for the Indian Trigonometrical Survey. 373 



lbs. 



(1) Standi with Levelling-apparatus complete 108 



(2) Iron Circle of Stand with 3 Centering-Screws 156 



(3) Tribrach, with Horizontal Circle, Guard ditto, Vertical Axis, 



and Relieving-apparatus 180 



(4) Vertical-Axis Socket, with 5 Horizontal-Microscope arms, 



Elliptical Table, Pillars, 2 Azimuthal Levels, 2 Vertical 

 ditto, 2 Vertical Microscopes, 2 Pointer ditto, Horizontal 



Tangent-Screw plates 235 



(5) Telescope, with Vertical Circle 64 



(6) Miscellaneous appliances and spare parts (about) 130 



Weight of Instrument 873 



(7) Aggregate weight of field packing-cases, assumed at f the 



weight of contents 582 



w . Total weight of Instrument in field-cases .... 1455 



The heaviest package (4) packed in field order will be, — instrument 

 235 lbs. -f case (f) 157 lbs. : total 392 lbs. This is less by about 100 lbs., 

 according to my recollection, than the heaviest package of Troughton's 

 Great Theodolite, long in use with the Indian Survey, and with which I 

 myself worked for five years in a country parts of which were extremely 

 rugged and mountainous. Expense apart, the portability of an instrument 

 is of course defined by the weight of its heaviest portion. 



Nevertheless the instrument is considerably heavier than I should wish. 

 There are three ways of diminishing this inconvenience : — first, by di- 

 minished dimensions ; second, by mode of construction ; third, by choice 

 of materials. I will say a few words on each. 



When the designing of this instrument was placed in my hands by Sir 

 Andrew Waugh, the late Surveyor- General of India, he left the question 

 to my decision whether the Horizontal Circle (which governs the whole) 

 should be 30 or 36 inches in diameter. With my five years' experience of 

 the larger size, and with ample experience of smaller ones, I deliberately 

 preferred and adopted the larger, and am responsible for that decision. I 

 believe myself to be in the minority on this question ; but can only say 

 that, if I had to use the instrument myself, I should still prefer the larger 

 size, attaching, as I do, great importance to the greater optical power and 

 the superior stability which are secured by it : for these I would myself 

 willingly sacrifice convenience to a great extent. 



By " mode of construction," I mean particularly the degree in which the 

 main parts are cast in single masses. That principle, when the design was 

 commenced, had been notably illustrated by the present Astronomer Royal 

 and President of this Society, in the new instruments designed by him for 

 the Royal Observatory and now in use there. Like many others, I was at 

 the time fascinated by it ; but I am now disposed to think that this prin- 

 ciple has been carried too far in my own design under notice. Discretion 

 is requisite in applying even an unquestionable principle. And I now 



VOL. XX. 2 E 



