1TO. 7.] 



HORIZONTAL INTENSITY. 



65 



The two distances at which the deflectors can be placed, are, as pre- 



viously stated, 



e = 29-840 cm. 



E = 39-638 



so that the proportion between e and E is as 1 : 1'33. By experiments in 

 Hamburg during the construction of the instrument, the distances were 

 chosen with a view to giving the deflection angles in the polar regions, where 

 the horizontal intensity is very small, a suitable, not too great, value. 

 According to the formula 



sin > = 



Hsin 



where H indicates the horizontal intensity in Hamburg, and gn the angle 

 of deflection found during the experiments there with the apparatus, Dr. 

 NEUMAYER calculated the angle of deflection for Kristiania, Tromso, and the 

 polar regions, assuming for these localities a horizontal intensity of respec- 

 tively 0-1616, 0-1228, and 0-0500 (C. G. S.), with the following result: 



After this, it would always be possible in the regions which the Fram 

 might traverse, to obtain efficient deflection-observations with magnet V at 

 both distances, while magnet VI would in most cases probably only be capable 

 of being employed at the greater distance E. It is also probable that it 

 would be more advantageous on the whole for magnet V to use the greater 

 distance. These hints were followed. During the expedition, there were, on 

 the whole, 163 angles of deflection determined, 132 of them being with 

 magnet V, and 31 with magnet VI. In the 132 cases in which magnet V 

 has been used, q> was determined 79 times with the deflector at the distance 

 E, and 53 times with the deflector at the distance e, both distances being of 

 course employed simultaneously when opportunity offered. Out of the 31 

 determinations of the angle of deflection with magnet VI, the short distance e 

 has been used in only 4 cases. 



