To Adjust the Vertical Axis to Verticality, however, level 

 up over either set of leveling screws, revolve the instrument 180 or 

 thereabouts, correct half of the bubble displacement with the level- 

 ing screws and the other half with the vertical control, C. Repeat 

 the operation over both sets of leveling screws until the bubble 

 remains centered during an entire revolution. 



This process secures a normal position for the vertical control 

 and the vertical axis with respect to the axis of the bubble, and for 

 accurate work it is necessary that this adjustment should be 

 accomplished and maintained within the limit of a few divisions on 

 the bubble tube. The H. I. is determined by the position of the 

 pivots at D, and it will be readily seen that if the axis VX is not 

 truly vertical, or very nearly so, the elevation of the point D will 

 change when the instrument is reversed. 



Having determined the normal position of the vertical control, 



as directed above, the zero of 

 the graduated drum should 

 coincide with the index blade. 

 If not, clamp the set screw 

 just above the vertical control 

 and turn the drum itself 

 against a concealed inner 

 spring until a coincidence is 

 effected. Clamping the set 

 screw may slightly disturb 

 the bubble, but it will center 

 itself again on being released. 

 The little index line on the indi- 

 cator blade, as located by us, 

 should now cut across the top 

 of the graduated drum. 



In using the instrument, 

 the V. C. should be constantly 

 used to center the bubble at 

 the instant of observation 

 without moving i n one's 

 tracks. By this means, resid- 

 ual errors in the adjustment 

 of the vertical axis are quickly 

 overcome, and, as shown in the 

 diagram, the sight-line and 

 pig , 3 bubble axis can be brought 



