ON HYDRAULIC PRESSURE. 231 



surprising, when we consider the true mode of action of continuous fluids, 

 'since all the motion which is produced by the fluid in the solid or by the 

 solid in the fluid is communicated much more by means of pressure than 

 by immediate 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 motion, the pres- 

 sure is increased before the moving substance, and diminished behind it ; 

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

 and the more as the velocity is greater. Now it is obvious that the 

 pressure must be greatest where the elevation is greatest, and hence a 

 perforation at the centre of the surface indicates a greater pressure than at 

 the circumference. Behind the body this pressure becomes negative, and 

 has sometimes been called nonpressure ; hence it happens that a tube, 

 opening in the centre of the posterior surface, exhibits the fluid within it 

 depressed below the level of the general surface of the water. Thus, if we 

 suppose the velocity of a body, terminated by perpendicular surfaces, to be 

 8 feet in a second, it will require the pressure of about a foot, to produce 

 such a velocity, and we may, therefore, expect an elevation of about a foot 

 before the body, and an equal depression behind it ; consequently an 

 equivalent difference must be found in the pressure of the water at any 

 equal depths on the anterior and posterior surfaces of the body. The water 

 elevated before the body escapes continually towards each side, and the 

 deficiency behind is also filled up in some measure by the particles rushing 

 in and following the body : but there is in both cases, a certain quantity of 

 water which moves forwards, and constitutes what is called the dead water : 

 before, where it is usually most observable, it forms an irregular triangle, 

 of which the sides are convex inwards. If the posterior part of the body 

 be formed like a wedge, the water on each side will be advancing to fill up 

 the vacuity, even while it remains in contact with the sides, and the nega- 

 tive pressure will be considerably diminished. For this reason the bottoms 

 of ships are made to terminate behind in a shape somewhat resembling a 

 wedge ; and the same economy may be observed in the forms of fishes, 

 calculated by nature for following their prey with the greatest possible 

 rapidity. In general, fishes, as well as ships, are of a more obtuse form 

 before than behind, but it is not certain that there would be any material 

 difference in the resistance in a contrary direction, although some experi- 

 ments seem to favour such an opinion. Perhaps if the natural form of the 

 dead water moving before an obtuse body, were ascertained, it might serve 

 to indicate a solid calculated to move through the water with the least 

 resistance ; for the water must naturally assume such a form for its own 

 motions, and the friction of fluids on solids being less than that of fluids 

 moving within themselves, the resistance would be diminished by substi- 

 tuting a solid of the same form for a fluid.* (Plate XXI. Fig. 275.) 



Supposing a body to move through a fluid at a considerable depth below 

 



* Consult Russel, Trans, of the Roy. Soc. of Edin. vol. xiv. p. 47. 



