1872.] 



for the Indian Trigonometrical Survey. 



323 



the plate or tongue. The lateral force generated by turning the instru- 

 ment in azimuth, when taking observations, is greatly within this limit. 

 This arrangement may be found useful in other cases. It is perfectly 

 successful ; but it is only available where a moderate range of vertical 

 movement is needed. In the present instance, as the stand on which, the 

 instrument is supported is always first made sensibly level, the vertical 

 range of the foot-screw need not be more than jfo of an inch ; but in 

 practice this is capable of being extended, without fear of injury to the 

 parts, to about ^ of an inch. 



In the actual construction, the tongue above described as formed by 

 slitting the tribrach, is, in fact, a separate plate, similar in plan to the 

 tribrach, and firmly bolted to the centre of the instrument. 



The second desideratum in the feet-screws was delicacy and certainty of 

 action ; this I attained by applying to them a clamp and tangent-screw 

 arrangement very similar in principle to that sometimes applied to circles. 

 Although the feet-screws themselves are very coarse, having about eight 

 threads only per inch, the arrangement is such that one entire revolution of 

 the slow-motion tangent-screw alters the level only one second of arc. Hence 

 the foot-screw, though coarse and strong enough to bear the great weight 

 imposed on it, is for the first time in keeping, in point of refinement, with 

 the most delicate parts of the instrument. 



The Horizontal Circle and its Micrometers. — The disposition of these 

 most important parts has been the subject of much controversy. Two 

 arrangements are possible: — (1) that called "the flying-circle" plan, 

 in which the microscopes are immovably attached to the basis of the in- 

 strument, the circle revolving with the telescope ; and (2) " the flying- 

 micrometer " plan, in which the circle is fixed and the micrometers 

 revolve. The question was carefully considered, and the balance of advan- 

 tages appearing to be in favour of the second plan, that was prescribed in 

 the project, and has been adopted. I have no doubt as to its superiority 

 for instruments of this kind. 



The horizontal circle is attached at the centre to the tribrach, and is 

 elsewhere perfectly free. 



The Guard Circle. — This addition was made to another instrument by 

 Sir A. S. Waugh, late Surveyor-General, some time before the present de- 

 sign was commenced. I saw it subsequently in an Altazimuth by Brunner, 

 of Paris, in the Great Exhibition of 1862. It was, I believe, arrived at in 

 both cases quite independently. It consists of a second horizontal circle, 

 exterior to and concentric with the circle carrying the working graduation 

 already mentioned. There is a space of about -fa of an inch all round be- 

 tween the two circles, and the upper plane of the outer guard circle stands 

 about the same quantity above that of the inner principal or working 

 circle. The guard circle has supporting radii of its own, quite independent 

 of those of the inner circle. The guard circle has four objects : — (1) it 

 protects the working circle from accidental injury ; (2) it tends to distri- 



