FLU 



practice ; because it supposes first, that the medium in which the body' 

 moves falls in behind the body in motion, as fast as this moves forward, 

 which is not the case, except the velocity is very small ; and secondly, 

 that the particles are so constituted, that after the body strikes them 

 their action entirely ceases j whereas the particles, after they are struck. 

 must necessarily diverge, and act upon other particles behind them. 

 Hence will arise some difference between theory and experiment. 



1. Required the resistance to a plane, moving- in a fluid, in a direction 

 perpendicular to its surface. 



Let a area of the plane ; v its velocity, w its weight, S the density 

 of the fluid; g 32% feet. R the resistance, R' the retarding- force, 

 then 



T? - 



~ 



Cor. If the body be a cylinder (rad. r) moving in the direction of its 

 axis, 



2. If the direction of motion be not perpendicular to the face of the 

 plane, but inclined to it at any angle fl, the resistance perpendicular to the 

 plane, is 



a 3 sin.g fl 



2# 

 And the resistance in the direction of its motion, is 



a 1 1> sin.s 9 

 g ' 



And in a direction perpendicular to that of its motion, is 

 Sfsin.0Xcos. e 



2g 



Ex. At what / must the rudder of a vessel be inclined to the stream, 

 that the effect produced may be a maximum ? 

 The effect varies (by the 3d Formula) as sin. 8 8 X cos. = max., .% 



sin. 6 = J |. 



3. If a plane figure, or a solid generated by the revolution^ a plane 

 figure round its axis, move in a fluid in the direction of its axis ; to deter- 

 mine the ratio of the resistances on the curve or surface, and on the base. 



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