TRANSACTIONS OF SECTION E. RZ 
distant ; he also found that everywhere between the Himalayas and the ocean, the 
excess of density of the land of the continent as compared with the water of the 
ocean would combine with the Himalayan attraction and increase the deflection of 
the plumb-line northwards, towards the great mountain ranges, and that under the 
joint influence of the Himalayas and the ocean the level of the sea at Kurrachee 
would be raised 560 feet above the level at Cape Comorin. 
But as a matter of fact the Indian are gave a value of the earth's ellipticity 
which agreed sufficiently closely with the values derived from the arcs measured in 
all other quarters of the globe, to show that it could not have been largely distorted 
by deflections of the plumb-line ; thus itappeared that whereas Everest might have 
slightly underestimated the Himalayan attraction, Pratt must have greatly over-- 
estimated it. His calculations were however based on reliable data, and were in- 
dubitably correct. For some time the contradiction remained unexplained, but 
eventually Sir George Airy put forward the hypothesis that the influence of the 
Himalayan masses must be counteracted by some compensatory disposition of the 
matter of the earth’s crust immediately below them, and in which they are rooted ; 
he suggested that the bases of the mountains had sunk to some depth into a fluid 
lava which he conceived to exist below the earth’s crust, and that the sinking had 
caused a displacement of dense matter by lighter matter below, which would tend 
to compensate for the excess of matter above. Now Pratt's calculations had 
reference only to the visible mountain and oceanic masses, and their attractive in- 
fluences—the former positive, the latter negative—in a horizontal direction ; he had 
no data for investigating the density of the crust of the earth below either the 
mountains on the one hand, or the bed of the ocean on the other. The pendulum 
observations furnished the first direct measures of the vertical force of gravity 
in different localities which were obtained, and these measures revealed two broad 
facts regarding the disposition of the invisible matter below ; first, that the force of 
gravity diminishes as the mountains are approached, and is very much less on the 
summit of the highly elevated Himalayan table lands than can be accounted for 
otherwise than by a deficiency of matter below ; secondly, that it increases as the 
ocean is approached, and is greater on islands than can be accounted for otherwise 
than by an excess of matter below. Assuming gravity to be normal on the coast 
lines, the mean observed increase at the island stations was such as to cause a 
seconds’ pendulum to gain three seconds daily, and the mean observed decrease in 
the interior of the Continent would have caused the pendulum to lose 24 seconds 
daily at stations averaging 1,200 feet above the sea level, 5 seconds at 3,800 feet, 
and about 22 seconds at 15,4U0 feet—the highest elevation reached—in excess of 
the normal loss of rate due to height above the sea. 
Pratt was strongly opposed to the hypothesis of a substratum, or magma, of 
fluid igneous rock beneath the mountains; he assumed the earth to he solid 
throughout, and regarded the mountains as an expansion of the inyisible matter 
below, which thus becomes attenuated and lighter than it is under regions of less 
elevation, and more particularly in the depressions and contractions below the bed 
of the ocean. And certainly we seem to have more reason to conclude that the 
mountains emanate from the subjacent matter of the earth’s crust than that they 
are as wholly independent of it as if they were formed of stuff shot from passing 
meteors and asteroids; any severance of continuity and association between the 
visible above and the invisible below appears, on the face of it, to be decidedly 
improbable. 
The hypothesis of sub-continental attenuation and sub-oceanic condensation of 
matter is supported by the two ares of longitude on the parallels of Madras and 
Bombay ; for at the extreme points of these arcs, which are situated on the opposite 
coast lines, the horizontal attraction has been found to be not landwards, as might 
have been anticipated, but seawards, showing that the deficient density of the sea 
as compared with the land is more than compensated by the greater density of the 
matter under the ocean than of that under the land. 
While on the subject of the constitution of the earth’s crust, I may draw 
attention to the circumstance that the tidal observations which haye been carried 
on at a number of points on the coasts of India, as a part of the operations of the 
