ON SIPHONS AND JETS OF WATER. 401 



cerncd in this experiment, by attending to the mutual effects of the 

 water and of the atmosphere. The water entering the orifice must 

 immediately acquire a velocity equal to that of the whole water in 

 the pipe, oti 'here would be a vacuum in the upper part of 



the pipe, which the pressure of the atmosphere will not permit; 

 and this pressure, considered as a hydrostatic force, is equal to that 

 which would be derived in any other way from a column of the 

 same height with the pipe, since the weight of the water in the pipe 

 is wholly employed in diminishing the counterpressure of the at- 

 mosphere below, not only in the beginning, when it is at rest, but 

 also while it is in motion ; for that motion being uniform through- 

 out its descent, the power of gravitation is expended in producing 

 pressure only; so that the pressure of the almosphere on the water 

 in the f\innel becomes completely analogous to the pressure of a 

 reservoir of water, of the same height with the pipe. The circum- 

 stance, which causes the appearance of paradox in this experiment, 

 exists also in the simplest case of the discharge of water ; for it 

 may be shown, that the portion of accelerating force actually em- 

 ployed in generating the velocity with which a stream is discharged 

 through a small orifice, is twice as great as the pressure of the fluid 

 on a part of the vessel equal in extent to the orifice ; and in the 

 same manner the quantity of force exerted by the atmosphere on 

 the water in the funne), as well as that with which the descending 

 fluid impels the air below, is equal to twice the weight in the quan- 

 tity existing at any time in the pipe. 



There is, however, a limit, which the mean velocity in such a 

 pipe can never exceed, and which is derived from the magnitude of 

 the pressure of the atmosphere. For the water cannot enter the 

 pipe with a greater velocity than that with \vhicb it would enter an 

 exhausted pipe, and which is produced by the whole pressure of the 

 atmosphere ; and this pressure being equivalent to that of a column 

 of water 34 feet high, the velocity derived from it is about 47 feet 

 in a second : so that if the vertical pipe were more than 34 feet 

 long, there would be a vacuum in a part of it near the funnel. 



Wherever a pipe of considerable length descends from a funnel, 

 if the supply of the fluid be scanty, and especially if it approach 

 the orifice obliquely, the pressure of the atmosphere, and the cen. 

 trifugal force of the particles which must necessarily revolve round 



VOL. in. 2 D 



