TERRESTRIAL MAGNETISM. 175 



year 1819 to the present time, that we are principally in- 

 debted for the knowledge of the variations of the magnetic 

 intensity over the whole surface of the globe, and for their 

 laws so far as we are yet able to infer them. After having 

 himself vibrated the same needles in the vicinity of the 

 North American pole, in Greenland, in Spitzbergen, on the 

 coast of Guinea, and in Brazil, he has been constantly 

 engaged in collecting and co-ordinating all the materials 

 capable of elucidating the great question of the isodynamic 

 lines. The first sketch of an isodynamic system divided 

 into zones was given by myself, for a small portion of South 

 America. The isodynamic lines are not parallel with the 

 isoclinal lines; the intensity is not, as was first supposed, 

 weakest at the magnetic equator, nor is it even equal at all 

 parts of that line. If we compare Erman's observations 

 (0*706), in the southern part of the Atlantic ocean, where a 

 zone of weak intensity extends from Angola past the Island 

 of St. Helena to the Coast of Brazil, with the most recent 

 observations of the great navigator James Clark Ross, we 

 find that on the surface of our planet the force augments 

 almost in a ratio of 1 : 3. The highest intensity which has 

 been measured is 2*071, in lat. 60 19' and long. 131 20' 

 E. from Greenwich. ( 16 ) These values are expressed in 

 terms of the scale of which the unity is the intensity which 

 I observed on the magnetic equator, in tiie north of Peru. 

 In Melville Island (74 27' N.), in the neighbourhood of 

 the northern magnetic pole, the force was found by Sabine 

 only 1*624, while at New York in the United States, almost 

 in the same latitude as Naples, he found it 1*803. 



The brilliant discoveries of Oersted, Arag9, and Earaday, 



have established intimate relations between the electric 

 VOL. i. o 



