GREAT TRIGONOMETRICAL SURVEY OF INDIA. 63 



step was the final reduction and harmonising of the results, giving 

 to each measurement and observation its proper weight, and 

 nothing more or less. Strictly speaking, this undertaking should 

 have been postponed until the completion of the whole of the 

 operations, and then all the observations should be reduced 

 simultaneously, because every fact of observation is more or less 

 dependent on, and connected with, every other fact. But the 

 simultaneous reduction of the vast number of such facts acquired over 

 all India, by many individuals and during a period of many years, 

 was obviously impossible. Thus it became necessary to divide the 

 triangulation of India proper into five sections, and even then the 

 simultaneous reduction of the numerous facts of observation 

 collected together in each group was a work of enormous labour, 

 necessitating, as remarked by Colonel Clarke, C.B., of the Ordnance 

 Survey, one of the most eminent of living geodesists, in his recent 

 treatise on geodesy, " the most elaborate calculations that have 

 " ever been undertaken for the reduction of triangulation." The 

 division of the work into sections necessitated the maintenance of 

 the results determined for the sections first reduced in the 

 contiguous sections, when they, in turn, came to be reduced and 

 this necessitated commencement with the section, which in all its 

 parts was of the highest accuracy. The section of which the field 

 work was first completed was the North-east Quadrilateral, but as 

 many of its angles had been measured with instruments of inferior 

 accuracy to those employed in the sections which were subsequently 

 completed, the reductions were performed in the following order : — ■ 

 first, the North-west Quadrilateral ; secondly, the South-east Quadri- 

 lateral ; and thirdly, the North-east Quadrilateral. The reductions 

 were commenced in the year 1869; the final results of the first 

 section are given in Volumes II., III., and IV. of the Account of 

 the Operations of the Great Trigonometrical Survey, published in 

 1879 ; those of the second in Volume VI., published in 1880 ; and 

 those of the third in Volumes VII. and VIII. The fourth section 

 selected for treatment was the Southern Trigon. 



The stations of the principal triangulation were 3,665 in number 

 in 1885. They have been constructed with a view to being as lasting 

 and permanent as possible. On the plains they take the form of 

 towers rising from 20 to 40, and even 60 feet above the ground 

 level, and usually about 16 feet square at base, with an isolated 

 central pillar — always of masonry — for the instruments to rest on. 



