Lithological Survey of Scheliallien. 369 
The attraction of each of the twenty rings being thus com- 
puted, their sum gives the attraction of the quarter cylinder.* 
From the projection of the columns in Fig. 1. it appears 
that the first six rings in the NW. quadrant are entirely of 
quartz, that the five following are mixed, being partly quartzy 
partly micaceous, and that the nine remaining columns are 
wholly micaceous. The little table that follows contains the 
proportions of quartzy and micaceous rock in the five rings 
just mentioned. 
Sectors. 
CO 
tuQ 
2 
This table is constructed only for that part of the north-west 
1 
2 
3 
4 
5 
6 
7 
8 
9 
10 
1 1 
1 2 
7 
_JL yyi 
1 0 
to q 
TO m 
to q 
To"! 
q 
q 
q 
q 
q 
q 
q 
q 
q 
8 
m 
m 
m 
i q 
i m 
t q 
-§• m 
1 1 
1*^1 
q 
q 
q 
q 
q 
q 
9 
m 
m 
m 
m 
m 
m 
1 q 
f m 
q 
q 
q 
q 
TT q 
TT*n 
10 
m 
m 
m 
m 
m 
m 
m 
I? 
t m 
t q 
t m 
q 
q 
i q 
1 1 
m 
m 
m 
m 
m 
m 
m 
m 
m 
i q 
i m 
i q 
| m 
m 
• It was most convenient to compute the attraction of the quarter cylinder in this 
way, though merely an approximation, because the columns of which it consisted are 
not all of the same specific gravity. In the case of their being homogeneous, the 
attraction of the quarter cylinder might be computed exactly by the second theorems 
given above. Indeed I investigated that theorem for the purpose of examining into 
the degree of accuracy that this approximation actually possessed ; and I had the 
satisfaction to find, that when the two methods were applied to the same half or quar- 
ter cylinder, (supposed homogeneous,) the difference of the results did not exceed 
a two thousandth part of the whole. This demonstrated in a very satisfactory manner 
the accuracy of the method pursued by Dr. Hutton. 
MDCccxr. 3 B 
