194 



EIGHTH REPORT— 1838. 



These positions are marked in the Map, and the isoclinal line 

 of 70° is prolonged to the westward in correspondence with 

 the mean of M. Erman's observations thus corrected for 

 epoch. 



To connect the isodynamic line of TOS with intensities of 

 the same value in the adjacent meridians, it is necessary to ex- 

 press the value of this line in terms of the arbitrary scale em- 

 ployed by Continental observers, in which the force in Lon- 

 don = 1*372. In this scale the line of 1'03 corresponds in value 

 to (1-03 X 1-372 = ) 1-413. The portion of this line which is 

 represented in the Map by an unbroken line has been deter- 

 mined by the observations contained in this report. Its pro- 

 longation to the eastward is traced in conformity with M. 

 Hansteen's observations in Norway, and with MM. Han- 

 steen's and Erman's in Russia. The station mai'ked in lat. 

 60° 11' and long. 10° 20' is the mean geographical position of 

 a group of six stations in Norway, not far removed from each 

 other, for which M. Hansteen's observations in 1821, 1823, 

 and 1825, gave a mean intensity of 1*414 (7th Report, British 

 Association, page 49). At Gros Novgorod (lat. 58° 31', long. 

 31° 19') the determinations of MM. Hansteen and Erman ac- 

 corded in assigning 1*412 as the value of the force (7th Report, 

 British Association, page 51), and the line has been still fur- 

 ther extended, in conformity with the observations of the same 

 gentlemen at Moscow, in lat. 55° 46', and long. 37° 36', their 

 mean determination being 1*405. The position of the line in 

 its western prolongation has been drawn in conformity with the 

 values of the intensity at the islands of Terceira and Madeira, 

 contained in the general table of the memoir on the magnetic 

 intensity already referred to, viz. 



Terceira . Fitz Roy . . 1836 . . . 1*457 



,- , . rSabine ... 1822 .. . 1-373"! , ^^^ 

 Maden-a j^j^^ ^ _ ^ IS^g ^ ^ ^ i.3.^|l*3/5. 



Both stations are included in the Map. The values of the 

 force at M. Erman's dip stations in the same quarter, deter- 

 mined by the same excellent observer, are also inserted in the 

 map, as affording corroborative evidence of the correct position 

 of the isodynamic line in this its western extension. 



