198 Things not generally Known. 



excited a ta^te for the study of terrestrial magnetism in Europe, 

 by means of his observations on the progressive increase of 

 western declination in receding from that line. 



The first chart showing the variation of the compass,* or 

 the declination of the needle, based on the idea of employing 

 curves drawn through points of equal declination, is due to 

 Halley, who is justly entitled the father and founder of terres- 

 trial magnetism. And it is curious to find that in No. 195 of 

 the Philosophical Transactions, in 1683, Halley had previously 

 expressed his belief that he has put it past doubt that the 

 globe of the earth is one great magnet, having four magnetical 

 poles or points of attraction, near each pole of the equator two; 

 and that in those parts of the world which lie near adjacent to 

 any one of those magnetical poles, the needle is chiefly go- 

 verned thereby, the nearest pole being always predominant 

 over the more remote. 



" To Halley" (says Sir John Herschel) " we owe the first ap- 

 preciation of the real complexity of the subject of magnetism. 

 It is wonderful indeed, and a striking proof of the penetration 

 and sagacity of this extraordinary man, that with his means of 

 information he should have been able to draw such conclusions, 

 and to take so large and comprehensive a view of the subject 

 as he appears to have done. " 



And, in our time, " the earth is a great magnet," says 

 Faraday : "its power, according to Gauss, being equal to that 

 which would be conferred if every cubic yard of it contained 

 six one-pound magnets ; the sum of the force is therefore equal 

 to 8,464,000,000,000,000,000,000 such magnets." 



THE AURORA BOREALIS. 



Halley, upon his return from his voyage to verify his theory 

 of the variation of the compass, in 1700, hazarded the conjec- 

 ture that the Aurora Barealis is a magnetic phenomenon. And 

 Faraday's brilliant discovery of the evolution of light by mag- 

 netism has raised Halley's hypothesis, enounced in 1714, to 

 the rank of an experimental certainty. 



EFFECT OF LIGHT ON THE MAGNET. 



In 1854. Sir John Ross stated to the British Association, in 

 proof of the effect of every description of light on the magnet, 

 that during h ; s last voyage in the Felix, when frozen in about 

 one hundred miles north of the magnetic pole, he concentrated 



* The fivst Vnr'aHon-Oomnnss was constructed, before 1525, by nn ingenious 

 apothecary of Seville. Pelisse Guillen. So earnest were the endeavours to learn 

 more exactly the. direction of tlie curves of magnetic declination, that in 1585 

 Juan Jayine sailed wirli Prune! sco Gali from Manilla to Acsipnlco, for the sole 

 purpose of trying in the Pacific a declination instrument which lie had invented. 

 Humboldt. 



