324 



Lieut. -Col. A. Strange on a new Theodolite 



[May 2, 



bute changes of temperature uniformly over the circumference of the work- 

 ing circle ; (3) it receives the clamp and tangent-screw, leaving the working 

 circle absolutely free from contact at all times ; and (4) it bears a strongly 

 cut set of divisions, more visible to the naked eye than those of the work- 

 ing circle, which are exceedingly fine, and therefore inconvenient for setting 

 the instrument approximately in azimuth. 



Relieving Apparatus. — The moving parts of the instrument, namely, 

 the pillars and their foundation- plate, the vertical-axis socket and hori- 

 zontal-microscope arms, the telescope and the vertical circle, with its ad- 

 juncts, constitute a very ponderous mass, weighing 284 lbs.* If the whole 

 of this great weight were allowed to bear on the flange of the vertical axis, 

 the friction would be so great as almost to cause cohesion. To obviate 

 this in similar cases, as in that of Troughton's Great Theodolite, internal 

 rollers were introduced, pressed upwards by means of spiral springs against 

 the lower surface of the moving mass. But this arrangement was imper- 

 fectly carried out in Troughton's instrument ; and it is clear that at that 

 time the action of sucli an antifrictional apparatus was not fully understood. 

 There were three soft brass rollers with central holes revolving on axes fixed 

 immovably in a circular ring, which ring was pressed upwards by three 

 spiral springs. 



The late Sir George Everest, in his work on the f Measurement of the 

 Meridional Arc of India,' p. cvii, states that when the instrument was first 

 received " angles taken with it could not be depended on to within 

 50 seconds of the truth," — a defect which was found to be mainly attribu- 

 table to the inequality of tension of the three spiral springs, which exerted 

 forces respectively in the proportions 8, 1 1, 15. Shortly after I joined the 

 Survey Department, I, with the consent of its head, made a new set of 

 rollers myself, the axes of which were fixed to them, instead of to the frame, 

 the axes revolving in Y bearings. This improved the levelling of the in- 

 strument ; but it was not possible, with every care, to maintain perfect 

 equality of tension of the three springs ; and this part of the instrument 

 always caused anxiety. 



The arrangement adopted for the present instrument is as follows : — The 

 three rollers are of larger diameter than before, and they are of steel, finished 

 with extreme care. They do not support the weight on their axes at all, 

 but on their circumferences, precisely as is now customary with heavy 

 revolving observatory-domes. The friction is therefore no longer of the 

 rubbing, but of the rolling kind. The three rollers are connected, as in 

 domes, by a light ring, which performs no function but that of keeping them 

 equidistant. Means are also provided for restricting the path of the rollers 

 to a circle perfectly concentric with the axis of motion of the instrument. 

 Finally, the rollers are pressed upwards by no less than forty, instead of 

 only three, spiral springs, by which multiplication a generally uniform 

 tension is much more easily maintained, the mean tension of each spring 

 * This has been recently increased to 299 lbs.' by additions. 



