(a) Sight some fixed point as high above the instrument as 

 can conveniently be observed. Depress the telescope and mark a 

 point on the ground a short distance beyond the instrument There 

 is no need of going to the nadir. It an error exists, the amount 

 would increase constantly with depth but the angular value would 

 not change. Reverse the instrument in altitude and azimuth and 

 re-center the telescope on the test point aloft. Depress the tele- 

 scope a second time and compare with the first located point 



If a coincidence occurs, the adjustment is complete. If not, 

 recollect that the horizontal axis is at right angles to the 

 direction of the last sight. If the last sight struck to the right, the 

 right axis is too high and 'vice versa. A mean position between 

 the two lower test points will be vertically below the upper one. 

 Move the adjusting block up or down, as the case may be, until 

 the vertical wire cuts the central point. 



The adjusting block, in the upper end of the standard, is held 

 in position between a capstan head adjusting screw beneath and 

 the bearing cap above. It is always best therefore to bring the 

 adjusting block into place by an upward movement of the capstan 

 screw, but in case it has been moved upward a trifle too far, 

 the excess can frequently be corrected by tightening the dowel 

 screws in the bearing cap. ' This association of parts, however, 

 ought not to be strained. Some skill and judgment, in fact, will be 

 required to get the bearings nicely seated on each side without 

 undue strain. 



(b) A second method is a modification of the first. Bisect a 

 well defined high point a\ depress the telescope to the horizon, or 

 ground, and mark a reference point, A, as indicated by the vertical 

 wire. Reverse in altitude and azimuth and again sight the point, a\ 

 depress the telescope as before and locate a third point, c. By 

 means of the slow motion screw, turn the alidade until the vertical 

 wire is midway between b and c then elevate the telescope on this 

 alignment to the locality of the point a. This upward movement 

 will describe a path parallel to the last downward plunge. There- 

 fore correct the whole error indicated in order to get the cross wires 

 vertically over the lower median point. 



When in 1886, A. V. Lane was Ass' t Prof, of Math, in the Univ. ot 

 Texas, he published a Handbook of Adjustments in which he successfully 

 proved, by descriptive geometry, that the correction point was not 

 necessarily midway between the two test points. This abstruse 

 principle, however, has not been generally noticed, for adjustments 

 in nearly all cases are matters of trial and no one will be satisfied 

 with a test until it has been verified by repetition. 



73 



