61 



and the point previously bisected. Again bisect the first point selected, reverse the 

 telescope and find a new point precisely in the new line of sight of the telescope ; 

 these two points will now remain 'bisected when the instrument is pointed upon them 

 in the manner described above, if the adjustment is correctly made. If the two points 

 are not now both bisected, the adjustment must be repeated until this be the case. 



To determine whether the standards are of the same height, suspend a plumb-bob 

 by means of a long cord from a height say of from thirty to forty feet. The plumb- 

 bob may swing in a bucket of water to keep it steady. Level the instrument carefully, 

 and point upon the plumb-line at its base. If the plumb-line remains bisected through- 

 out its entire length when the telescope is moved in altitude, and then the telescope 

 reversed and again made to bisect the line throughout its length from its base upward, 

 the adjustment is correct. Otherwise make the adjustment by means of the capstan- 

 headed screw directly under one of the telescope wyes. Loosen the screws in the pivot 

 caps and turn the adjusting screw right handed until it is in the required position. If 

 it is already too high, it should be unscrewed a little and again raised by a right hand 

 turn to the correct position. If this is not done the support is likely to stick and not 

 rest on the adjusting screw, thus causing liability to error. The screws in the pivot 

 caps should then be turned down just enough to. prevent looseness in the bearings. 



It now remains to adjust the vernier of the vertical arc to read zero when the 

 telescope is level, to complete the adjustments of the transit. Bring the telescope 

 level bubble in the middle of its tube, and with the bubble in this position, set the 

 zero of the vernier to coincide with the zero of the vertical arc; loosening the 

 capstan-head screws, which secure the vernier to the standard, in so doing. 



NOTE. If the vernier for the vertical arc is single, made to read both ways, in reading it proceed to the 

 right or left on the upper line of figures in the direction of the graduation used, and if the coincident line of the 

 vernier is beyond the 15' line, continue on the lower line of figures on the other half of the vernier, so that the 

 whole graduation from o' to 30' lies in the same direction. Messrs. C. L. Berger & Sons now make double 

 Terniers for their vertical arcs, similar to those described on pp. 34 and 35. 





