37 6 
Mr. Playfair's Account of a 
M, acid the total attraction at P = (2467.675) O + (2466. 
347) M; then the total attraction by which the direction of 
gravity is altered by the mountain, is (4942.064) O -{- 
( 3870.853) M. Hence as before D = Z 49 : — 
or D = (1.0053) Q + (°-7 8 743) M. 
Here if we make as before Q = 2.639876 and M= 2.81039, 
we shall have D = 4.866997. This therefore is the mean 
density of the earth, on the supposition that the interior of 
Schehallien, on a lower level than the observatories, consists 
of micaceous schistus. The measure thus obtained, for the 
mean density or mean specific gravity of the earth, is above 
that of any of the precious stones, and is nearly a mean be- 
tween the results of Dr. Hutton and Mr. Cavendish. Ac- 
cording to the former, D = 4.481 ; according to the latter, 
D = 5.48, the mean of which is 4.98. The difference between 
this and the last of our results is nearly = .1, or less than a 
forty-fifth part. 
If we are to consider the experiments on Schehallien singly, 
it seems highly probable that the mean density of the earth 
is contained between the limits deduced from the two different 
suppositions concerning the structure of the mountain, so 
that it cannot be less than 4.5588, nor greater than 4.867. 
The mean of these is nearly 4713. 
It is however desirable that an element so important in phy- 
sical astronomy, as the mean density of the earth, should be 
the result of many experiments. The principle on which 
those at Schehallien were made seems the most likely to lead 
to accurate conclusions. In the selection of the places fit for 
§uch observations, the homogeniety of the rock is a condition 
