THE UNIVERSE.- DISCOVERIES IN THE CELESTIAL SPACES. 333 



Baffin, Hudson, James Hall, and Schouten), sketched, in 

 1683, his theory of four magnetic poles or points of at- 

 traction, and of the periodical movement of the magnetic 

 lines of no variation. In order to test this theory, and to 

 render it more perfect by the aid of new and more exact 

 observations, he was permitted by the English Government 

 to make (1698 1702) three voyages in the Atlantic Ocean, 

 in a ship of which he was given the command. On one of 

 these voyages he proceeded as far as 52 south latitude. 

 This undertaking forms an epoch in the history of terres- 

 trial magnetism. A general " variation chart/' or a chart 

 on which the points at which the navigator had found the 

 same amount of declination were connected by curved lines, 

 was its result. Never before, I believe, did any Govern- 

 ment equip a naval expedition for an object, which, whilst 

 its attainment promised considerable advantages for prac- 

 tical navigation, yet so properly deserved to be entitled 

 scientific or physico-mathematical. 



As no phenomenon can be examined by an attentive in- 

 vestigator without being considered in its relation to others, 

 Halley, as soon as he returned from his voyages, hazarded 

 the conjecture that the Aurora Borealis is a magnetic 

 phenomenon. I have remarked, in the picture of nature 

 contained in the first volume of this work, that Faraday's 

 brilliant discovery of the evolution of light by magnetism 

 has raised this hypothesis, enounced in 1714, to the rank of 

 an experimental certainty. 



But if the laws of terrestrial magnetism are to be tho- 

 roughly sought out, that is to say, if they are to be inves- 

 tigated in the great cycle of the periodical movement in 

 geographical space of the three classes of magnetic curves, 



