53S Proceedings of the Asiatic Society. [No. 5, 



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 &'\ in the northern section of the arc 

 and 3"f- 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 Himalayas 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 



