INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 17 



some prominent point, as the tip of church spire, peak of gable roof, 

 sharp corner of building, etc., and note the azimuth reading of this 

 point. Take particular care that all subsequent azimuth readings 

 during this observation are made from the same horizontal vernier. 

 A little time before culmination occurs, say half to three-quarters of 

 an hour, a little practice should be gained by sighting upon the upper 

 of the two stars and rapidly shifting 

 the sight to the lower one. By the 

 time culmination occurs, if the prac- 

 tice of raising and depressing tele- 

 scope has been carried out, the ob- 

 server will have gained considerable 

 proficiency in the act, and the final 

 movement at time of culmination 

 will be performed with little or no 

 difficulty. 



41. Have the cross hairs illumi- 

 nated as mentioned above and the 

 telescope properly focused. Engage 

 both vertical and horizontal tangent 

 screws and bring the intersection of 

 the cross hairs centrally over the 

 star in question. Quickly note the 

 readings on the respective verniers 

 and rapidly depress the telescope to 

 elevation of the lower star by turn- 

 ing the vertical tangent screw, but 

 do not disturb the horizontal tan- 

 gent screw during the depression. 

 The lower of the two stars will appear 

 to the left of the vertical cross hair, but it will gradually approach the 

 vortical hair as the time of culmination is approached. Raise the 

 telescope to the upper of the two stars again, reset, read the angles 

 from the same two verniers, and immediately depress the telescope, 

 as before. Repeat the foregoing operation until it is observed that 

 the Jower of the two stars also falls upon the vertical cross hair when 

 the telescope is depressed. When this is obtained, raise the telescope 

 to the altitude position of Polaris, but do not disturb the azimuth setting 

 or the result of the observation will be of no avail. As a check, note 

 that Polaris culminates just 10 minutes after the culmination of Delta 

 Cassiopeia and Mizar. Note and record the azimuth setting, then 

 depress the telescope to sight upon some conveniently accessible 

 object where a distinct point coincident with the intersection of cross 

 hairs will be placed. This point so placed will be true north. 



Example 1: Suppose the theodolite is first sighted upon the cross 

 of a church spire to the right of north, and the azimuth bearing, read 

 from the right horizontal vernier, is 126.15°. Let 98.4° be the read- 

 ing from the same vernier when Delta Cassiopeia and Mizar are in 

 culmination. The difference between these two readings will give the 

 angle at observation point between true north and the reference 

 point, or the bearing of reference point from north: 126.15°— 98.4° = 

 27.75°; thus, when theodolite is set up with zero of base plate on 

 north, the azimuth bearing of cross on church spire will be 27.75°. 



Figure 9.— Constellations of Ursa Major and 

 Cassiopeia. 



