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=M—wm. 
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 (7) by C, and the azi- 
muth of the star, calculated from 
N through FE, by A, the angle 
x= (S— C)— A, 
and we obtain 
D=M+(C—S+A)—m 
=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 
Fig. 1. such a case, retaining the same 
signs as above (see Fig. 2), 
D=M—S— (180°— A)=M+(A— 180°) —S, (2) 
and the setting at the mark only serves as a check N 
upon the stability of the instrument, while the 
observation was being made. 
The calculation of 
the angle C— S-+ A, or A — 150° — S, I have 
received from Professor GErELMUYDEN in whose 
paper, “Astronomical Observations 
(Norw. Pol. er 
Exp. No. 6), List C. Determination of Azimuth’, all 
the data for the present observations will be found. 
Fig. 2. 
