446 THE INTELLECTUAL RISK IN ELECTRICITY. 



/ by Sellers in 1667. * It was of course old to magnetize 

 iron needles by rubbing them wit'li the lodestone ; and 

 that even a succession or chain of armatures could be 

 rendered magnetic by induction from a single stone, both 

 by actual contact and through simple location in the field, 

 had been known for ages. Sellers, however, had been 

 rubbing needles on the stone to find out the conditions 

 under which they would become most strongly mag- 

 netized ; and he made up his mind that the needle's 

 strength or direction did not depend so much upon 

 "fainter or stronger touches on the stone nor the mul- 

 tiplicity of strokes" as upon "the nature of the steel 

 whereof the needle is made, and the temper that is given 

 thereunto." So he tried all sorts of steel, .and finding the 

 magnetism apparently permanent in his needles, easily 

 made the succeeding step which was to regard the mag- 

 ^Xnetized steel itself in the same light as the lodestone ; or, 

 in other words, as an artificial magnet 'which "shall take 

 up a piece of iron of two ounces weight or more ; and 

 give also to a needle the virtue of conforming to the mag- 

 netic meridian without the help of a lodestone or anything 

 else that has received virtue therefrom." 



As the century drew to its close, the growing commerce 

 of England created an urgent demand for more definite 

 knowledge concerning the variation of the compass. In 

 1580, William Burrowes determined the variation in Lon- 

 don to be 11 I5 V to the East. Edmund Gunter, the in- 

 ventor of the scale and rule which bears his name, fount 

 that, in 1622, it had diminished some five degrees. Gel- 

 librand, Gunter's successor in the Chair of Astronomy at 

 Gresham College, observed that it had become reduc< 

 some two degrees more. In 1640, Henry Bond, a teach< 

 of navigation in London, published 'his Seaman's Calen- 

 dar, showing the progressive nature of this secular vari; 

 tion, and in 1668 issued a table predicting, though in- 

 accurately, its changes in London for the uext forty-eight 



'Phil. Trans., No. 26, 478, 1667. Abridg., vol. i., 166. 



