Lithological Survey of Schehal lien. 371 
sine of E being = . 3137, and the attraction due to the quartz 
= iL f° 6 ■ 3>57 = ( ™9-9&3) Q ■ 
In this manner the attraction of the whole cylinder on the 
plummet at O is readily computed ; but it muct be diminished 
on account of the part by which this cylinder rises above the 
surface of the ground. The quantity that is to be subtracted 
is computed from the sines of the depressions of the tops of 
the different columns below the observatory O; and DlV 
Hutton's paper either actually exhibits* those sines, or fur- 
nishes us with the means of readily computing them.-f When 
a ring is wholly of the same species of rock, the sum of the 
sines of the depressions of all the columns in that ring is given 
in the tables, and needs only to be multiplied by to give 
the coefficient of O or M as far as that ring is concerned. 
Again, when in the same ring some columns are of quartz 
and others of mica, the sines of depression must be computed 
trigonometrically for each column by help of the data con- 
tained in the tables above referred to. The sum of those 
sines for the quartz columns being multiplied by b d gives the 
coefficient of Q. 
Where the same column is of two different kinds of rock, 
the sine of the depression, or of E, must be multiplied into b d , 
and divided in the proportion of the numbers prefixed to Q 
and M in the cell belonging to the column. 
All this may be illustrated by the calculation of the attrac- 
tion of the columns belonging to the above table. In the 
seventh ring the first columns are mixed, the next three are 
• Phil. Trans. Vol. LXVIII. from p. 769 to 776. 
f Ibid, from p. 759 to 765. 
3 B 2 
