I and of the Altitude and Azimuth Instrumeni. 11 



is equal to about two-thirds of the diameter of the circle ; the 

 outer part of which, being pertbrated from end to end, be- 

 comes the socket for an axis of tlie same length, to which the 

 index of the circle and telescope is attached in front. The 

 index has tour branches placed at half right angles to the tele- 

 scope, each of which subdivides the divisions of the circle into 

 spaces of lO". To the middle of the cross axis is fixed a socket, 

 which receives about two-thirds of the length of the centre- 

 work: and the exterior surface of the remaniing third of that 

 work becomes the axis for another telescope and a level to re- 

 volve contiguous to the back of the circle. This is a com- 

 plicated matter, difficult to be described or understood with- 

 out a figure : it will however be sufficient, if it is conceived 

 that there are tliree concentric motions in planes parallel to 

 that of the circle: nameh', a general one within the socket of 

 the cross axis, which carries round together, the circle, level 

 and two telescopes : another, by wliich, upon the exterior part 

 of the centre-work the level and back telescope revolve; and 

 a third, that gives motion to the fore telescope and the verniers, 

 so as to make them advance upon the circle, which is produced 

 from the interior axis. These motions are independent of each 

 other, and are all furnished wi|;h clamping and tangent screws. 

 A counterpoise is placed upon the exterior end of the centre- 

 work, which, by balancing the circle, telescopes, and level, 

 keeps them stationary in any position. The greatest part of 

 these instruments, M-hich have been constructed in London, 

 have the back telescope on one side of the axis, and the level 

 parallel to it on the other side, which latter, being made heavier 

 than would be otherwise required, becomes a counterpoise for 

 the former, a thing not attended to in the earlier constructions 

 of the repeating circle. The azimuth circle of this instrument, 

 only just named above, was in the first construction small, and 

 of no other use than to point out roughly when the upper cir- 

 cle had been tui'ned half round; but, in most of those made 

 in London, to that circle has been given the snine radius, and 

 the same attention paid to its execution as to the upper one. 

 In the best construction of this part the circle is attached 

 to the tripod, and tlu'ee indices fastened to the vertical pillar 

 revolve round it; ihiis may a horizontal fingle be taken on 

 three ecjuidistant parts of the circle, and, what is of ecjuai im- 

 portance, by simpiv reversing the jiosition of the telescope and 

 turning half roiuid in azimuth, a similar obstrvation maybe 

 made, in which the readings will tall at 60" tlistauce from the 

 former ones. Bv tliis double operation simple errf)rs of divi- 

 sion may be considered as verv much diminished, ench sight 



Vt 2 havmg 



