ON HYDRAULIC PRESSURE. 301 



When a part of the weight of any fluid is expended in producing a motion 

 in any direction, an equal force is deducted from its pressure on the vessel 

 in that direction: for the gravitation, employed in generating velocity, cannot 

 at tlie same time be causing pressure; and when the motion produced is in 

 any other direction than a vertical one, its obliquity must be immediately de- 

 rived from the reaction of the vessel, or of some fixed obstacle; for it is ob- 

 vious that a vertical force, like that of gravity, cannot of itself produce an 

 oblique or a horizontal motion. 



If a small stream descends from the bottom of a vessel, the weight expend- 

 ed in producing its motion is equal to that of a column of the fluid standing 

 on a base equal to the contracted orifice, and of twice the height of the 

 vessel. Thus, if the vessel be 16 feet high, the velocity of the stream will 

 be 32 feet in a second, and a column 32 feet in length will pass through the 

 orifice in each second, with the whole velocity derivable from its weight 

 acting for the same time; so much, therefore, of the pressure of the fluid in 

 the reservoir must be expended in producing this motion, and must of course 

 be deducted from the whole force with which the fluid acts on the bottom of 

 the reservoir; in the same manner as when two unequal weights are connected 

 by means of a thread passing over a pulley, and one of them begins to des- 

 cend, the pressure on the pulley is diminished, by a quantity, which is^as much 

 less than the sum of the weights, as the velocity of their common centre of 

 gravity is less than the velocity of a body falling freely. If the stream issue 

 from the vessel in any other direction, the effect of the diminution of the 

 pressure in that direction will be nearly the same as if the vessel were sub- 

 jected to an equal pressure of any other kind in a contrary direction; andif 

 the vessel be moveable, it will receive a progressive or rotatory motion in 

 that direction. Thus, when a vessel or pipe is fixed on a centre, and a stream 

 of water is discharged from it by a lateral orifice, the vessel turns round at 

 first with an accelerated motion, but on* account of the force consumed in 

 producing the rotatory motion, in successive portions of the water, the velocity 

 soon becomes nearly stationary. (Plate XXI. Fig. 272.) 



From similar reasoning it appears, that the effect of a detached jet on a 

 plane surface perpendicular to it must be equivalent to the weight of a portion 

 of the same stream equal in length to twice the height which is capable of pror 



