XVIII GEELMUYDEN. ASTRONOMICAL OBSERVATIONS. (NORW. POL . E XP. 



If a be the azimuth of the star at the moment of observation, reckoned 

 from south through west, as computed from the given declination, latitude 

 and clock error, 



A = 180 + a + i tg h c sec h 



will be the azimuth, from north through east, corresponding to the circle rea- 

 ding 5 for the star; here h is the apparent altitude of the star, the double 

 sign of c corresponding to obj. right and obj. left. 



It has not been necessary to take account of the collimation. For the 

 high stars the effect is eliminated in the mean, as the observations were taken 

 in both positions of the instrument and the altitudes were nearly the same 

 on both occasions. This is, however, not visible from the circle-readings, which 

 ought to differ by about 180, but do not do so, the observer having always 

 added 180 to the second circle-reading. When the small altazimuth was used 

 the difference of 180 has been retained. 



For some low stars, observed only in one position of the instrument, the 

 effect of collimation will be very nearly the same as for the terrestrial mark, 

 supposing both to have been observed in the same position of the objective 

 relative to the observer, which has not always been expressly stated. 



The accuracy of angle-measuring with the magnetic theodolite being infe- 

 rior to that of the large altazimuth, a few seconds of arc are of no impor- 

 tance in the determination of azimuth. 



The values of the angle C S-\-A, where C is the circle-reading for the 

 mark in the magnetic observatory, were transmitted to Mr. STEEN for applica- 

 tion in the reduction of the observations of declination. 



On several occasions the Sun was observed directly with the magnetic 

 theodolite. 



Lieut. SCOTT-HANSEN also made a great number of independent determi- 

 nations of the magnetic declination by means of the azimuth compass, which 

 was for this purpose mounted on the ice at a distance of at least 60 paces 

 from the ship. The observations then consisted in simply noting the time 

 when the Sun or a star passed the plane of the sights, and reading off the 

 card of the compass. The reduction of these observations does not call for 

 any further remark. 



Most of the azimuth-observations have been computed by Mr. ALEXANDER 

 and Mr. GRAARUD. 



