a)}d of the Altitude and .izi ninth Instrument. 15 



quite equal to tlie puqiose, is to look with the front telescope 

 at any elevated object, whether remote or near, and having 

 made the middle vertical wire bisect it, look at the same object 

 when i-eflected from the surface of a fluid. If the wire does not 

 cut the reflected image, the circle must be turned round the 

 cross axis, to bring the wire as nearly as can be estimated half 

 way towards that image ; now by turning the intrument in 

 azimuth, make the bisection, then elevate the telescope to the 

 object, and if the bisection is not perfect, the operation of esti- 

 mating and turning in azimuth must be repeated. A level, 

 which is placed parallel to the axis of the circle, must now be 

 adjusted so that the bubble may stand in the middle of its tube; 

 which afterwards becomes the index for the vertical position of 

 the circle. Another adjustment, which is not however of so 

 much importance as either of the former, is to make the cross 

 axis at right angles to the vertical one ; which is indeed the 

 business of the maker. If, when the vertical axis is adjusted, he 

 brings the upper circle horizontal by means of a pocket-level, 

 whicli is to be placed upon the face of the circle at right angles 

 to this axis, then, by placing the level parallel to the axis, he 

 will see which of the supports wants to be shortened bv the 

 file. 



Adjustments of the Altitude and Azimuth Instrument. 



The axis of azimuth is rendered vertical by means of the 

 level and feet-screws ; exactly in the manner that was required 

 in the other instrument ; and it may be stated that either or 

 both of the levels belonging to it may be used for this pur- 

 pose. That adjustment, which answers to the second for the 

 repeating circle, or setting the line of collimation perpendicular 

 to tlie axis, is no other than the usual way practised for doing 

 the same thing in a plain transit; namely, by moving in azi- 

 muth bring the middle vertical wire to any object, then reverse 

 the horizontal axis end for end upon its sujjjiorts, and if in this 

 position the wire does not cut the same object, alter one half 

 the error by turning in azinmth, and the other by means of 

 the screws whicli act upon the wire-plate. The transit axis 

 is brought to the horizontal position simply by ]:)lacing tlie 

 level uj)on tlie j)ivots of the axis, and observing if the air-bulb 

 changes its place on turning the level end for end. If it does, 

 nothing more is wanted to effect the adjustment, than with the 

 screw Ijclow either of tlie pivots, to bring tlie bubble, accord- 

 ing to the indication of the dividetl ivory scales, just halfway 

 towards the place which it occujiied in the first position. 



Hotli the circles under consideration recjuire many more 

 adjustments : but as those belong to the minor jiarts, and are 



common 



