342 Mr. Nixon's Description of a Bepeating Circle. 



G, G, fixed to the divided circle. When the instrument is 

 placed in a horizontal position, the line of collimation of the 

 telescope can be rendered level (in the nsual manner) by the 

 adjustments of the level and those of the cross-wires. It will 

 also be parallel to the plane of contact of the wheel to the 

 circle in case the latter can be moved half-round in azimuth 

 without disturbing the bubble (of the telescope). The circle 

 C and the wheel 'W have their circumferences cut into teeth. 

 One pinion J (fixed to the wheel) serves to move the circle, 

 and the other L (fixed to the immoveable plate) turns the 

 wheel and circle, clamped together by the nut K. The wheel 

 and plate are secured together by the nut M. 



Supposing the instrument fixed by the plate P to the side 

 of a vertical wall, or mounted as a French repeating circle, 

 with a movement in azimuth ; the circle and wheel, clamped 

 together with the zero lines of the divisions of the circle coinci- 

 dent with those of their respective verniers, are moved together 

 by the pinion L, until the middle of the bubble is brought to 

 its reversing point. The line of collimation of the adjusted 

 telescope will now point level ; and if we wish to take the alti- 

 tude of a nearly horizontal star, we must disengage the circle 

 from the wheel, and direct the telescope exactly at the star by 

 turning the pinion J. Having clamped the circle to the wheel, 

 the angle of elevation is finally read off by the two verniers. 

 In the event of the subsequent obscuration of the star, note 

 the position of the bubble, and having freed the wheel from 

 the fixed plate, depress the telescope by the pinion L until 

 the bubble attains the reversing point, or that degree of its 

 scale at which the line of sight points level. Clamp once more 

 the wheel to the plate, and having disengaged the circle from 

 the wheel, elevate the telescope (and with it the divided circle) 

 by means of the pinion J, until the bubble reverts to the two 

 points of its scale between which it stood when the telescope 

 bisected the star. The angle now to be read off will, it is 

 evident, be double the altitude of the star ; and by continuing 

 the same process of measurement, a multi]:)le of the angle, suffi- 

 cient to obviate the errors of graduation and reading oflf^ may 

 be procured leisurely and accurately. 



When the altitude of the object is beyond the range of 

 the scale of the level, recourse must be had to the additional 

 level Z mounted on a toothed wheel V, which can be moved 

 by the pinion U about a short horizontal axis projecting from 

 the divided circle, or fixed by the opposite clamps X, Y. 

 Having pointed the telescope parallel to the horizon, and af- 

 terwards on the elevated star precisely after the manner above 

 indicated, clamp together the circle C and wheel W, and then 



