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Imperial Gazetteer of India, v. 4, p. 485.

xv] SUR VLE YS 485 in all subsequent operations of the great Trigonometrical Survey. Everest further completed the Great Arc Series, closing it on the Dehra Dun base by continuing the main triangulation northwards from the point near Ellichpur to which Colonel Lambton had brought it. He likewise com- pleted the Bombay Longitudinal Series, and revised a con- siderable portion of Lambton's work on the Great Arc, re- measuring the bases. The design and partial completion of the meridional series of the 'gridiron' in Bengal and Bihar are also due to him. Sir Andrew Waugh, who became Surveyor-General and By Sir A. Superintendent of the Trigonometrical Survey in 1843, on the Waugh and retirement of Sir G. Everest, designed a great quadrilateral General figure of chains of triangulation, which was to enclose and Walker. intersect the lately-conquered territory of the Punjab. It was between the years 1845 and a85o, while the main chain of triangles which runs along the base of the Himalayas from Dehra Dun to Jalpaiguri was in progress, that the correct heights of the main Himalayan peaks were determined. Rennell, in his Memoir, notices their great elevation, but the first attempts to measure them were those of Colonel Crawford (Surveyor-General in 1814-6) while in Nepal in 1802, and again in 1805, during the course of a route-survey from Bihar to Rohilkhand. General J. T. Walker succeeded Sir Andrew Waugh as Superintendent of the Trigonometrical Survey in I861, when the two offices held by the latter were separated, and Colonel H. L. Thuillier was appointed Surveyor-General. Under the guidance of Walker the geodetic work of Lambton and Everest was widely expanded. The Vizagapatam, Bangalore, and Cape Comorin base-lines were remeasured, while portions of the Great Arc and of the Calcutta-Karachi Longitudinal Arc were revised. The execution of sixteen principal series of the 'grid- iron,' and the complete revision of the Great Arc, saw the practical close of main triangulation in India proper, and sup- plied fresh data for determining the figure of the earth. The deduction of the work as a whole entailed the most elaborate calculations that have ever been undertaken in geodesy. The time had arrived for determining the procedure by which the fallible values of the several angles and base-lines, as obtained by actual measurement on the ground, were to be rendered consistent; and final values had to be determined for the lengths and azimuths of the sides of the triangles, and also for the latitudes and longitudes of the stations. The chains divide