66 



formula were determined by means of observations from 90 

 different points on the earth-surface and corresponded so 

 well with the really existing magnetic state that the mag- 

 netic force could be calculated to its intensity and direc- 

 tion approximativeJy for nearly all points on the earth-sur- 

 face. This great work has been taken up in låter times 



Pia 



by Erman and Petersen, who by means of new observa- 

 tions from a great many points have determined the con- 

 stants with higher degree of exactness, by which the cal- 

 culations of the magnetic elements can be executed well 

 enough for practical use. 



