252 Mr. J. Nixon on the Trigonometrical Height 



for the greater certainty, oblique to the horizon, and on the 

 repeating principle. As the new data, although partially 

 explanatory of the previous discrepancies, proved unsatisfac- 

 tory in some minor respects, the principal angles were re- 

 measured the following year by my repeating circle, recently 

 fitted up by Mr. Lealand with new axes, Ys, and telescope. 



The theodolite is identical in construction with the one 

 figured at page 8 of Mr. Simms's Treatise on Surveying-In- 

 struments, and would measure horizontal angles by repetition, 

 as described at pages 15 — 16 of the same work, were not the 

 tangent screw of elevation so powerful as to disturb the feeble 

 clamping-apparatus of the two circles. When the multiple of 

 an oblique angle is required, adjust the upper circle and the 

 line of coliimation to be parallel to the horizon, and conse- 

 quently to each other. The circle must then be got into the 

 plane of the two signals by the two pair of screws of the pa- 

 rallel plates; and when this is effected the telescope will bisect 

 in succession both signals as the upper plate moves about its 

 axis, and the process of repetition may be conducted in every 

 other respect precisely the same as for horizontal angles. The 

 circle would generally keep in the plane required, but in some 

 instances the deviations, which are most sensible in large an- 

 gles, would gradually become so important as to require a 

 renewal of the adjustments (which may be effected without 

 interrupting the series of repetitions). 



Mr, Nixon^s Repeating Circle. 



In giving a description and drawing of the repeating circle, 

 the object is more to exhibit its defects and objectionable pe- 

 culiarities than to hold it out as superior, or even equal, to 

 those of the usual construction. In the figure which represents 

 (on a scale of quarter size) a vertical section of the instru- 



