196 AKSEL S. STEEN. TERRESTRIAL MAGNETISM.  [NORW. POL. EXP. NO. 7.] 
The principle here followed for the grouping of the observations is, as 
I have already said, arbitrarily chosen, and I have therefore not considered 
it necessary to deduce and put down the difference between the mean values 
belonging to each group, and the corresponding theoretically calculated values, 
as I have thought it possible that others might find a better mode of group- 
ing than the one here adopted. Moreover, for the sake of conciseness, I have 
given all single observation-results the same weight in the formation of the 
mean values, without regard to the circumstances under which they were 
produced. Thus, side by side with all the detailed data previously given in 
the present paper, the foregoing table on pages 183—188 will become the 
natural basis for the employment of the magnetic observations of Dr. NANsEN’s 
Norwegian Polar Expedition, in a closer inquiry into the general theory of 
terrestrial magnetism. 
CurisTiAnta. December, 1900. 
AKSEL S. STEEN. 
