TERRESTRIAL MAGNETISM. 35 



Should this turn out to be really the case, it would be a colla- 

 teral result of no small practical value, being in effect a new 

 and general means of ascertaining the longitude^ at least of 

 places on land. 



But although the ancient methods of observation are super- 

 seded for the nicer purposes of theorj^,by themore refined methods 

 above alluded to, it by no means follows either that the results 

 to which they have led are to be thrown aside as useless, or the 

 methods themselves rejected, when the object is not to attain the 

 last degree of numerical accuracy, still less when these more re- 

 fined processes are impracticable, as they are at sea, where, 

 nevertheless, it has been shown by Erman and others that the 

 dip and intensity, as well as the variation, can be observed with 

 a considerable approximation. 



It has been, and must ever continue to be, the peculiar pro- 

 vince of these ruder ordinary processes, to trace out by actual 

 visitation of every accessible spot on the globe, those important 

 curves which, when laid down on charts, express to the eye 

 general relations, such as theory must exercise itself in render- 

 ing account of ; and, following them through all their intricacies, 

 must find its first application in showing how they originate in 

 the relations of the forces in action. These curves are in fact 

 no other than approximative expressions of those inductive 

 laws above alluded to, and will play the same part in a strict 

 general theory of terrestrial magnetism which Kepler's laws do 

 in that of gravitation, or the polarized figures do in that of 

 light. 



A glance at the best and most approved charts, of varia- 

 tion and intensity, will show how much is wanting to render 

 this precursory knowledge complete. However, in all easily 

 accessible regions of the globe the investigation is proceeding 

 with rapidity, nor is there any desirable information of this 

 nature in such regions, which the diligence of navigators, sur- 

 veyors and travellers, in the pursuit of their ordinary objects, 

 will not adequately furnish. The case is otherwise with those 

 difficultly accessible regions, in which, unfortunately, lie the 

 most characteristic, critical, and important points and inflexions 

 of the curves in question, viz. the points of greatest intensity, 

 and those in which the needle points vertically dovvnwards, and 

 which are usually known by the name of the magnetic poles. 

 Two such points of greatest intensity are known to exist in the 

 northern hemisphere, one in Siberia, and the other in the terri- 

 tories of the Hudson's Bay Company ; the existence of the 

 first has been proved by the voyages and travels of Hansteen, 

 Due, and Erman ; and of the other, by the observations made 



D 2 



