1898-1902. No. 6.] TERRESTRIAL MAGNETISM. 



B. DECLINATION. 

 THE NEEDLES. 



There were two declination needles belonging to the apparatus, one 

 double needle, P, and one rather shorter needle, L, to be placed on a 

 pin as in an ordinary Neumayer's declinatorium. Both needles were fur- 

 nished with a mirror, but only P could be reversed; and it was this 

 needle that was always used for determinations of declination, while the 

 small needle, L, was only employed as a deflected magnet in deflection- 

 observations for the determination of horizontal intensity. 



The position of the double needle was noted as "Skr. op" {heads 

 of the screws up) and "Skr. ned" (heads of the screws down), and these 

 two positions will be indicated in the following pages respectively by 

 the signs P, and P^. 



The needle's total error of collimation (mirror and magnetic axis) 

 was + 30.1' during the first Fram Expedition, so that the correction 

 — 30.1' was applied to the readings in the position P,, which gave too 

 large an easterly declination, and -)- 30.1' in the position Pj, which 

 gave too small an easterly declination. 



The following values for the error of collimation were obtained from 

 the first determinations of declination at Station I a, during the last few 

 days of November, 1898: 



1898. November 25, + 24.7' 

 — 28, + 31.1' 



Mean + 27.9', 



which shows, when we consider that the observations on the 28th Novem- 

 ber were made under particularly quiet magnetic conditions, that at the 

 starting of the Expedition, the needle had practically the same error of 

 collimation as before. 



After the return of the Expedition, a series of constant determina- 

 tions were made with the apparatus, as stated in the introduction, at 

 Potsdam by Roald Amundsen. The value for the error of collimation 

 given by the determination of declination made there, simultaneously 

 with readings of the declination variometer, was: 



