CATALOGUE. — THEORY OF HYDRAULICS, HYDRAULIC PRESSURE. 229 



resistance of the flat side of a hemisphere moved in air to 

 that of the convex side 2.45:1. Ed. tr. ; tothat of the whole 

 globe as 2-1: 1. Math. diet. Whence we might conclude, 

 that the resistance of a globe is greater than that of a hemi- 

 sphere. This,- however, cannot be the fact. 



The experiments of the society for naval architecture con- 

 tain some valuable remarks on the difl'erent efi'ects of the 

 form of the different parts of a body moving in water. The 

 form nearest to the shape of a fish appears to move with 

 least resistance. Soaked planks were more resisted than 

 planks not soaked. 



A furfacc of 4S square feet, moving in its own direction 

 with a velocity of I nautical mile per hour, produced a 

 friction of .563 pounds, with a velocity of 2 miles 1.992, 

 of 4, 6.642, of 6, 12.839, of 8, 19.856. 



The direct resistance appeared to vary as k''"*. The 

 same body having prows differently inclined, the resistance 

 at different angles was thus. 



Hutton makes the resistance -JjCi)'-"^ i '•«<'<■, c being 

 the cosine, s the sine. Thus, for 32 square inches or | 



foot moving 13 feet per second, .840' ■''"f. 



Calculations from the resistance to plane surfaces make 

 the resistance to curved surfaces in general greater than ex- 

 periments; the particles gliding more easily along these 

 surfaces. The resistance calculated from the curves in 

 which the particles of a fluid were observed in Sir Charles 

 Knowles's experiments to move, was ,^ less than the ob- 

 served resistance ; perhaps on account of the adhesion, 

 which was not calculated. A quadruple velpcity had no 

 effect on these curves. Robison. 



In Bossut's experiments, the resistance was nearly (cos. 



x)'-f3.i53( — ) , jr being the complement of half the 



angle at the prow, in degrees. Robison. 



The direct impulse of the wind on a surface of a square 



foot in pounds is nearly ^— , k being the velocity in a se» 



cond in feet ; or more nearly .00229t)', when v is 10, 

 .00228711^ when loo. In grains I6u'. Robison, 



Remarks on the Resistance of Fluids. By Dr. Yot-'^o. 

 See Journ. R. I., II. 14, 78. 



The first approximation to a determination of the eflfect 

 of the resistance to a body of a given section, terminated by 

 eblique planes, is to suppose each patticle of the fluid to 

 impinge once on the surface, and then to retire for ever : on 

 this supposition, the resistance ought to vary as the square 

 of the cosine of the angle of intidence. 



Another part of the resistance is occasioned by the adhe- 

 sion of the particles of the fluid'; this m;iy be supposed to 

 vary, as the product of the secant and the sine of the angle 

 of incidence ; that is, as its tangent. This portion appears 

 in fact to be but small, it may however be taken into con- 

 sideration, in order to facilitate the computation. 



A third part depends on the form of the posterior surface 

 ef the btxly, and upon the unknown irregularities produced 

 in the motions of the particles of the fluid, by the difference 

 of the forms of its anterior part. It may be expected, that 

 this negative pressure will be nearly liniform, when the 

 shape of the posterior part of the body remains unaltered, 

 as in Bossut's experiments ; but that, when a thin surface 

 is employed, as in Mr. Vince's apparatus, it will be some- 

 what diminished by the obliquity of that surface, even sup- 

 posing the trans\erse projection of the surface to remain 

 tmaltercd. This portion, however, may naturally be ex- 

 pected to be liable to great irregularities ; and it appears to 

 be somewhit increased, when the thin surface is inclined 

 in a small angle only. 



Mr. Romme has remarked, that the facility, with which 

 the particles of the fluid can escape before the moving body, 

 is proportional to the angular space of the fluid which re- 

 mains open to admit them, and that therefore the resistance 

 must vary in proportion to this angle. Without allowing 

 the truth of the observation in its whole extent, we may 

 with propriety inquire, whether or no the portion of the 

 pre-ssure derived from impul.se may not in part depend on 

 some simple function of the angle of incidence; and whe- 

 ther the whole resistance to an oblique surface may not be 

 considered, as comjioscd of a constant portion, a portioa 



