268 Mr. Knight on the Attraction of such Solids 
Prop. 21. 
To find the quantity and direction of the attraction of a right 
prism, whose base is any triangle whatever, on a point anv 
where on its surface. 
Let the triangle uvu, fig. 11, be a section of the prism, pa- 
rallel to its base, and through the attracted point p. Let fall 
the perpendiculars pi', pr', on the opposite sides : and the solid 
may be divided into four prisms, whose bases are the right 
angled triangles pur, prv, pvr', pr'u', and the attraction of each 
of these, both in quantity and direction, is given by Prop. 20. 
It is plain that there may be other cases of this problem, 
besides the one here considered ; for instance, one of the per- 
pendiculars may fall beyond the base ; but it would be end- 
less, in a subject of this kind, to consider every particular 
case, and in none can the intelligent reader find the smallest 
difficulty. 
Prop. 22. 
To find the attraction of any prism, fig. 12, whose base is 
a convex polygon uvuV, on a point q any where within it. 
As such a solid may be divided into triangular prisms, like 
those in Prop. 20 and its corollaries, with the attracted point 
on the common edge pm, the problem is already solved. 
If the point be at p', in the line mp produced, the action on 
it may still be found, being the difference of the actions of two 
prisms, like that in the figure. 
Prop. 23. 
Let vuv', fig. 13, be the section of an isosceles prism ; pv 
a line passing through the vertex v perpendicularly on the 
