228 Of the SOLIDS 
Hence this rule, multiply the fine of the greateft elevation, 
into the fine of the greateft azimuth of the folid ; the arch of 
which this is the fine, multiplied into the thicknefs of the fo- 
lid, is equal to its attraction in the direction of the perpen- 
dicular from the point attracted. 
Tue heighth and the length of the parallelepiped, are, there- 
fore, fimilarly involved in the expreflion of the force, as they 
ought evidently to be from the nature of the thing. 
XIX. 
Tus theorem leads directly to the determination of the at- 
traction of a pyramid, having a rectangular bafe, on a particle 
at its vertex. For if we confider EM (Fig. 11.) as a flice of a 
pyramid parallel to its bafe, A being the vertex, then the flice 
behind EM fubtending the fame angles that it does, will have its. 
force of attraction = 7 ¢, n' being its thicknefs, and fo of all the 
reft; and, therefore, the fum of all thefe attractions, if p denote 
the whole height of the folid, or the perpendicular from A on 
its bafe, will be pg. But as 7@ is only the attraction of the 
part HB, it muft be doubled to give the attraction of the whole 
folid EM, which is, therefore, 2” 9; and this muft again be 
doubled, to give the attraction of the part which is on the fide 
of AB, oppofite to EM; thus ‘the element of the attra@tion of 
the pyramid is 4'7 , and the whole attraction oe to 
the depth pf, is 4 pig OU 
Ir the folid is the frudum ofa pytamid Ae depth i is p! ; ane 
vertex A, the angle ¢ being determined as before, the attraction 
on A is 479. 
Ir 
