1898-1902. No. 6.] TERRESTRIAL MAGNETISM. 69 



D. INCLINATION. 



As I mentioned in the Introduction, the Fox circle belonging to 

 the Neumayer apparatus was the only means for the determination of 

 inclination that the Expedition possessed. There were two needles with 

 it, only one of which, that indicated by the letter B, was in a condition 

 to be used. It was the needle that was principally used during the first 

 Fram Expedition. 



After the meridian reading of the horizontal circle had been deter- 

 mined in the usual manner by four settings in the magnetic prime ver- 

 tical, three or four settings and readings were made of the north and 

 south ends of the needle in both positions of the instruments, "circle E" 

 and "circle W". The same observations were then generally repeated 

 with the needle reversed in its bearings, that is to say, the one time 

 with the marked side of the needle outwards, and the other time with 

 the marked side inwards. I have called the two positions of the needle 

 respectively B^ and B^. 



Previous to a direct determination of the inclination such as this, 

 an indirect determination would also often be made by the aid of the 

 deflectors belonging to the apparatus. 



The alhidade on the back of the vertical circle, with one deflector 

 screwed in, was first adjusted with its zero-point at that point of the hmb 

 answering to (/ — 30°), when 1' stands for the mean of the inclination 

 readings, which had previously been obtained with the free needle in 

 the same position of the instrument, B^ or B^, "circle E" or "circle W". 

 When the north and south ends of the deflected needle had been sighted 

 and read three or four times, the alhidade and deflector were moved 

 across to the position (/' -f 30°), whereupon corresponding sights and 

 readings of the needle, now deflected to the other side, were made. 

 In this way, a new value of T was obtained for each position of the 

 instrument, being the mean of the readings made with the deflected 

 needle. 



