112 TERRESTRIAL MAGNETISM. 



is thus as yet an entire absence of magnetic determinations. 

 My own observations of inclination and force through the 

 whole interior of South America (from Cumana to Rio 

 Negro, as well as from Cartagena de Indias to Quito) were 

 confined to the tropical zone north of the geographical 

 equator ; those in the southern hemisphere, from Quito to 

 Lima, only extended over the narrow district adjacent to the 

 western coast. 



The movement of translation of the African node to the 

 westward from 1825 to 1837, noticed in a preceding page, 

 is confirmed, on the east coast of Africa, by a comparison of 

 the inclination observed by Panton, in 1776, with Rochet 

 d'Hericourt's observations. The last-mentioned traveller 

 found the line of no dip much nearer the Straits of Bab-el- 

 Mandeb, i. e. 1 south of the island of Socotora in 8 40' 

 N. lat. Thus there would appear to have been in 49 years 

 only an alteration of 1 27' in latitude : the alteration in 

 longitude in the same interval of time, by the movement of 

 the node as estimated by Arago and Duperrey, would 

 amount to 10 to the westward. The direction of the 

 movement due to secular change on the eastern side 

 of Africa has therefore been quite the same as on 

 the western, but the quantity or amount of the move- 

 ment still requires to be determined by more exact 

 results. 



The periodicity of the variations in the magnetic inclina- 

 tion, in reference to hours and seasons, the existence of 

 which has been already remarked, has only been established 

 definitively, and in its entire character, within the last twelve 

 years, or since the formation of the British magnetic stations 

 in the two hemispheres. Arago, to whom so much is due 



