16 



AKSEL S. STEEN. TERRESTRIAL MAGNETISM. [NORW. POL. EXP. 



If the reading (corrected for the needle's error of collimation) on the 

 magnetic apparatus, with the telescope directed towards the mirror of the 

 declination needle, be termed M, and the reading on the same apparatus, 

 with the telescope directed towards the mark (U), m, the angle 



y = Mm. 



If, in the next place, the reading on 

 the astronomical instrument, with 

 the telescope directed towards a star 

 s, be indicated by S, the reading 

 with the telescope directed towards 

 the mark (T) by C, and the azi- 

 muth of the star, calculated from 

 N through E, by A, the angle 



a>-(8-C)-A, 

 and we obtain 



= M-S-(180 



= M+B. (1) 



Occasionally the azimuth deter- 

 minations were made directly with 

 the magnetic apparatus by obser- 

 vation of the sun or a planet. In 

 such a case, retaining the same 

 signs as above (see Fig. 2), 



\ = M + (A-180)-S, (2) 



and the setting at the mark only serves as a check 

 upon the stability of the instrument, while the 

 observation was being made. The calculation of 

 the angle C S + A, or A 180 S, I have 

 received from Professor GEELMUYDEN in whose 

 paper, "Astronomical Observations (Norw. Pol. 

 Exp. No. 6), List G. Determination of Azimuth", all 

 the data for the present observations will be found. 



Fig. 2. 



