342 
MR. AIRY’S ACCOUNT OF THE HARTON EXPERIMENT. 
62. Without pretending to extreme accuracy, we may consider the thickness and 
specific gravity of the beds as represented, with sufficient accuracy for the present 
purpose, by the following numbers : — 
1211 feet of rocky and shaly beds . . specific gravity 2*56 
30 feet of coaly beds specific gravity 1-43 
15 feet completely worked out . . specific gravity O’OO 
From these the mean specific gravity is found to be 2’50. 
63. Substituting this for d in the expression at the end of the last Section, we get 
for the mean density of the Earth, as determined from these experiments, 
6-566+-0182, 
where the last term (expressing the probable error) ought to be at least doubled. 
64. The value thus obtained is much larger than that obtained from the Schehal- 
lien experiment, and considerably larger than the mean found by Baily from the 
torsion-rod experiments. It is extremely difficult to assign with precision the causes 
or the measures of the errors of any of these determinations ; and I shall content 
myself with expressing my opinion, that the value now presented is entitled to com- 
pete with the others, on, at least, equal terms. 
