304 LECTURE xxr. 



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 witliin 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, there- 

 fore, expect an elevation of about a foot before the body, and an equal de- 

 pression 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 tlie 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 negative pressure will be considerably diminished. For this 

 reason, the bottoms of ships are made to terminate behind in a shape some- 

 what resembling a wedge ; and the same economy may be observed in the 

 forms of fishes, calculated by nature for following their prey with t.he great- 

 est 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 ma- 

 terial 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 indi- 

 cate 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 substituting 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 



