GREAT TRIGONOMETRICAL SURVEY OF INDIA. 59 



of India and the Court of Directors for a new instrumental equip- 

 ment, much superior to what had hitherto been employed. During 

 his absence from India a small party of surveyors was engaged 

 in carrying a longitudinal chain of triangles eastwards from the 

 point reached by the Great Arc in Central India to Calcutta. 



On his return from Europe in 1830, Colonel Everest recommended 

 the abandonment of the network system of triangulation, and the 

 substitution instead of what he called the "gridiron" system, 

 consisting of meridional chains which were intended to be constructed 

 at intervals of about one degree apart, while the longitudinal chains 

 would follow the parallels of Calcutta, Bombay, and Madras, and 

 thus run at intervals of from five to six degrees apart. The external 

 chains of the gridiron were to follow the British frontier lines and 

 the coast lines. The entire triangulation was to be grounded on 

 base-lines measured with the Colby apparatus of compensation bars 

 and microscopes — in terms of a fixed standard of length — which 

 were to supersede the old base-lines that had been measured with 

 chains of comparatively rude construction and of uncertain length. 

 This programme of operations was approved by the Government of 

 India and the Court of Directors, and it has furnished the guiding 

 lines on which the principal triangulation has been executed during 

 a period of almost exactly half a century. 



For convenience of treatment in the final reduction, the whole 

 of the chains situated within the limits of India proper have been 

 grouped into five sections. Four of these are roughly four-sided in 

 outline and are respectively called the North-East, North-West, 

 South-East, and South-West Quadrilaterals, names in which the 

 cardinal points have reference to the Kalianpur Observatory in 

 Central India, which Colonel Everest adopted as the origin of the 

 operations subsequent to 1832. The fifth is three-sided, and is called 

 the Southern Trigon, and embrace the southern portion of the 

 peninsula, below the parallel of Madras. The North-East Quadri- 

 lateral was completed first of all, and here it will be seen, on reference 

 to the Chart of the Principal Triangulation, that the meridional 

 chains of triangles lie at intervals of about one degree apart, as 

 originally designed by Colonel Everest. But in the sections sub- 

 sequently executed the intervals between the meridional chains were 

 materially increased, as the minor triangulations which in course 

 of time came to be executed by the topographical surveys were of 

 such accuracy that a smaller amount of principal triangulation was 



