86 BENJ. PIKE'S, JR., DESCRIPTIVE CATALOGUE. 



The third adjustment is that -which makes the azimuthal 

 axis, or axis of the horizontal limb, truly vertical. 



Set the instrument as nearly level as can be done by the 

 eye, fasten the centre of the lower horizontal limb by the 

 staff-head clamp, H, leaving the upper limb at liberty, but 

 move it till the telescope is over two of the parallel plate- 

 screws ; then bring the bubble of the level under the tele- 

 scope, to the middle of the tube, by the screw P ; now turn 

 the upper limb half round, that is 180, from its former po- 

 sition ; then, if the bubble returns to the middle, the limb is 

 horizontal in that direction ; but if otherwise, half the differ- 

 ence must be corrected by the parallel plate-screws over 

 which the telescope lies, and half, by elevating or depress- 

 ing the telescope, by turning the tangent-screw of the ver- 

 tical arc ; having done which, it only remains to turn the 

 upper limb forward or backward 90, that the telescope 

 may lie over the other two parallel plate-screws, and by 

 their motion set it horizontal. Haying now levelled the 

 limb-plates by means of the telescope level, which is the 

 most sensible upon the instrument, the other air-bubbles 

 fixed upon the vernier-plate, may be brought to the middle 

 of their tubes, by merely giving motion to the screws which 

 fasten them in their places. 



The vernier of the vertical arc may now be attended to ; 

 it is correct, if it points to zero when all the foregoing ad- 

 justments are perfect ; and any deviation in it is easily rec- 

 tified, by releasing the screws by which it is held, and tight- 

 ening them again after having made the adjustment ; or, 

 what is perhaps better, note the quantity of deviation as an 

 index error, and apply it, plus or minus, to each vertical 

 angle observed. This deviation is best determined by re- 

 peating the observation of an altitude or depression in the 

 reversed positions, both of the telescope and the vernier 

 plate : the two readings will have equal and opposite errors, 

 one half of their difference being the index error. Such a 

 method of observing angles is decidedly the best, since the 

 mean of any equal number of observations taken with the 

 telescope reversed in its Y's, must be free from the effects 

 of any error that may exist in the adjustment of the vernier, 

 or zero of altitude. 



The theodolite, as constructed in the manner we have 

 described, is not inconveniently heavy, as the diameter of 

 the horizontal limb seldom exceeds five inches ; but when 



