Of GREATEST ATTRACTION. 239 
fing’ = = cof ( 1" — n”) — = cof (n'+ 2”) 
fin oN = = cof ("— 1 ) = - cof ("++ 4 ve 
By either of thefe antehods: the determination of the attrac-. 
tion is reduced to a very fimple trigonometrical calculation. 
XXII. 
THE preceding theorems will alfo ferve to determine the at- 
traction of a parallelepiped, of any given dimenfions, in the di- 
rection perpendicular to its fides. 
Let BF (Fig. 16.) be a parallelepiped, and A, a point in BK, 
the interfection of two of its fides, where a particle of matter is. 
fuppofed to be placed; it is required to find the attraction in 
the direction AB. . 
Tuoucu the placing of A in one of the interfections of the. 
planes, feems to limit the inquiry, it has in reality no fuch ef- 
fect ; for wherever A be with refpect to the parallelepiped, by 
drawing from it a perpendicular to the oppofite plane of the fo- 
lid, and making planes to pafs through this perpendicular, the 
whole may be divided into four parallelepipeds, each having 
AB for an interfection of two of its planes; and being, there- 
fore, related to. the given particle, in the fame way that the: 
parallelepiped BF is to A. 
Let GH be any fection of the folid parallel to EC, and Iet it. 
reprefent a plate of indefinitely fmall thicknefs. 
Let AB =x, B%é, the thicknefs of the plate =%. Then 9 
being fo determined, that fin 9 = fin BAH x fin BAG, the at- 
graction of the plate GH is 9%, which, therefore, is the ele- 
ment 
