GREAT TRIGONOMETRICAL SURVEY OF INDIA. 61 



means, and it happens to be the most important of all the great 

 chains of triangles, because it furnishes bases for no less than 

 14 meridional chains lying to its north and south. Partial revisions 

 have been made in other quarters of work executed with inferior 

 instruments, which it was deemed necessary to raise to a higher 

 standard of accuracy. Outside the limits of India proper the more 

 recently completed chain of triangles called the Eastern Frontier 

 Series is a valuable contribution to geodesy as well as geography. 



The whole of the triangulation rests on ten base-lines which have 

 been measured with the Colby apparatus of compensation bars and 

 microscopes, which was constructed in England under Colonel 

 Everest's superintendence. The relations of the length of the 

 Indian standard to the principal European standards of length have 

 been very exactly determined. Considerations of symmetry would 

 suggest the introduction of an additional base-line near Bombay, 

 on the same parallel as the Bidar and Vizagapatam base-lines, and 

 measured with the same apparatus. But it so happened that a 

 chain base-line had been measured on the Karleh plain, near 

 Bombay, in the year 1828, by Captain Shortrede, the calculated 

 value of which, through the longitudinal series from the Bidar 

 base-line, agrees very closely with the measured value. It was 

 commended by Colonel Everest, who, however, some years after- 

 wards, in 1848, made preliminary arrangements for the measurement 

 of another line in the neighbourhood with the Colby apparatus, but 

 he did not carry out this project. Eventually the idea was 

 abandoned, as the distance from the Bidar base is comparatively 

 small, and no material advantage at all commensurate with the 

 labour and expense would be derived from the measurement of a 

 new base; for to measure a base-line with the Colby apparatus 

 occupies two full-strength trigonometrical parties for an entire field 

 season, unless there happens to be other employment for the survey 

 officers in the neighbourhood of the base. There is some uncertainty 

 as regards the unit of length, adopted by Captain Shortrede in 

 measuring the Karleh base, consequently this base has not been 

 employed m the final reductions, though no new base has been 

 measured. 



Thus the great work of the principal triangulation of India 

 became an accomplished fact. Commenced in 1800, under the 

 auspices of the Madras Government, it was carried on by Major 

 Lambton, almost single-handed, until the year 1818, when the 



