392 



t is the mean time of the observations in every group ; n the number of 

 complete observations in every mean ; d the probable error of every mean ; 

 i the mean dip of the n observations. As the dip i in a not too long interval 

 of time t — t^ may be expressed by the following formula, — 



where a,h, and c are constants, I have taken t^ = 1820, and by the method 

 of least squares found 



a = 72° 41'. 1 + 1'.935 



h = — 3'.63978 +_ 0'.2I654 



c = + 0'.056166 jtO'-008749; 



and from these constants calculated the dip in the 5th column. A is the 

 difference between observation and formula. The instrument of DollonJ 

 was too little, but the great number of observations (102 in 5.3 years), and 

 the great variety of methods which have been employed, I suppose may have 

 destroyed all constant errors. Since better instruments were employed, the 

 quantity of A is much less ; and I think that all these differences are not 

 really errors of observation, as it is evident that the dip has its irregular 

 variations as well as all other magnetic phenomena of the earth. The for- 

 mula — 



i = 72° 41'. 1 — 3'.63978 {t — 1820) + 0'.056166 {t — 1820)^ 

 with the probable errors of a, h, and c, gives 



Minimum in Christiania = 71° A2'.2'± 11'.? 18 



when t =. 1 852.4 _t. 5.4 years. 



