16 INSTRUCTIONS FOR MAKING PILOT BALLOON OBSERVATIONS 



ing the telescope by use of the rack and pinion, will complete the 

 orientation and setting of the theodolite for observation work. 



35. Orientation of theodolite. — In single theodolite work the azimuth 

 should be set on zero when the telescope points toward the south. 



In double-theodolite work, both instruments are set with zero 

 azimuth along the base line that is, the thedolite at station A is set 

 with 0° on station B, and the theodolite at station B is set with 180° 

 on station A. This method of orientation greatly simplifies compu- 

 tation as will be brought out in a later paragraph. 



However, before orientation can be accomplished, the exact posi- 

 tion of the north point must be determined. 



36. Determination of north point. — Three methods are here given 

 for the determination of the north point. The first method is by 

 the culmination of Delta Cassiopeia and Mizar, the second by setting 

 on Polaris, and the third by determining the time at which the sun 

 is on the meridian, 



37. The culmination method is much the simplest of the three, re- 

 quiring neither computation nor tables; it is necessary to know only 

 the approximate time of culmination. However, during certain 

 periods it will be inconvenient to determine the north-south line by the 

 culmination method, due to clouds obscuring one or both constella- 

 tions, or culmination occurring at a time when the sky is so w^ell 

 lighted that the stars can not be seen. 



38. Whichever method is used, the theodolite must be in perfect 

 adjustment and the actual point of observation selected and per- 

 manently marked. The observer's watch will be compared with the 

 standard of time in local use, and corrections made as become neces- 

 sary. The theodolite will be placed centrally over the point to be 

 determined. Much care and attention should be given to adjustment 

 and leveling of the instrument and determination of angles. Ail 

 angles should be read to the nearest hundredth of a degree when 

 extreme accuracy is desired. 



In either the first or second method, it will be necessary to provide 

 a means of illuminating the cross hairs. Any method whereby a beam 

 of light can be reflected or thrown into the object end of telescope 

 giving sufficient illumination to set forth the intersection of cross hairs 

 and not flood the field with light to the extent that the image of the 

 star is lost will answer the purpose. 



39. First method. — Delta Cassiopeia is the lower left-hand star in 

 the constellation Cassiopeia, figure 9, when this constellation is in 

 the position of the letter "W". During culmination this star crosses 

 the north-south line 10 minutes in advance of Polaris and at the 

 same time as Mizar, or the middle star in the handle of Ursa Major. 

 These two stars mentioned are on opposite sides and nearly equidis- 

 tant from Polaris. Culmination of these two stars occurs twice in 

 24 hours, and is followed within 10 minutes by Polaris crossing the 

 same meridian. These facts, with the aid of an instrument, afford 

 a simple means of determining the north-south line. 



40. Having determined the approximate time of culmination of 

 Delta Cassiopeia and Mizar, the theodolite is set over the exact point 

 for which the meridian is to be determined, and leveled very carefully. 

 It is well to do this while it is yet light. Be sure that the base plate 

 is firmly locked and that both vertical and horizontal tangent screws 

 can be turned freely without resistance. Sight the telescope upon 



