538 
[No. 5, 
Proceedings of the Asiatic Society. 
extension to points where there would probably be no attraction, and 
he predicted that it would then give a figure coinciding with those 
obtained in Europe. These suggestions have been entirely verified by 
the subsequent remeasurement and prolongation of LaCaillies’s arc by 
Sir Thomas MacClear, the Government Astronomer at the Cape. 
Major Walker mentioned these circumstances to shew that the 
officers entrusted with the survey of India had not been blindly 
ignoring the influence of mountain attraction. 
It was believed to have been avoided, in great measure, by placing 
the northern extremity of the arc at Kaliana, a distance of upwards 
of sixty miles from the Himalayas. Colonel Everest considered that 
the residual errors were about 5' in the northern section of the arc 
and 3"|- in the southern section, by which amounts he conceived the 
astronomical amplitude to be less than the geodesic in the upper 
section, and greater in the lower. 
Major Walker observed that Archdeacon Pratt’s early investiga¬ 
tions shew that the Plimalayas may have a far greater effect in 
disturbing the plumb-line than had formerly been supposed, thus 
raising a doubt of the scientific accuracy of the survey operations and 
questioning the correctness of the relative situations of places, as 
given in the maps. But the Archdeacon’s last paper has dispelled 
this doubt, by proving the following elegant theorem that the 
length of an actual arc, measured on the surface of the earth, however 
altered its form may be by geological changes, is nevertheless sensibly 
equal to what would have been obtained had the original curvature 
been undisturbed ; or, in other words that no possible change of 
curvature can disturb the normal length of the arc. Hence the 
relative mapping of a country is free from all error arising from local 
attraction. If the positions on the map are too far north or south, 
they will all be so to an equal degree, and consequently are relatively 
accurate. 
The Archdeacon’s investigations are further useful in establishing 
the fact that while the positive attraction of the Himalayas draws 
the plummet northwards, the negative attraction of the Indian Ocean 
has a similar effect. Thus, in moving from Cape Comorin to the 
Himalayas, the influence of the ocean diminishes, while that of the Hills 
increases, and hence there is a tendency to equalize the resultant at¬ 
traction, at every point between the ocean, and the Himalayas. Major 
