MAGNETIC SURVEY OF GREAT BRITAIN. 



193 



Extension of the Isoclinal and Isodynamic Lines into Meri- 

 dians East and West of the British Islands. 



Having thus completed the representation of the principal 

 lines of dip and intensity passing across the British Islands, it 

 appears desirable to trace their prolongation on either side, 

 until they are brought in connexion with the lines of the same 

 value in adjacent meridians to the east and west, as determined 

 by recent and satisfactory observations. As a single line of 

 each of the phenomena will suffice to exhibit this connexion, I 

 have selected for that purpose the isoclinal line of 70°, and the 

 isodynamic line of 1 "03. 



In Plate III. the portion of the isoclinal line, which is repre- 

 sented by an unbroken line, has been determined by the ob- 

 servations contained in this report. In its eastern prolongation 

 it passes through countries where its position is well assured 

 by observations of higher amount on the one side, and of lower 

 amount on the other, too numerous for insertion in a map on 

 so small a scale, and too well known to need a recapitulation 

 here. Towards the north-eastern extremity of the map, the 

 position of Gros Novgorod is marked in lat. 58° 31' and long. 

 31° 19', where M. Erman observed the dip 70° 26'-l on the 

 13th of July, 1828. This observation, reduced to January 

 1837, by allowing an annual diminution of 3', becomes 70° O0'"6: 

 the line of 70° is therefore made to pass through this station. 

 To the west of the British Islands, the line is prolonged until 

 it is brought in connexion with M. Erman's observations on 

 his homeward passage, in August 1830. For this purpose I 

 have formed M. Erman's observations into two groups, each 

 of three stations, as follows : 



Allowing, as in Britain, an annual decrease of £'-4, the dips in 

 January 1837, corresponding to the mean positions of these 

 groups, are as follows : 



