PART II. POLAR MAGNETIC PHENOMENA AND TERREU.A EXPERIMENTS. CHAP. (. 321 



however, arc useless in this connection, just because the perturbing forces themselves then come into 

 the diurnal variation, and it was these we wanted to eliminate. By taking a sufficient number of obser- 

 vations, it might be thought that the perturbing forces would be eliminated, as the oscillations would 

 possibly be as frequent to one side as to the other; but this is not the case. The oscillations, when 

 they occur, generally have a definite direction for every distinct time of day. Perturbations, for instance, 

 that occur about midnight, local time, at most of the polar stations, in horizontal intensity, will almost 

 exclusively show negative values of P h . By the addition of all the values, too low a mean value of H 

 will therefore be found here. If we would use such a determination, perturbing forces would often be 

 found, for instance, at times when the conditions were quite normal. 



At several stations, however, the diurnal variation has been calculated exclusively from observations 

 on quiet days. If the days used in these calculations really were 'quiet' we might apply these determi- 

 nations. A quiet day in the Polar regions is, however, a very rare occurrence, and in most cases, on 

 the majority of the 'quiet' days made use of, deviations having the character of minor perturbations occur. 

 When these perturbations are not eliminated, the result would not always be applicable to our purpose. 



For us, in the calculation of the perturbing forces, the best means of obtaining an approx- 

 imately accurate determination of the diurnal variation on the day in question is, by means of the 

 hourly observations made daily at the various stations, to draw the magnetic curves for the nearest 

 quiet days before and after the fixed day; by comparing these we can draw a normal line, that is in 

 correspondence with only the quiet parts of the curves, from which consequently the perturbations are 

 eliminated. This is the method that has mainly been followed. 



In Kingua Fjord not a single really quiet day is to be had, especially in the afternoon, Greenwich 

 time ; the conditions are always more or less disturbed. In the forenoon, however, the conditions are 

 very often fairly undisturbed. From the most quiet days found in the material, it seems, however, to 

 become clear, that the diurnal variation in the afternoon is but small, and that consequently the disturbed 

 conditions here must be regarded as perturbations. As normal line, we have therefore here drawn a 

 fairly straight line, and as the variations as a rule are somewhat considerable, the error in the position 

 of the normal line will be of less importance. 



This circumstance, that magnetic perturbations occur much more frequently at this station than at 

 the other polar stations, is a fact of very great importance for our theory, and we will return to this 

 later on. 



At the stations where the hourly observations have not been taken, namely, Christiania, Gottingen, 

 and Kasan, the determination of the diurnal variation becomes considerably more difficult and to some 

 extent rather uncertain. We here have only the more or less quiet term-days to go upon, in addition 

 to the comparisons we can draw with observations of recent years and adjacent stations. The determi- 

 nation of the normal line at these two stations may therefore sometimes be somewhat arbitrary, espe- 

 cially in the case of the vertical intensity of Gottingen, where it has occasionally been impossible to 

 make any determination. 



On the whole we may remark, that the diurnal variations that we have used must of course not 

 be regarded as entirely correct, when the oscillations attain a certain amplitude, however, the uncer- 

 tainty in the normal line is of smaller significance. 



With regard to the vertical intensity, the observations are often a little unreliable, and it may 

 perhaps be doubtful on the whole whether any conclusions at all may be drawn from these observations, 

 especially in the case of those stations at which the method employed was that of induction in bars of 

 soft iron. 



We have thus made no use of the vertical intensity observations from Ssagastyr, as the perturbing 

 forces constantly appearing there are of an altogether different order of magnitude to that which we find 



