LAWS OF MAGNETIC PHENOMENA. 53 



wards this point in the adjacent regions. We shall 

 hereafter have to consider the more complete and 

 connected views which have been taken of terres- 

 trial magnetism. 



In 1633, Gellibrand discovered that the varia- 

 tion is not constant, as Gilbert imagined, but that 

 at London it had diminished from eleven degrees 

 east in 1580, to four degrees in 1633. Since that 

 time the variation has become more and more 

 westerly ; it is now about twenty-five degrees west, 

 and the needle is supposed to have begun to travel 

 eastward again. 



The next important fact which appeared with 

 respect to terrestrial magnetism was, that the posi- 

 tion of the needle is subject to a small diurnal 

 variation : this was discovered in 1722, by Graham, 

 a philosophical instrument-maker, of London. The 

 daily variation was established by one thousand 

 observations of Graham, and confirmed by four 

 thousand more made by Canton, and is now con- 

 sidered to be out of dispute. It appeared also, by 

 Canton's researches, that the diurnal variation un- 

 dergoes an annual inequality, being nearly a quarter 

 of a degree in June and July, and only half that 

 quantity in December and January. 



Having thus noticed the principal facts which 

 belong to terrestrial magnetism, we must return to 

 the consideration of those phenomena which gra- 

 dually led to a consistent magnetic theory. Gilbert 

 observed that both smelted iron and hammered 



