206 ASTROMOMICAL CIRCLE. 



•—and of the A level remains constantly suspended on the horizontal axis, 



nonzomalaxis. ^i^j^h is verified in the same manner as in a transit instl-ument. 

 There are forcing screws for this purpose, which pass through 

 the bar on which the vertical columns stand, and these by pres- 

 sing against (he long axis, produce a small change in the in- 

 clination of the upper part of the ir.strument, without altering 

 the position of the azimuth circle or its axis. 



The application of the plumb-line, as already described, is 

 peculiar to the instruments made by Mr. Troughton: it regards 

 the vertical axis rather than any other part, and is, in fact, 

 exactly analogous to the usual verification of a zenith sector. 

 Oil the adjust- During the period in which I was engaged in making obser- 

 ment of circu- vation with circular instmments, I was led to consider the ad- 

 lar instruments , ,. . p,, \ ^-l j c j- .. 



vantages and inconveniences ot the usual method or adjust- 

 ing them; and it appeared to me, that the essential part of their 

 construction, which relates to their adjustment, was capable of 

 being improved. 



In order to render the nature of the improvement, which I 

 wish to propose, more intelligible, I ought previously to re- 

 mark, that there are, at present in use, two modes of adjusting 

 these instruments, which are founded on different principles. 

 Two methods In the one, two points are taken on the limb of the circle ; 

 T°'o Doi'T' ^^^ when these are brought into a given position, by means of 

 the circle are a plumb-line passing over them, the microscope or index is 



brought into a ^lade to coincide with the zero point of the divisions: by this 

 vertical hne by , , , . „. . . , -r i 



means of the method, the error in collimation remams constant; and it the 



plumb hne, adjustment is by any accident deranged, it can easily be recti- 



-is then brought ^®^> ^""^ there vs'ill be no absolute necessity for frequently re- 



to zero. versing the instrument: so that this method seems well adapted 



for large instruinents, particularly if placed on stone piers. But 



it is liable to this defect, that the adjustment cannot be exa- 



Objeciion.The mined at theraoir.ent of observation ; and if any change should 



5!^^'!?^,-?^?'^'^ , take place in the general position of the frame work, the obser- 

 may v.iry, and ^ at „ . 



render the ob- vation will be erroneous without the means of detection. It 

 servation eiro- ^^^^ probably to avoid this inconvenience, that Mr. Troughton, 

 in most of his instruments, particularly if they were intended 

 to move freely in azimuth, has preferred the other method. 

 Or. 2. The In this case, the plumb-line is attached to one of the pillars, 



plun^b-linc be- which support the microscopes in the way above described ; and 

 the Support of '^^ has no reference to any fixed points or divisions on the limb 



of 



