INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 15 



the rod when placed on N; if in adjustment the line of sight should 

 fall on point 4. If in error, set line of sight on point 4, then shift the 

 vernier and carefully adjust it to read zero in the new position. 



31. Setting up and leveling theodolite for observation. — Place the 

 theodolite over the observation point so that the base plate of the 

 instrument is nearlj^^ level and centered over the exact point selected. 

 To do this, see that the tripod is well opened, with legs firmly and 

 symmetrically implanted about, and equidistant from, the exact 

 point. It is well to arrange the theodolite, when setting up for obser- 

 vation, with plate clamp screw, P, figure 2, on the opposite side of 

 vertical axis, VA, from the orientation point which is being sighted 

 upon. The significance of this will be understood later. 



32. To level the theodolite, turn the telescope upon the horizontal 

 axis HA, figure 2, until it is about perpendicular to the base plate 

 (the vertical circle set at or near 90°, see reading of VV, fig. 4). 

 Disengage the horizontal tangent screw HT, figure 2, and turn the 

 instrument about its vertical axis until one of the levels, preferably 

 the standard level SL, is parallel with the fine joining any two of the 

 leveling screws, LS. See that the shifting center tension spring is 

 sufl&ciently loosened to allow ample adjustment of leveling screws, 

 then bring the bubble between the marks of the standard level by 

 turning the two leveling screws in opposite directions; that is, both 

 in or both out as the occasion demands. While in this position, 

 adjust plate level, PL, by raising or lowering with third leveling screw. 

 If necessary, relevel the instrument by the above method, noting 

 that each bubble is equally spaced between the marks on the appropri- 

 ate tube. Now turn the instrument about its vertical axis successively 

 through 90°, 180° and 270°, and observe that the bubbles are still 

 in the central positions. If they are not, then return the telescope 

 to the initial position and readjust untU this is accomplished. 



33. The theodolite now being leveled, turn the instrument about 

 its vertical axis until either the front horizontal vernier, HV, figure 

 2, or the 45° or right horizontal vernier, hv, figure 2, (depending 

 upon which vernier is to be read), is set on the azimuth bearing of 

 the reference point, then lock by throwing in the horizontal tangent 

 screw, HT. Set the vertical circle of the telescope at or near zero, 

 loosen plate clamp screw and turn the locked telescope and base plate 

 about the vertical axis until the telescope is sighted upon reference 

 point of orientation, accomplished by means of the ball and V sights 

 along the main tube of the telescope. Be sure that the azimuth set- 

 ting on base plate for the particular reference point has not been dis- 

 turbed, then lock base plate to the horizontal axis by tightening the 

 plate clamp screw P. 



Upon sighting through telescope, if it is found that intersection 

 of cross hairs is not coincident with reference point, raise or lower by 

 means of the vertical tangent screw, VT, and shift horizontally by 

 means of the slow-motion or base-plate adjustment screw. This final 

 horizontal adjustment must not be made with the horizontal tangent 

 screw, since this would disturb the orientation setting of the particu- 

 lar reference point. 



34. Adjustment of the eyepiece, by turning the aperture disk either 

 in or out, to obtain the maximum sharpness of cross hairs, and focus- 



