. Mr . Knight on the Attraction of such Solids 
25% 
Prop. 3. 
To find the attraction of a triangle umv, fig. 3, on a point p 
any how situated. 
Let fall from p the perpendicular pm', on the plane of the 
triangle; join m'm, m'u, m'v. Find, by the last Prop, the attrac- 
tions of the triangles m'uv, m'um, ra'vm, on the point p; and 
resolve them into others in the directions of any three rectan- 
gular co-ordinates : and, when thus resolved, let the actions of 
f m'uv 1 . . _ 
I , I in the directions ol these 
«< m'um > 
co-ordinates be 
Lm'vmj 
It is plain, that the actions of the triangle umv, on p, in the 
directions of the same co-ordinates, will be 
A — A' — A"; B — B' — B" ; C _ C' — C". 
There may be other cases of this proposition, in which the 
triangle and point are placed differently, with respect to each 
other, from what I have represented in fig. 3 ; but the reader, 
who understands the case that has been considered, will have 
no difficulty in any other that may occur. 
Though the preceding propositions contain every thing that 
is necessary, for finding the attraction, both in quantity and 
direction, of any plane bounded by right lines ; yet there are 
some cases worthy of a particular notice : as 
Prop. 4. 
To find the attraction of a rectangle mrvr', fig. 4, on a point 
p situated perpendicularly over one of its corners as m. 
Draw pg, po, parallel to mr, mr', the sides of the rectangle; 
put h = rm, b' = r'm, r ~ tang, rmv, r' — tang, r'mv : then, if 
fA , B , C] 
< A',B',C' l 
! l A", B", C"J 
