80 LEVELING 



2. To make the line of sight parallel to the axis of the 

 level tube, so that the line of sight will be horizontal when 

 the level bubble stands in the center of its scale. 



First Adjustment. Plant the tripod firmly and level up the 

 instrument, using each pair of leveling screws. With the tele- 

 scope over one pair of leveling screws, center the bubble accu- 

 rately, then reverse the telescope end for end over the same pair 

 of leveling screws. If the bubble runs toward either end, 

 bring it half way back by means of the capstan nuts at one 

 end of the level tube; then center it with the leveling screws. 

 Repeat the operation over each pair of leveling screws alter- 

 nately until the bubble will remain centered perfectly through- 

 out a complete revolution of the telescope. 



Second Adjustment. The second adjustment is effected by 

 establishing a horizontal line and adjusting the cross-hairs to 

 agree with it while the bubble is at the center of the tube. 

 To establish this line, drive two pegs into the ground several 

 hundred feet apart and determine the true difference in ele 

 tion of these pegs. This can be accomplished even with 

 unadjusted instrument by setting it up at a place having 

 same distance from each peg and then taking rod readings and 

 subtracting them. Next, set up the instrument over one peg 

 with its center at a distance from the peg horizontally equal to 

 about one-half the length of the telescope; bring the level 

 bubble to the center of the tube, and with the leveling rod 

 measure the exact height of the intersection of the cross-hairs 

 above the peg. To determine this height on the rod, hold the 

 graduated face of the rod about a half-inch from the eye end 

 of the telescope, and by looking into the object end of the 

 telescope bring the point of a pencil in the center of the small 

 field of view on the face of the rod. Set the target at this 

 height, plus or minus the difference in the elevations of the 

 pegs, according as the rod reading on the distant peg was more 

 or less than on the peg over which the instrument is set; theo 

 direct the telescope toward the rod held on the distant peg aod 

 adjust the cross-hairs so that the horizontal cross-hair wili 

 exactly bisect the target when the level bubble stands in the 

 middle of its scale. 



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