C 347 3 
XVIII. Account of a Lithological Survey of Schehallien , made in 
order to determine the specific Gravity of the Rocks which compose 
that Mountain. By John Playfair, Esq. F.R.S. 
Read June 27, 1811. 
9 
I he astronomical observations made on the mountain Sche- 
hallien, in 1774, were confessedly of great importance to 
science. They ascertained the power of mountains to produce 
a sensible disturbance in the direction of the plumb-line ; of 
consequence, they proved the general diffusion of gravity 
through terrestrial substances, and afforded data for determin- 
ing the medium density of the earth, compared with that of 
the bodies at its surface. 
The skill with which this very delicate experiment was 
conducted by Dr. Maskelyne, and the ingenuity with which 
the results were deduced by Dr. Hutton, were worthy of 
the objects in view, and of the reputation which these distin- 
guished men have acquired in their respective departments of 
the mathematical sciences. 
One thing only seemed wanting to give to the determination 
of the earth’s density all the accuracy that could be obtained 
from a single experiment, namely, a more precise knowledge 
of the specific gravity of the rock which composes the moun- 
tain, as being the object with which the mean density of the 
earth was immediately compared. The specific gravity of 
that rock was assumed to be to that of water as 5 to 2; 
Y y 2 
