530 Prof. Riicker. On the Magnetic Permeability of 



40. A mass of magnetic matter in the shape of a frustum of a cone 

 of which the vertex was in the surface, and the angle between the 

 generating and median lines tan" 1 v / 2, and of which the upper plane 

 surface was 450 feet from the surface of the earth, would exert 

 at the vertex a vertical force 231, thus bringing the total calculated 

 disturbance up to 279 + 231 = 510, as against 384 + 80 = 464, 

 observed at Market Weighton. Such a cone gives a maximum effect ; 

 but it must be remembered that, although no magnetic rocks exist 

 near the surface at Market Weighton, yet the fact that the older 

 rocks approach the surface near that town led Professor Judd to 

 advise its being selected as a station, and it may well be that the 

 underlying magnetic masses rise steeply in its neighbourhood. 



It is a little difficult to summarise an argument of this kind, but if 

 we start from the near mean negative disturbance in the " valley" as 

 a datum, and distinguish the other stations as being over an under- 

 ground plateau, mountain, and peak respectively, we get the following 

 results : 



It is thus possible to imagine a distribution of magnetic matter at 

 depths between 450 feet and 12 miles, and of no greater permeability 

 than Mull basalt, the mere presence of which in the earth's magnetic 

 field would produce vertical disturbing forces of the same order as 

 those actually observed. 



The maximum calculated horizontal forces are larger than any of 

 those which Lave been measured in England or Wales. Case I 

 (see Table, p. 527) has been adopted as the basis of calculation, and 

 the horizontal disturbing force at the edge of the plateau is thus 227. 

 Forces of this magnitude are found in Scotland, but the largest value 

 in England is 124 at Melton Mowbray. 



At the middle of the edge of the smaller mass, on the side on which 



