xMAGNETISM. 187 



tensity affecting the whole Earth, is especially due, since 1819. 

 to the unwearied activity of Edward Sabine, who, after hav- 

 ing observed the oscillations of the same needles at the Ameri- 

 can north pole, in Greenland, at Spitzbergen, and on the coasts 

 of Guinea and Brazil, has continued to collect and an'angc 

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

 namic lines. I have myself given the first sketch of an isody- 

 namic system in zones for a small part of South America 

 These lines are not parallel to lines of equal incHnation (iso- 

 clinic 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 Clark Ross, we shall find that 

 on the surface of our planet the force increases almost in the 

 relation of 1 : 3 toward the magnetic south pole, where Vic- 

 toria Land extends from Cape Crozier toward the volcano 

 Erebus, which has been raised to an elevation of 12,600 ieet 

 above the ice.* If the intensity near the magnetic south pole 



615): "The observations on the variation of terresti'ial magnetism, to 

 which I have devoted myself for thirty-two years, by means of instru- 

 ments which admit of comparison with one another, in America, Europe, 

 and Asia, embrace an area extending over 188 degrees of longitude, 

 from the frontier of Chinese Dzoungarie to the west of the South Sea 

 loathing the coasts of Mexico and Peru, and reaching from 60° north 

 lat. to 12° south lat. I regard the discovery of the law of the decre- 

 ment of magnetic force from the pole to tlie equator as the most im- 

 portant i-esult of my American voyage." Although Tiot absolutely cer- 

 tain, it is very probable that Condorcet read Lamanou's letter of July, 

 1787, at a meeting of the Paris Academy of Sciences; and sucli a sim- 

 ple reading I regard as a sufficient act of publication. {Annvaire dit 

 Bureau des Longitudes, 1842, p. 463.) The first recognition of the law 

 belongs, therefore, beyond all question, to the companion of La Peiouse ; 

 but, long disregarded or forgotten, the knowledge of the law that the 

 intensity of the magnetic force of the Earth varied with the latitude, 

 did not, I conceive, acquii^e an existence in science until the [)ublica- 

 tion 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 started in 

 connection with it, and who, from their own experience, are aware 

 that w^e are apt to attach some value to that which has cost us the un- 

 interrupted labor of five yeai's, under the pressure of a tropical climate, 

 and of perilous mountain expeditions. 



* From the observations hitherto collected, it appears that the max- 

 imum of intensity for the wliole surfoce of tlie Earth is 2*052, and the 

 minimum 0.706. Both phenomena occur in the southern hemisphere; 

 ^e former in 73° 47' S. lat., and 169° 30' E. long, from Paris, ueai 



