454 The Surveys of India. [Oct., 



thence, in 1830, provided with geodetical instruments and appa- 

 ratus of every description executed by the most skilful artists of 

 the day, including a complete base-line apparatus, the invention 

 of Colonel Colby, precisely similar to that employed on the Ord- 

 nance Survey; a great theodolite, 36 inches in diameter; two 

 18-inch theodolites ; and a variety of smaller instruments from 12 

 inches diameter downwards. The signals, all of the most efficient 

 kind, and recently invented, consisted of heliotropes, reverberatory 

 lamps, and Drummond's lights, of which the two former have been 

 exclusively used. 



In addition to the duties of Superintendent of the Trigonome- 

 trical Survey, Colonel Everest had, on his return to India, to per- 

 form those of Surveyor-General of India. In 1833 the offices of 

 Deputy Surveyor-General at Madras and Bombay were abolished, 

 and their duties devolved upon the Surveyor-General, so that Colonel 

 Everest had now to perform the work which had hitherto occupied 

 the undivided attention of four officers. 



By the end of 1832 a longitudinal series of triangles had been 

 completed from Seronj to Calcutta, where another base line was 

 measured. Upon the completion of that work the measurement of 

 the great arc was re-commenced, after a cessation of seven years. 

 It was carried on from that time unremittingly till December, 1841, 

 when the whole Indian arc from Cape Comorin to the Himalayas, 

 forming the main axis of Indian geography, was finally completed. 

 The area comprised by the great arc operations, principal and secon- 

 dary, aggregated 56,997 square miles, including the revision of the 

 section from Beder to Kalianpoor, and the measurement of three 

 base lines, each from 1\ to 8 miles in length, viz. those of Beder, in 

 latitude 18° ; Seronj, near Kalianpoor station, in latitude 24° ; and 

 the Dehra base, about 70 miles north of Kaliana station, in latitude 

 29° 30', where the great arc actually terminates ; this distance being 

 observed on account of the proximity of the Himalayas. On com- 

 paring the actual measurement of the Debra Dhoon base by Colley's 

 apparatus with that calculated from the Seronj base, measured by 

 the chain in 1824, a difference of nearly %\ feet was found. In 

 former times this would have been considered a very satisfactory 

 agreement, seeing that the length of the base is 1\ miles, and its 

 distance from the new base upwards of 400 miles in a straight line ; 

 but Colonel Everest considered the difference as indicating a much 

 larger error than ought to exist, regard being had to the precision 

 of the new methods. In order to set the question at rest, he re- 

 solved to re-measure the old base with the more complete apparatus 

 he now had at his command. This operation was completed in 

 January," 1838, when it appeared that the length given by the 

 chain measurement of 1824 was too short by nearly 3 feet, as 

 compared with the new result. 



