72 AKSEL S. STEEN. TERRESTRIAL MAGNETISM. [NORW. POL. EXP. 



to take observations in the polar regions. He therefore recommends the 

 making, if possible, of a direct determination of this coefficient upon the field 

 of observation itself. There was no opportunity, however, of making such 

 delicate investigations in the difficult natural conditions with which the 

 expedition had to contend ; and it is thus only a question of deducing indirectly 

 from the series of observations a reasonable value for a, and this only for 

 magnet V, which was the one most frequently employed. 



With this object in view, I first took all the cases in which both the 

 vibrations and the deflections had been observed with magnet F on the 

 same day, and the double needle had been employed as deflected magnet. 

 Assuming for the time being that the horizontal intensity on the above- 

 mentioned days had been the same during both the deflections and the 

 vibrations, I was able, for the determination of a, to draw up by formula (6), 

 39 equations in the form 



* V sin (p 



which fall into two groups, 15 with the deflector at the distance e, and 24 

 with the deflector at the distance E. As the constant value of /t, I employed 

 the mean of the values found in Hamburg and Wilhelmshaven, in 1897. 

 The mean value of a from the first group of equations was 



= 0-000431, 



from the second 



= 0-000567, 



and from the entire 39 



a = 0-000514. 



With this value for , and the mean values for /w and C, found by the 

 determinations in Hamburg and Wilhelmshaven in 1897, a temporary value 

 for H was calculated by formulae (4) and (5) (page 64) separately by 

 vibrations and deflections in the above-mentioned 39 cases. It then appeared 

 that in only 7 of these cases did the values for the horizontal intensity 

 deduced from the vibration and deflection observations made on the same 

 day, agree so far that there could be any question of using the observations 

 for determinations of constants. These 7 days were: 



in 1894, December 7th, 

 1895, May 24th, 



