AECHDEACOjS' PEATT ON" THE INDIAN AEG OP MEEIDIAN 
593 
have been known, or perhaps suspected, if the actual calculation of the attractions had 
not been made. 
16. It will be observed that the four several elliptic arcs which I have examined, as 
representing the Indian Arc, have been compared, not with the mean ellipse itself, but 
with an ellipse equal to the mean elhpse, and supposed to be drawn through the extre- 
mities of the arc. By the calculations of one part only of a meridian line it is impos- 
sible to determine the position of the centre of the mean ellipse, and therefore to ascer- 
tain how much the arc may have been upheaved or depressed with reference to the 
original centre of the earth when in a fluid state. It would require the survey of the 
whole meridian from pole to pole to determine this. 
^ 5. Conclusions from the whole investigation regarding the Indian Arc. 
17. The results finally arrived at may be stated as follows : — 
(1) Colonel Eveeest discovered that the astronomical amplitudes of the two portions 
of the Indian Arc between Kaliana and Kalianpur, and between Kalianpur and Damar» 
gida, are, the fii’st less by 5"-24, and the second greater by 3"-79, than the geodetic am- 
plitude calculated with the mean semi-axes and ellipticity of the earth. 
(2) The geodetic amplitudes of these two portions of the arc, calculated from the 
measured lengths and with the mean axes, will come out sensibly the same, even should 
the curvature of the arc differ from that of the mean meridian within reasonable but 
vide limits — a thing which geology teaches us to be very likely the case. 
(3) Hence the geodetic measurements of the Survey being without sensible error, as 
is known by the tests applied, the discrepancy mentioned in (I) can arise only from local 
attraction affecting the vertical line, and so changing the astronomical amplitudes. 
(4) Two great Hsible causes of disturbance of the vertical by attraction are, the 
Mountain Mass on the north of India, and the Ocean on the south. The influence of 
both of these is felt all over India ; the first . producing a northerly deflection varying 
from 27"’98 at Kaliana to a sensible angle (probably about 3", but this I have not 
calculated) at Cape Comorin ; the second producing also a northerly deflection, varying 
from about 1 9"’ 7 1 at Cape Comorin to 6^'T8 at Kaliana. 
(5) The combined effect of these two visible causes is to make the astronomical ampli- 
tudes of the upper arc I3"TI too small, and of the lower 3"-82 also too small. They 
are therefore insufficient to account for the discrepancies pointed out by Colonel 
pass through A and D, the extremities of the whole arc. The ellipse about the centre O' is the mean ellipse 
itself, the position of which with respect to our starting line (viz. the arc ABCD) is not known, except 
that its axes are parallel to those of the other ellipses. The values of JE above deduced show ho’w very 
little the Indian Arc differs in ciu’vature from the curvature of the mean ellipse in the same latitudes : and 
that it is very slightly flatter. How much other parts of the meridian may differ in curvature from the 
mean ellipse it is impossible to say, and therefore how much the Indian continent may he bodily upheaved 
or depressed below the mean ellipse. To determine this wordd require, as stated in the text, the survey of 
a whole meridian line. 
