of Terrestrial Magnetism. 119 



the southern to the west — in a periodic revolution, of which the 

 cycle might perhaps be roughly estimated at 700 years*. 



Since the time of Halley, several maps of the declination or 

 isogonic lines have been published from time to time from the 

 observations of successive epochs. There have also been pub- 

 lished maps of the other two magnetic elements; and these, 

 which embody facts unknown at the date when Halley wrote, 

 have confirmed in a remarkable degree his inference, drawn from 

 the declination only, of the existence of four poles or points of 

 principal magnetic attraction. In the higher latitudes of both 

 hemispheres the isoclinal and isodynamic lines have the form of 

 lemniscates, each loop of which encloses and surrounds a point 

 of maximum force. In each one of the three elements the maps 

 corresponding to successive epochs indicate a secular change in 

 remarkable accordance, both in character and degree, with the 

 views entertained by Halley. When carefully studied, the changes 

 in all parts of the globe are seen to concur in indicating a gra- 

 dual and progressive translation of his two moving poles in the 

 directions assigned by him. The gradual and extremely regular 

 character of this movement has recently been strikingly illus- 

 trated by the results of observations, conducted with suitable 

 care and under particularly favourable circumstances, which en- 

 able us to trace the progress of the secular change, not merely 

 from year to year, but actually from week to week, each week 

 showing an equal aliquot part of the yearly change. Whether 

 we regard the connected generality of the phenomena, or their 

 minute regularity, it appears difficult to resist the impression that 

 we see in thern the indication of some yet unknown cosmical re- 

 lation. Be this, however, as it may, the path of inductive research 

 lies clearly before us : it is our part to advance in it by continuing 

 diligently and perseveringly the investigation of the facts which 

 are accessible to us on our own globe, comprising (should the 

 supposed periodicity be confirmed) the duration of its cycle and 

 the characters of its epochs. The accomplishment of these objects 

 is being sought by what are termed magnetic surveys, repeated after 

 the lapse of a suitable number of years, and disclosing by their 

 comparison the changes that have taken place in the areas over 

 which they extend. Such a survey was made in our own coun- 

 try in 1837, at the instigation of the British Association, and 

 was speedily followed by similar undertakings in the Austrian 

 and Bavarian states, in the British possessions in North America, 

 in the Indian Archipelago, and more recently in British India. 

 Amongst the most important, and certainly the most arduous of 

 these labours, must be reckoned the determinations of the mag- 



* In this hypothesis the two fixed poles may be considered to be those 

 proper to the earth itself as a magnetic body, and the two moving poles 

 to be those of a superinduced magnetism. 



