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



The deflection-observations were made in the usual manner, the deflector 

 being placed successively in the well-known 4 positions. In each of these, 

 the telescope was pointed towards the mirror of the deflected needle, where- 

 upon the two verniers of the horizontal circle were read off, and also a 

 reading was taken each time of the thermometer placed above the deflector. 



If we call the two circle-readings (mean of the two verniers) with the 

 N-end of the declination-needle deflected in an easterly direction, u t and u%, 

 and the readings with the N-end deflected in a westerly direction, u 3 and u 4 , 



we obtain 



g> = <p' + <J , 



where 



while <5 is the correction for angular inequality expressed in minutes. 



If we put u v 2 = ^i a d U 3 M 4 = z/ 2 , and express J l and 4% 

 in degrees, then 1 



d = -A[4\ + 4}. 



As we know, the factor A has the following form: 

 A = 0-5236 [ tan q> + cot q>] , 



and according to this formula, I have drawn up a table for A for each 

 single degree from g> = 3 to q> = 70, in order to simplify the calcu- 

 lation of <5. 



The hour was unfortunately not noted for the separate settings of the 

 needle during the actual deflections, but was noted at the setting of the free 

 declination-needle before and after the deflections, and I have therefore tried 

 by interpolation to fix the hour corresponding to the angle of deflection found. 

 This interpolated hour is also assumed to be applicable to the reading of 

 the needle's position in the magnetic meridian, calculated by the deflection- 

 readings, this calculated reading being entered, as mentioned on page 17, 

 in the series of directly observed declination-readings. 



1 LAMONT. Handbuch des Erdmagnetismus, p. 31. 



