LAWS OF MAGNETIC PHENOMENA. 219 



have been since corrected and improved by various persons. The 

 most recent are those of Mr. Yates in 181 7, and of M. Hansteen. The 

 dip, as well as the variation, was found to be different in different 

 places. M. Humboldt, in the course of his travels, collected many 

 such observations. And both the observations of variation and of 

 clip seemed to indicate that the earth, as to its effect on the magnetic 

 needle, may, approximately at least, be considered as a magnet, the 

 poles of which, are not far removed from the earth's poles of rotation. 

 Thus we have a magnetic equator, in which the needle has no dip, and 

 which does not deviate far from the earth's equator ; although, from 

 the best observations, it appears to be by no means a regular circle. 

 And the phenomena, both of the dip and of the variation, in high 

 northern latitudes, appear to indicate the existence of a pole below 

 the surface of the earth to the north of Hudson's Bay. In his second 

 remarkable expedition into those regions, Captain Ross is supposed tc 

 have reached the place of this pole ; the dipping-needle there pointing 

 vertically downwards, and the variation-compass turning towards this 

 point in the adjacent regions. We shall hereafter have to consider 

 the more complete and connected views which have been taken of 

 terrestrial magnetism. 



In 1C 33, Gellibrand discovered that the variation is not constant, as 

 Gilbert imagined, but that at London it had diminished from eleven 



O ' 



degrees east in 1580, to four degrees in 1G33. 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. 



O 



The next important fact which appeared with respect to terrestrial 

 magnetism was, that the position of the needle is subject to a small 

 diurnal variation : this was discovered in 1722, by Graham, a philoso- 

 phical instrument-maker, of London. The daily variation was esta- 

 blished by one thousand observations of Graham, and confirmed by 

 four thousand more made by Canton, and is now considered to be out 

 of dispute. It appeared also, by Canton's researches, that the diurnal 

 variation undergoes 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 gradually led to a consistent magnetic theory. Gilbert observed 

 that both smelted iron and hammered iron have the magnetic virtue, 



