MAGNETISM. 181 



coasts of Guinea and Brazil, has continued to collect and arrange 

 all the facts capable of explaining the direction of the isodyna- 

 mic lines. I have myself given the first sketch of an isodynamic 

 system in zones for a small part of South America. These lines 

 are not parallel to lines of equal inclination (isoclinic lines), and 

 the intensity of the force is not at its minimum at the magnetic 

 equator, as has been supposed, nor is it even equal at all parts of 

 it. If we compare Erman's observations in the southern part of 

 the Atlantic Ocean, where a faint zone (0'706) extends from 

 Angola over the island of St. Helena to the Brazilian coast, 

 with the most recent investigations of the celebrated navigator 

 James Clarke Ross, we shall find that on the surface of our planet 

 the force increases almost in the relation of 1 : 3 towards the 

 magnetic south pole, where Victoria Land extends from Cape 

 Crozier towards the volcano Erebus, which has been raised to 

 an elevation of 12,600 feet above the ice.* If the intensity 

 near the magnetic South Pole be expressed by 2'052 (the 

 unit still employed being the intensity which I discovered on 

 the magnetic equator in Northern Peru), Sabine found it was 

 only 1-624 at the magnetic North Pole near Melville Island 

 {74 27' north lat.), whilst it is 1'803 at New York, in the 

 United States, which has almost the same latitude as Naples. 



Earth varied with the latitude, did not, I conceive, acquire an existence 

 in science until the publication of my observations from 1798 to 1804. 

 The object and the length of this note will not be indifferent to those who 

 are familiar with the recent history of magnetism, and the doubts that 

 have been startedun connexion with it, and who, from their own expe- 

 rience, are aware that we are apt to attach some value to that which has 

 cost us the uninterrupted labour of five years under the pressure of a 

 tropical climate, and of perilous mountain expeditions. 



* From the observations hitherto collected, it appears that the 

 maximum of intensity for the whole surface of the Earth is 2 '052, and 

 the minimum 0'706. Both phenomena occur in the southern hemi- 

 sphere : the former in 73 47' S. lat., and 169 30' E. long, from Paris, 

 near Mount Crozier, west-north-west of the south magnetic pole, at a place 

 where Captain James Eoss found the inclination of the needle to be 87 1 1', 

 {Sabine, Contributions to Terrestrial Magnetism, 1843, No. 5, p. 231); 

 the latter, observed by Ennan, at 19 59' S. lat., and 37 24' W. long, from 

 Paris, 320 miles eastward from the Brazilian coast of Espiritu Santo 

 (Erman, Pliys. Beob., 1841, s. 570), at a point where the inclination is 

 only 7 55'. The actual ratio of the two intensities is therefore as 1 to 

 2*906. It was long believed that the greatest intensity of the magnetic 

 force was only two and a half times as great as the weakest exhibited 

 on our Earth's surface. (Sabine, Report on Magnetic Intensity, p. 82.) 



