590 
AECHDEACON PEATT ON THE INDIAN AEG OF MEEIDIAN. 
Ocean — have been calculated approximately, and are found to produce the deviations 
and 3"'82, making the astronomical amplitudes so much less than those calculated 
geodetically. But the Survey makes these deviations 5"‘24 and — 3"-79. There must, 
therefore, be some other source of attraction which increases the amplitudes by the 
differences of these, viz. by 7"’87 and 7''’61. We must attribute this to those hidden 
and unknown causes which lie below in the crust of the earth, where, as I have shown, 
causes, sufficient to produce a sensible deflection in the plumb-line, calculation proves 
may easily be supposed to reside. The following appears to me to be the simplest hypo- 
thesis regarding the variation below to account for these quantities, 7"'87 and 7"‘61, 
which, it will be observed, are nearly equal to each other, that a] 3 pertaining to the 
northern portion of the arc being somewhat larger than the other. 
If the density of the crust deviates by ps-id fi’om the density given by the fluid- 
law through a cubic space, measuring 200 miles parallel and at right angles to the 
meridian and 200 miles deep, and situated with the centre of its upper surface at 
Kalianpur, then the Table in par. 2 shows that the deflections caused by the attraction 
of the upper and lower halves of this mass at a distance 379 miles from the centre of 
the upper surface on a point on the surface of the earth, along the chord of the arc, are 
r'-94+r-62 = 3"-56. 
If the deviation in density be twice this, viz. 5 -oth part of the fluid-density, this 
deflection becomes 7"T2, very nearly the quantities to be accounted for. Now Kaliana 
is 371 miles, and Damargida is 430 miles from Kalianpur: these do not differ much 
from 379. 
It is very conceivable, therefore, that the deflections 7"'87 and 7"‘61, which have to 
be accounted for, arise from a slight excess of density of about -^th part prevailing 
through a circuit of about 100 or 120 miles around Kalianpur, and to a depth of abont 
200 miles. Of course an indefinite number of other similar hypotheses might be 
fi'amed to account for the deflections, but hardly one so simple as this. If Ave adopt 
the hypothesis of deficiency of matter beneath the Mountain Mass, we must suppose a 
similar deficiency to exist south of Damargida towards Cape Comorin and the Ocean ; 
but as two independent hypotheses are here necessary, this solution is not so simple as 
the one I have adopted above. 
13. Had I foreseen the result of the demonstration given in this communication, 
that a deviation in the curvature is altogether inadequate to account for any part even 
of the errors in the amplitudes, it would have been at once perceived, as it is now, that 
the errors 5"-24, — 3"-79 brought out by the Survey, must arise solely from local 
attraction affecting the plumb-line ; and these errors would have been taken, as they 
now must be, to be the accurate measures of the differences of total local attraction at 
the three stations. 
The calculations of Himmalayan and Ocean Attraction are nevertheless of consider- 
able importance. Without them we should most probably have remained ignorant of 
the large amount of deflection due to that cause. 
