1872.] 



for the Indian Trigonometrical Survey, 



319 



whole results up to the date of the arrival of the Sectors ; and ten years (a 

 comparatively short period for which to arrange a system of observation on 

 a matter of this magnitude) will see us in a position to look back on the 

 arrival of the Sectors as on the commencement of a new era." 



I now pass to the Great Theodolite, which I do myself the honour this 

 evening to expose to the scrutiny of the Royal Society. A brief history of 

 the design of this instrument may here with propriety be recorded. 



There are at present at the disposal of the India Survey Department 

 two instruments of the same order as the one under consideration, namely, 

 Theodolites having Horizontal Circles 3 feet in diameter, the largest 

 dimension hitherto employed, — hence the term Great Theodolite. There 

 are, I believe, not more than four in existence of that size — one by Rams- 

 den, with which the greater part of the principal triangulation of Great 

 Britain was executed, the two Indian instruments, and the one now before 

 the Society. 



Of the two Indian instruments, one was made by Messrs. Troughton and 

 Simms about forty -five years ago, its Horizontal Circle bearing divisions cut 

 by the hand of the celebrated Edward Troughton himself. The other was 

 constructed and divided in India by the late Henry Barrow, then attached 

 as instrument-maker to the Survey, its design being the work of the late 

 Sir George Everest, then Surveyor-General of India. There are also in 

 use with the Department several theodolites of 2-feet diameter, and of 

 course many of smaller sizes. 



The principles of construction, the advantages, and the defects of all 

 these instruments were a constant theme of discussion to the officers of the 

 Department, who being accustomed, according to the excellent system of 

 the Great Trigonometrical Survey of India, to take every observation 

 personally, have thereby at all times been led to display the most critical 

 fastidiousness in every thing relating to the apparatus with which use has 

 made them perfectly familiar. 



Sir Andrew Scott Waugh, R.E., F.R.S., then Surveyor-General of 

 India, having collected all the experience of his officers, including of course 

 his own, gathered during many years of actual work in the field, combined 

 the whole into a " Project and Specification of a new Great Theodolite^ 

 which in 1855 he placed in my hands for the purpose of guiding me in the 

 preparation of a design, with working drawings and a fully detailed speci- 

 fication. The project in question fixes all the general principles intended 

 to be kept in view, and is ample for that purpose ; but it enters into very 

 few details, merely pointing out what is required, without definitely stating 

 how it is to be attained, and it contains no drawings whatever, and only 

 three or four slight diagrams. The part I had taken in the previous 

 discussions had supplied me with a knowledge of the desiderata not actu- 

 ally described, knowledge which of course was also in great measure de- 

 rived from the fact that I had worked for five years with Troughton' s Great 

 theodolite, and subsequently with other excellent instruments. 



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