84 Mr. Nixon on the Measurement by Trigonometry of the 



The excellent twenty-inch telescope of this instrument is fur- 

 nished with cylindrical rings, a delicate spirit-level, and very 

 fine adjustable (steel ?) cross-wires. — It is supported in the 

 Ys of a cradle (common to theodolites of this description) 

 having a horizontal axis four inches in length. A clamp and 

 tangent screw, fitted to one of the supports of this axis, serve 

 to give a slow vertical motion to the telescope*. The lower 

 of the six-inch horizontal circles is divided (on silver) into 

 thirds of a degree, capable of being read off to half-minutes by 

 means of the two opposite verniers of the upper circle and a 

 pair of double magnifiers. The latter are very conveniently 

 fixed to a ring moveable about the rim of the under circle. 

 The upper circle is also provided with a clamp and tangent 

 screw for slow horizontal motion, and has a couple of spirit- 

 levels placed at right-angles to each other, which with the 

 four screws of the parallel plates enable the observer to render 

 the plane of contact of the two circles parallel to the horizon. 

 On the upper parallel plate is fixed a third clamp and tangent 

 screw, intended, no doubt, to bring the cross-wires of the te- 

 lescope, the circles being clamped together, with some parti- 

 cular degree coincident with the zero of one of the verniers, 

 exactly on the observed object. 



When the ground at the station proved impenetrable, heavy 

 stones were piled against the feet of the tripod (set up with its 

 legs at an angle of 45° to the horizon), in such a manner as to 

 render it fixed and inflexible. The summit of the hill being 

 swampy, the legs (set up more erect) were forced below the 

 surface of the turf to some depth, and large stones were heaped 

 on the ground immediately under the centre of the instru- 

 ment f. 



Either of the levels of the upper circle being brought ex- 

 actly over any two of the screws of the parallel plates, the lat- 

 ter served to place its bubble at the mark. The circle being 

 then moved half round, one half of the deviation of the bubble 

 from its mark was corrected by the two screws, and the other 

 by the adjusting nuts of the level. The level at right angles 

 having undergone a similar verification and correction, the 

 plane of contact of the two circles would consequently be ren- 

 dered parallel to the horizon J. The intersection of the cross- 



* The instrument has no vertical arch. 



f Noughtberry Hill excepted, the ground at the theodolite-stations 

 proved hard and firm. 



\ The correctness of this adjustment was confirmed when the upper 

 circle could be moved quite round without displacing the bubble of the 

 level of the telescope, previously brought to its mark. 



wires 



