g66 Mr. Playfair’s Account of a 
Again, putting S' and V'to express the same things for the 
part of the mountain to the south of O, the whole attraction 
of that part equal S' — V', and this acting in an opposite 
direction to the other, or tending to restore the plummet to its 
mean position, is to be subtracted from the former quantity, 
so that the whole disturbing force by which the part of the 
mountain below the level of O acts upon the plummet at O, 
is S — V — - S' -f- V'. To this the attraction of the upper 
part of the mountain, or that which is above the level of O, 
being, as it happens, wholly to the north is to be added, and 
if it be called T, the whole disturbance on the plummet at 
O is S — S' -V + V'f T. 
In Dr. Hutton’s computation, S and S', or the attraction of 
the half cylinders on opposite sides of O are equal to one 
another, the cylinder being supposed to consist of matter of 
the same density throughout ; they must therefore destroy 
one another, and consequently, according to that hypothesis, 
they did not require to be calculated. The case here is not 
the same ; for the matter in the two semi-cylinders not being 
of uniform density, nor having its inequalities similarly dis- 
tributed, the attraction of each must be calculated in order 
that their difference, or S — S' may be found. 
If 2, 2', U, U', and T ' denote the same quantities for 
he observatory P on the north side of the mountain, then 
the disturbing force on the plummet at P, — 2 — 2' — 
U + U' + T' ; and so the whole force which alters the di- 
rection of gravity is S-S-V-J-V'-fT + S-S' 
— U + U' + T'. 
The computation of these quantities for the columns in the 
quadrant north-west of O, will serve to explain the method 
followed in all the rest. 
