746 
AECHDEACOX PRATT OA THE DEELECTIOX 
mv results, by comparing the curvature thus deduced ’uith the cun-ature of other arcs 
on the continent of India. But this proceeds upon the gratuitous hypothesis, and one 
^yhich for geological reasons is most lilcely not true, that the earth is at present an 
exact spheroid of revolution ; i. e. that all meridians are ellipses, and indeed the same 
ellipses, and that every arc of longitude is circular. There are only two ways of avoidmg 
the conclusion regarding the curvature of the Indian Ai-c to which I came in my paper 
of 1855 ; either by showing that my data and reasoning are wrong, or by pointing out 
that some other cause is in operation, which either m whole or in part counteracts ihe 
effect of the Himmalayan Mass. My calculation has been before the pubhc three years ; 
and, though some small numerical errors have been detected, they are not of sufficient 
importance to affect the result ; and the data I have every reason for behering to be 
correctly takeir, as the Surveyor-General — who first called my attention to the subject 
in 1852, as an unsolved difficulty in the operations of the Great Trigonometrical Sun'ey 
of Iirdia — has been requested to forward to me airy corrections 'u hich may appear to him 
to be advisable, and none have been seirt*. There remains, then, only the resource of 
looking for some counteracting cause to compensate for the large disturbance produced 
by the Hiinmalayas and the regions beyond. 
2. The Astronomer Boyal, in a paper published in the Trairsactions for 1855, 
suggested that immediately beneath the mountain-mass tirere vas most probabB a 
deficiency of matter, which would produce, as it were, a negative attraction, and so 
counteract the effect on the plumb-line. This hypothesis appears, however, to be 
* The whole Mountain Region (which I call the Enclosed Space, see par. 8) I clb ide into two portions 
by a cii-cular arc of about 350 mdes radius described about Kaliana, the northern station of the Great Arc, 
as centre. The portion of the mountain country within that arc I have called, in ui} former Paper, the 
Known Kegion, because the heights are all readily obtained from the Survey Maps. The remaining portion 
I designate the Doubtful Megion, because the heights cannot possibly he so weU determined. My chief 
source of information for the Doubtful Region is IlxiMBOLni’s ‘ Aspects of Nature. The Doubtful Region 
is, in superficial extent, about sixteen times as large as the Known Region ; hut, as I show in my former 
Paper, being more distant, produces nothing like a corresponding effect on the stations of the Arc. 
By the use of the Tables in paragraphs 8 and 11 it may be shown without much difficulty, that the three 
Deflections 27"'973, 12"-017, 6"-790 at the three principal stations would be reduced only to 21''-633, 
9"'937, 5"'010, if the whole of the Doubtful Region beyond a radius of about 700 miles from the northern 
station were left out of the reckoning. If the lohole Doubtful Region were considered to be a dead flat and 
to have no effect at all — an hypothesis clearly impossible — the Deflections would still be reduced only to 
12"-972, 3"‘219, l"-336. In these three cases the corrections to the amplitudes would be 
15"-926 and 5"-267 if the Doubtful Region be as I have taken it ; 
14!"-696 and 4"-927 if all beyond 700 miles from Kaliana be left out or annihilated ; 
9"-753 and l"-883 if the whole Doubtful Region be supposed non-existent. 
The errors to be accounted for (if the ellipticity of the Indian Arc he what Colonel Eweeesx assimies it to 
be, viz. the mean) are 5"-236 and -3"-789. This extravagant, and indeed impossible hypothesis, then, of 
the non-existence of the vast Mountain Region beyond the Himmalaya crest, will not account for these 
errors. Much less, then, Avill any mere correction of the heights of this Doubtful Region. A solution, if 
there be one, must be sought for in another direction. 
