— 108 — 



crust or at the centre of the globe. If there is something in the crust which at- 

 tracts the needle in Boothia; it ought to attract the needle in London; but the 

 needle in London is attracted neither to the crast at Boothia, nor to the centre. 

 The magnetic pole then appears to be in the atmosphere. 



« The fact that the declination-needle points constantly to the magnetic pole does 

 certainly seem to indicate that it is attracted; but it is impossible to believe this, when 

 the sanie needle and the same end, dipping, is manifestly repelled. The trae expla- 

 nation of the apparent attraction is, I believe, this: the needle places itself in the 

 magnetic meridian as a position of equilibrium, which the moving magnetic pole is 

 constantly disturbing. 



« Even the north end of the declination-needle manifests a certain amount of the 

 repulsion produced by the north magnetic pole of the earth, which is shewn by the 

 north end becoming apparently heavier directly it is magnetised. It is impossible 

 to believe that the mere tonching a needle with a magnet can give it weight. We 

 are then driven to the conclusion that the north end must be attracted or repelled. 

 Norman arranged the needle so that it should move fruly in a vertical direction and 

 discovered that this apparent heaviness was really a dipping. I have shewn that 

 the dip is produced by repulsion. 



« As the declination-needle was known long before the dipping-needle, such a 

 universal belief got abroad that the north end of the needle was attracted by the 

 north magnetic pole because it pointed to it, and that therefore that end of the needle 

 must be a south or austral pole, that when the dipping-needle was invented, it never 

 occurred to the acutest minds that the same end in the dipping-needle was pro- 

 claiming in the most distinct manner that it was not attracted, but repelled, — that 

 it was not an austral pole, but a boreal one. 



« In conclusion I vould remarli that is admitted that the phenomena of terre- 

 strial magnetism are most satisfactorily explained by four poles: two in the northern 

 hemisphere and two in the southern, as maintained by Halley and Hansteen. I 

 hope to shew hereafter that the two magnets as it were, having these poles, are not 

 in the earth, but in the atmosphere; that instead of two rigid magnets in the earth, 

 whose extremities come to the surface near the poles, there are two broad atmosphe- 

 ric magnetic belts, extending from the neighbourhood of the north pole to the equa- 

 tor, and two similar belts coming up from the south pole to meet them, the austral 

 magnetism of the northern belts uniting with the boreal magnetism ofthe southern 

 belts along the magnetic equator ; the magnetic poles in each case being the free 

 extremities of the magnetic bands. These bands revolve at slów and unequal rates 

 round the poles of the earth, producing the secular variations. 



« It is of the first importance then that it should be clearly settled whether the 

 magnetic pole, which Ross 50 years ago found in Boothia, is there stili, or has 

 moved to latitude 72° longitude 115° ». 



Il Socio Blaserna presenta a nome del Socio Cannizzaro la seguente Nota 

 del prof. M. Eileti, intitolata : Ricerche sulla Cinconina. - 



