as are terminated by Planes , &c. 269 
middle of the base. It is required to find the action of the 
prism on any point p in the line pv. 
This action is equal to the action of the prism whose base is 
the triangle v'pu (given by Prop. 20), less the actions of the 
prisms whose bases, or sections, are the triangles v'vp, uvp, 
which are found by Cor. 2, Prop. 20. 
Scholium. 
I considered here only an isosceles prism ; but the solution 
is just the same, if the section of the prism is any triangle 
whatever, as v*'uv, fig. 14, and the action on a point p, (situated 
in the line uv produced) is required. For the attraction wanted 
will be the difference of attractions of the two prisms whose 
bases are the triangles v'up, v'vp, and these are given by Cor. 2, 
Prop 20. 
Suppose the base of the prism, whose attraction is required, 
to be the trapezium v'utf/ 3 , fig. 14, the action of this on p, being 
the difference of the actions of the triangular prisms, whose 
bases are v'uv, / 3 av, is found by the case just now considered. 
In this manner, might cases be multiplied without end ; but 
I think it is sufficiently plain, that by means of the preceding 
propositions and scholium, we may find the action of any prism 
whatever, on a point either within or without it. 
§. IV. 
Of the Attraction of certain Solids not terminated by Planes. 
The expressions, arrived at in the first section, are useful in 
finding the attraction, not only of such solids as are bounded 
by planes, but of a great variety of others ; viz. of such as 
MDCCCXil. N 21 
