362 A NATURALIST IN THE PACIFIC chap. 



" great magnetic mountain " in the heart of Germany. He was then 

 director-general of the mines in two Franconian principalities ; and 

 in order to awaken the interest of German physicists and 

 mineralogists in this matter, he announced his discovery with an 

 air of mystery, and did not disclose the locality for many months. 

 He then placed his specimens in the mining-office at Bayreuth to 

 be sold at so much by weight for the relief of poor miners. His 

 plan succeeded, and this young savant who had yet before him his 

 great career, had soon enlisted the interests of several of the noted 

 scientific men in Germany, including Werner the mineralogist, 

 Voigt the mathematician, Blumenbach the naturalist, Charpentier, 

 and others. The amount of attention that this subject then excited 

 can be inferred from the pages of the " Intelligenzblatt der 

 Allgemeine Literatur-zeitung " for 1796- 1797 and from the con- 

 temporary publications. It has been almost forgotten now, and 

 the matter is indeed often approached " de novo." 



However, although by these means the data became largely 

 increased, no generally accepted explanation resulted. Opposing 

 views continued at various times to be advanced ; and it has only 

 in recent years come to be recognised that the magnetic polarity ^ 

 of rocks in exposed situations, as in the mountain-peak or in the 

 crested spur, often arises from atmospheric electricity inde- 

 pendently of the inductive action of terrestrial magnetism. This is 

 the conclusion to which the later evidence given by Zirkel is 

 directed and was that which Oddone and Sella formed from their 

 study of the magnetic rocks of the Central Alps. It is not, how- 

 ever, always necessary to suppose that the affected rocks have been 

 struck by lightning, although Sella and Folgheraiter have shown 

 that this is the result of such a contact. They may be found, as 

 indicated by Mr. Harker, in mountainous localities where thunder- 

 storms are remarkably rare, and where the peaks act, it is suggested, 

 as natural conductors. It is easy to show, remarks the same 

 author, that no lapse of time is required for rocks in exposed 

 situations to become magnetised. The stones of cairns erected a 

 few years before on the mountain-tops of the Isle of Skye become 

 invariably highly magnetic ; whilst the loose stones lying on the 

 ground display this property to a much less degree. Nor is it 

 requisite that the rocks affected should be basic volcanic rocks. It 



^ Nearly all volcanic rocks at all basic are magnetic, owing to the constant 

 presence of magnetite ; but magnetic polarity, when the rock-fragment has a 

 negative and a positive pole, is not directly concerned in volcanic rocks with 

 the mineral composition. 



