TERRESTRIAL MAGNETISM. 69 



an unmistakable influence in the animation and better 

 direction of researches in terrestrial magnetism. It was 

 followed by Hansteen's general maps of lines of equal 

 inclination and equal force for a considerable portion of the 

 earth's surface. 



1819. Observations of Admiral Roussin and Givry on the 

 coast of Brazil, between the mouths of the Amazon and the 

 Eiver Plate. 



1819 1820. Oersted's great discovery of the fact that a 

 conductor traversed by an electric current, forming a com- 

 plete and continuous circuit, exercises, so long as the current 

 continues, a definite influence on the direction of the mag- 

 netic needle dependent on their relative position. The 

 earliest extension of this discovery, as well as of those of the 

 separation of the metals from the alkalies, and of double 

 polarisation, the most brilliant of the discoveries of 

 the age,( 69 ) was Arago's observation that a connecting 

 wire through which an electric current flows, though made 

 of copper or platinum, attracts and retains iron filings like a 

 magnet ; and also, that needles placed inside a galvanic coil re- 

 ceive opposite poles according as the turns of the coil are given 

 an opposite direction (Annales de Chimie et de Physique, 

 T. xv. p. 93). The discovery of these phaenomena, which 

 were traced under a variety of circumstances, was followed 

 by Ampere's ingenious theoretical combinations on the 

 reciprocal electro-magnetic actions of the molecules of pon- 

 derable bodies. These combinations were supported by 

 much new and ingeniously-devised apparatus, and led to a 

 recognition of laws in many phsenomena of magnetism which 

 had previously appeared contradictory. 



1820 1824. Wrangel and Anjou's journeys to the North 



