37 2 Mr. Playfair’s Account of a 
entirely of quartz, and the remaining six are wanting, that is 
to say, their tops are not depressed below the level of O, as 
may be seen in Table V. of Dr. Hutton’s paper. From that 
table it also appears, that the depth of the summit of the first 
column of the seventh ring below the level of O is 250 feet ; 
of the second 240, of the third 200, of the fourth 150, of the 
fifth 60, and of the sixth go. From these measures the angles 
of depression may be computed. Thus, if 250 be divided by 
the radius of this ring, viz. —^2, we have .0577 f° r ^ ie tan " 
gent of the depression, or of E, and the sine which corresponds 
is .0568. As y— of this column consists of quartz, we must 
take f- of this sine for the proportional part of the coefficient 
of O. In like manner, the sine of the depression of the top of 
the second column is .0545, of which taking T 8 -, we get .0436 
for the part of the coefficient of Q belonging to this column. 
So also for the third ring, the proportional part of the sine is 
.04149. The fourth, fifth, and sixth columns being entirely 
of quartz, no proportional parts are to be taken ; their sines, 
computed as before, are .0346, .0138, .0069; and the sum of 
all these six numbers is .18015. 
Calculating in the same way for all the columns that are 
entirely or partly of quartz in the north-west quadrant, we 
have the amount of the whole — -2534. Now the total sum 
of the sines of the depressions in this quarter, is 13-534. (See 
Dr. Hutton’s computations, page 83). From this number, if 
.2534 be taken away, there will remain 13.2806 as the coeffi- 
cient of M, arising from the depressions of the micaceous 
columns. 
Now the sum of the sines belonging to the quartz in the 
