1864.] 



Prof. Erman — Magnetic Elements at Berlin, 



221 



Date of 



Inclination, 



observation. 





i. 



1800 -J-/* 



Observed. 



Calculated. 



1806 '0 



O 1 



69 53 



o / 



69 52-99 



xo^tj \jyj 



68 49-19 



68 44-62 





68 34-55 



68 34-17 



1832-50 



68 18-08 



68 21-40 



1 83fi-87 



68 7-43 



68 8-84 



XOOO 1 



68 2-04 



68 3-66 



1846-20 



67 43-25 



67 44-46 





67 35-48 



67 36-29 



1853-78 



67 29-81 



67 27-09 



1856-56 



67 20-50 



67 24-26 



1857-55 



67 20-30 



67 19-25 



1860-60 



67 15-75 



67 13-31 



1862-55 



67 7-63 



67 9-69 



The system of the above calculated values, which best agrees with the 

 observed ones, results from the expression 



(B) 2 = 70° 17'-42— 4'-1854#+0'-01893U-; 

 it leaves in each single equation a probable error of + 1''42 ; and accordingly 

 in the expression itself the probable errors appear to be 



in the absolute term + 2'* 1 7 ; 



in the coefficient of if ±0'- 121 1 ; 



in the coefficient of t"" +0'-001591. 

 This expression can be brought under the form 



(B*) 2=66° 26''09 + (^'-110-543y.0'-018931, 

 which would prove that at the place in question the inclination will come, 

 in 1910*543, to a minimum of 66° 26'-09. The aforesaid errors of terms 

 give +2*27 years for the uncertainty of the epoch of this minimum, and 

 + 3'*9 for the uncertainty of its value ; but as the expression (B) results 

 from observations between 1806 and 1863, its consequences ought not to 

 be extended as far as 1910. 



3. Declination, 



Four results of observations of this element, made by the late astrono- 

 mers Kirch in 1731, Bode in 1784 and 1805, and Tralles in 1819, have 

 been added to my own, which extend from 1825 to 1864. These latter 

 were obtained with the declination- and transit-instrument employed in my 

 voyage, which intermediately was frequently compared and found in per- 

 fect agreement with a large Gaussian magnetometer, whenever the indi- 

 cations of the latter were duly freed from the torsion of the suspending wires 

 and from the want of parallelism between the normal of the employed spe- 

 culum and the magnetic axis of the bar. My observations were all made in 

 the open air, with the exception of the two in 1849 and 1850, which, having 

 been executed in a room, were corrected for the influence of local attractions. 

 As the determination of this latter seemed exposed to a somewhat larger 



