1872.] for the Indian Trigonometrical Survey. 



321 



tend the carrying out of this design and that of the other instruments 

 already enumerated. 



It is now ten years since the work was placed in the hands of Messrs. 

 Troughton and Simms — a long period certainly. The delay has arisen 

 from several causes — from the removal of the factory of that firm to 

 Charlton, from the necessity for various modifications and some experi- 

 ments, particularly on aluminium bronze as the material for some portions 

 of the instrument, from the time occupied in the designing, supervising, 

 and testing the other numerous and important instruments which I have 

 enumerated, and, finally, from the time which my duty of superintending 

 the whole scientific instrument supply of India necessarily occupies. 



I cannot here attempt a full description of the instrument. I trust that 

 such a description may at some future time be executed by myself, or by 

 some officer of the Survey Department ; but, for my own part, the leisure 

 necessary for such an undertaking, although all the necessary materials are 

 at hand, is denied me at present. 



I will, however, indicate some of those salient features which may be 

 rendered intelligible without illustrative figures. 



The principal dimensions are as follows : — 



Horizontal Circle. — 3 feet in diameter, divided on silver to 5', and read 

 to tenths of a second by five equidistant micrometer microscopes. 



Vertical Circle. — 2 feet in diameter, similarly subdivided by two micro- 

 meter microscopes for ordinary terrestrial work ; but by four, whose posi- 

 tions can be shifted, for astronomical work. 



Telescope. — Aperture 3*25 inches ; focal length 36 inches ; supplied with 

 two distinct eye-ends, carrying respectively a vertical and a horizontal parallel 

 wire micrometer, and furnished with both bright and dark field illumination. 



Vertical Axis. — A truncated cone of steel, base downwards, 10*6 inches 

 high, and 3*31 inches and 2 - 06 inches in diameter at the base and summit 

 respectively, the flange being 4*4 inches in diameter, and of the " isolated " 

 form described by me in the Royal Astronomical Society's Memoirs, vol. 

 xxxi. 



Horizontal Axis. — 13*18 inches between the shoulders of the pivots, 

 cast in one piece of gun-metal with steel pivots. 



The Vertical-Axis socket and the five Horizontal-Microscope arms are 

 cast in one piece of aluminium bronze. 



The Stand has three massive mahogany legs, braced together with hori- 

 zontal and oblique bars of wrought iron, wooden braces having been found, 

 in great hygrometrical changes, to impart azimuthal disturbance. I 

 believe this to be the first stand for such instruments in which the means 

 for levelling approximately are completely worked out. Within each leg, 

 divided vertically for that purpose, is a long very substantial square- 

 threaded upright screw actuated by a ratchet-wheel and endless screw, 

 and capable of being immovably clamped, after adjustment, at points 

 15 inches asunder. On the upper conical ends of these three screws rest 



