MEAN DENSITY OF THE EARTH. 
BY 
Erasmus Darwin Preston. 
[Read before the Society, April 28, 1894.] 
The mean density of the earth has been determined several 
times by comparing its attraction with that of a mountain. 
Maskelvne first applied the method to Schehallien, a mount¬ 
ain 3,561 feet high in Scotland. The experiment was re¬ 
peated by the English ordnance survey officers in connection 
with their work around Edinburgh on a hill 823 feet high. 
The present paper is the result of an application of the 
same method on a much larger scale, the mountains used 
having altitudes of 10,000 and 14,000 feet respectively. 
Moreover, the Hawaiian Islands offer peculiar advantages 
for the solution of the problem, because the mountains rise 
directly from a deep sea and there is very little plain attrac¬ 
tion to complicate the solution. Reduced to its simplest 
expression, the method is this : The attraction of the mount¬ 
ain on a plumb line is determined by connecting two points 
on the north and south side of it by a system of triangles. 
The latitudes of these two points are determined by astro¬ 
nomical observations, and their positions, compared with 
those derived by triangulation, give the amount of deviation 
of the plumb line. This deviation evidently depends upon 
the relative attractions of the mountain and the earth. Ex¬ 
pressing the attractions in terms of the volumes, densities, 
and distances of the attracting masses and comparing the 
resulting ratios with the deviation of the plumb line, we 
48—Bull. Phil. Soe., Wash., Vol. 12. (369) 
