1895.] ln<Uct Contribution to Geodesy. 157 



IV. " India's Contribution to Geodesy." By General J. T. 

 WALKER, R.E., C.B., F.R.S., LL.D., late Surveyor-General 

 of India. Received April 27, 1895. 



(Abstract.) 



This paper gives a summary of the operations which have been 

 completed up to date in India to furnish the necessary data of the 

 lengths and the amplitudes of meridional and longitudinal arcs for a 

 mathematical determination of the mean figure of the earth. It 

 shows that geodetic investigation has already greatly influenced the 

 operations of the Great Trigonometrical Survey of India, from its very 

 commencement at the beginning of the present century. It describes 

 the principal triangulatinn which has been laid out over the whole 

 face of the country, mostly in meridional and longitudinal chains, 

 and shows how it has been finally reduced and made consistent 

 throughout by processes of calculation which are fully set forth in 

 the published accounts of the operations. Brief statements of the 

 discrepancies which were met with at the base-lines and the sides of 

 junction of the chains of triangles, and the average values of the 

 angular corrections which had to be applied to satisfy the discre- 

 pancies and produce harmony and consistency throughout, are given 

 to indicate the extent to which the triangulation may be relied on for 

 geodetic investigations. Then the astronomical determinations of 

 latitude, and those of differences of longitude by the electro- 

 telegraphic method, are described and the results are compared with 

 the corresponding geodetic determinations from the triangulation. 



All the facts of observation having been given, the question is 

 considered of the extent to which the astronomical determinations 

 have been influenced by local deflections of the plumb line. The 

 views which have been put forward as regards the attraction of the 

 great Himalayan Mountains on the plumb line are considered ; it is 

 shown that the magnitudes of the attractions computed on the theory 

 of gravitation are considerably greater than is necessary to explain 

 the discrepancies between the astronomical and the geodetic results, 

 and this fact is fully explained by the Indian pendulum operations, 

 which have thrown great light on the constitution of the earth's 

 crust. They indicate that there is an excess of density under the 

 sea-level and a deficiency above that level which increases to a very 

 notable magnitude at a high altitude in the Himalayas ; thus, 

 therefore, there must be a condensation of matter under ocean beds, 

 and an attenuation under mountains ; and consequently points must 

 be sooner or later reached at which the positive attraction of a moun- 

 tain mass is cancelled by the negative attraction of the mass under 



