254 J. T. Walker —On Indian Pendulum Observations. [Nov. 
to the sea-level, in order to obtain results from observations at different 
stations which will be strictly comparable with each other, it is necessarily 
assumed that the matter of the whole of that portion of the earth’s crust 
which lies directly below the sea-level—and which, from its proximity, 
materially influences the attraction at the sea-level—is of uniform density 
throughout, in all parts of the world, whether situated underneath moun¬ 
tains and continents, or underneath the bed of the ocean. On this hypo¬ 
thesis all pendulum observations have hitherto been reduced to a common 
level, and it is none the less an hypothesis that it has heen made tacitly. It 
implies that the matter of the visible masses above the sea-level is wholly un¬ 
connected with, and independent of, the matter of the invisible masses below ; 
thus the mountains and continents might consist of just so much stuff thrown 
off passing meteors and asteroids—having a density a- — ^ p the mean den¬ 
sity of the earth,—instead of being a continuation, or an expansion, of the 
matter immediately below them, which is the more natural supposition. 
That there actually is any such severance of continuity and disconnection 
between the visible above and the invisible below, appears, on the face of 
it, to be highly improbable. 
Seeing then that, do what we will, we must make some assumption, I 
cannot but think that Archdeacon Pratt’s hypothesis that the visible mass¬ 
es above may be regarded as so much matter abstracted from the invisible 
masses below, is the least difficult of the two hypotheses to accept. And if 
we proceed to consider the constitution of the crust below the bed of the 
ocean, it appears to me to be easier to assume, with the Archdeacon, that 
there the matter has been condensed down to a depth which bears some 
relation to the depth of the ocean above, than to assume it to be of the 
same density as the comparatively uncontracted matter at the level of the 
sea, on the coast lines. 
Data are available for estimating, with tolerable approximation, the 
relative magnitudes of the greatest horizontal attraction exerted by the 
Himalayas and the greatest vertical diminution of attraction under the 
Himalayas, that is to say, of the two forces by which the geodetic and the 
pendulum operations, in India, are respectively most influenced. The latitude 
of one of the trigonometrical stations in Delira Dun—beyond the northern 
extremity of the Great Arc has been determined, both astronomically, and 
by calculation through the triangulation from Kalianpiir, the astronomical 
origin of latitudes, 428 miles to the.south. Dehra Dun being at the foot 
of the Himalayas, a large deflection of the plumb-line must be expected 
there, and, in fact, a larger meridional deflection has been met with there, 
than at any other station of the Survey at which astronomical observations 
have been taken. The astronomical latitude at Dehra is 37"G in defect of 
