— 246 — 



nearly bisected by the meridians of noon and midnight, and the 

 areas of vipward and downward disturbing force nearly l)isected 

 by the line of separation of day and niglit. 



Finally it is to be remarked, that the foei of the noith- 

 hemisphere are strongest in snmmertime; a computation for 

 Jan Maijen gives a considerable reduetion for ^Yintertime. 



It is indeed desirable to extend the investig'ation of 

 the fleld of daily variation over the polar regions; but in 

 these regions, though the use of quiet days may give great 

 advantage, as is shovvn by Lüdeling in his paper quoted 

 above, both phenomena still remain intrieated. Besides a 

 day seleeted in the usual way as a quiet one, is not to be 

 considered as entirely free from disturbance. 



To prove this 26 couples of successive days for the sumniei- 

 months have been chosen out of the days selected by Wild 

 as quiet ones for Fetershurg — Pawlowsk. They are: 



The difïerences of corresponding hourly values of H and 

 D have been taken for earh conple and the iiiean difference 

 for each hour lias been calculateil. By meaiis of the latter 

 the diagram of the horizontal differential veetor has been 

 cbnstructed, which proved to be quite regular and to rotate 



