1879.] J. T. Walker— On Indian Pendulum Observations. 251 
ing that the effect of the mountains may be counteracted by their bases 
having sunk to some depth into the dense fluid lava below, on the surface 
of which the crust may usually be supposed to repose ; such sinking would 
cause a displacement of dense by lighter matter below, which would tend 
to compensate for the excess of matter above. While demurring to the 
form of this suggestion, Archdeacon Pratt followed up the idea, and reduced 
to calculation another hypothesis regarding deficiency of matter below 
mountains, viz., that the irregularities of the mountain surface have arisen 
from the expansion of the earth’s crust upwards, from dejsths below, which 
has upheaved the mountains and produced a slight but extensive attenua¬ 
tion of the mass below them. This attenuation he shows to be sufficient 
to produce a considerable amount of compensation for mountain attraction ; 
but he states that it does not clear up the difficulties ; and, being a mere 
hypothesis, nothing certain could be determined regarding it; see No. 
XXIX of Philosophical Transactions for 1858. Subsequently he investi¬ 
gated the influence of the Ocean on the plumb-line in India, and found 
that it also had a very sensible effect at the stations of the Arc, and in the 
same direction as the Himalayan Attraction (No. XXX, Philosophical 
Transactions, 1858). 
Hitherto the Archdeacon had been inclined to attribute the calculated 
deflections of the plumb-line, in some degree, to errors in the elements of 
the figure of the earth which had been employed in the geodetic computa¬ 
tions. But in I860 he satisfied himself that this was not the case, and 
that there are hidden causes,—in variation of density in the crust below 
the Indian Arc—which, taken in combination with the Mountain and Ocean 
Attraction, explain the smallness of the discrepancies that had been met 
with, (No. XXXIV, Philosophical Transactions, 1861). 
Thus far his attention had been directed only to horizontal attractions, 
producing, and measured by, deflections of the plumb-line. When the 
Indian pendulum operations were commenced, he watched their progress 
with great interest, to see whether their direct measures of vertical attrac¬ 
tion supported his views regarding attenuation of matter below mountains. 
The results of his calculations are given in his Figure of the Earth, 4th 
edition, 1871. He shows, in Art. 196, that the discrepancies between 
theory and observation become considerably reduced when it is assumed that 
beneath any portion or cap of the earth’s crust, which is raised above the 
sea-level, there is a uniform attenuation of matter equal to that of the cap, 
running down to a depth from fifty to one hundred times the thickness of 
the cap. This result was arrived at when as yet he had only been furnish¬ 
ed with the evidence of the stations between Minicoy and Kaliana, the 
highest of which is only about 3000 feet above the sea-level. Subsequent- 
