ON HYDRAULIC PRESSURE. 303 



the body forwards, in its own direction, is found to be very nearly pro- 

 portional to the height to which the jet would rise, if it were similarly in- 

 clined to the horizon. But when a plane is situated thus obliquely with 

 respect to a wide stream, the force impelling it in the direction of the stream 

 is somewhat less diminished by the obliquity, at least if we make allowance 

 for its intercepting a smaller portion of the stream: thus, if the anterior part 

 of a solid be terminated by a wedge more or less acute, the resistance, ac- 

 cording to the simplest theory of the resolution offerees, might be found bv 

 describing a circle on half the base of the wedge as a diameter, which would 

 cut off a part from the oblique side of the wedge that would be the mea- 

 sure of the resistance, tlie whole side representing the resistance to the sanie 

 solid without the wedge: but the resistance is always somewhat more than 

 this, and the portion to be added may be found, very nearly, by adding to 

 the fraction thus found one ten millionth of the cube of the number of de- 

 grees contained in the external angle of the wedge. (Plate XXI. Fig. 274.) 



The pressure of a fluid, striking perpendicularly on a plane surface, has been 

 found to be very different at different parts of the surface; being greatest at 

 the centre, and least towards the edges; so that if an aperture be made in 

 the centre of a circular plane, covering the mouth of a bent lube, the fluid 

 within it will rise half as high again as if the whole mouth were open. It h 

 also observable, that two bodies, equal and similar in the form of the part 

 meeting the fluid, undergo very <lifferent degrees of resistance according to 

 the forms of their posterior terminations, and that a thin circular plate is 

 much more retarded than a long cyHnder of the same diameter. These cir- 

 cumstances are utterly inexplicable upon the vague approximation of sup- 

 posing the resistance produced by the immediate impidse of separate particles 

 of the fluid on the solid ; but they are no longer surprising, when we consider 

 the true mode of action of continuous fluids, since all the motion which is j)ro- 

 duced by the fluid in the solid or by the solid in the fluid is communicated 

 much more by means of pressure than by innnediate impulse. The minute 

 operations of this pressure are too intricate to be accurately developed, but we 

 may observe in general, that when a body moves along the surface of a resisting 

 medium at rest, or when an obstacle at rest is opposed to a fluid in equable mo- 

 tion, the pressure is increased before the moving substance, and diminished be- 

 hind it; so that the surface is elevated at the one part, and depressed at the other, 



