fluid 

 acting on each other as if contained within the legs of a 

 syphon. Consequently the shorter column will be displaced 

 by the longer one, with a velocity proportionate to the excess 

 of the perpendicular depth of the latter, which is obviously 

 the same with the perpendicular depth of the nozzle F, below 

 C, the point at which the air enters the cask. But the velocity 

 with which the longer column G, D, E, F, displaces the 

 shorter column C, G, or with which it enables the air to dis- 

 place it, and with which it is itself thereby enabled to descend 

 through the lower perforation to the point of emission, must 

 evidently be the same with the velocity of* efflux. Of course 

 this last must be in proportion, to the excess of the perpendicu- 

 lar depth of the longer column, which is the same with the 

 depth of F, below C. 



