52 HISTORY OF MAGNETISM. 



became known, it was important to mariners to 

 register the variation in all parts of the world. 

 Halley was appointed to the command of a ship in 

 the Royal Navy by the government of Wiliam and 

 Mary, with orders " to seek by observation the dis- 

 covery of the rule for the variation of the compass." 

 He published Magnetic Charts, which have been 

 since corrected and improved by various persons. 

 The most recent are those of Mr. Yates in 1817. 

 and of M. Hansteen. The dip, as well as the varia- 

 tion, was found to be different in different places. 

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

 many such observations. And both the observa- 

 tions of variation and of dip 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 varia- 

 tion, 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 to have reached the place of this 

 pole; the dipping-needle there pointing vertically 

 downwards, and the variation-compass turning to- 



