526 Prof. Riicker. On tlie, Magnetic Permeability of 



and r{log e (cos0 1 sin# 2 + v/1 sin 2 6 2 s ^ n ~ 0i) 



logo (cos 02 sin 2 + \/l sin 2 O z sin 2 2 ) 

 Iog e (cos0 1 sin #! + A/1 sin 2 l sin 2 0^ 



+ loge (cos 2 sin 6 l + v/1 sin 2 ^ sin 2 2 ) } . 



The component parallel to the plane of the rectangle and to that of 

 the 0's is obtained from the last bj writing for and vice versa. 



In the particular case for which : = l = and 2 = w/2, the 

 expressions reduce to 



ff0 2 and fflogg cos 2 = fflog d sec0 2 . 



In the case under consideration these must be doubled, as the plate 

 is supposed to extend both north and south of the point. 



If we assume the magnetic susceptibility of the mass to be O0016, 

 which is about the mean value of the Mull basic rocks, and take the 

 vertical component of the earth's field as 0'46 C.G.S. units, we 

 get 



2<r = 2x0-46x0-0016 = 0-00147, 



so that if 02 i& expressed in degrees 



2*702 = 0-000025702. 

 also 2<r log, sec 2 = O'\)0338 Iog 10 sec 2 . 



Hence the disturbance produced is 2'57 2 and 338 Iog 10 sec 2 

 units of disturbing force respectively. 



Bj means of these' expressions we may calculate the attractions 

 exerted at points on the surface 10 kilom. apart by horizontal 

 plates of magnetic " matter " of opposite kinds 60 kilom. wide, and of 

 indefinite length at depths of 4 and 20 kilom. respectively. 



Subtracting the n ambers given by the lower from those deduced 

 from the upper plate, we get the resultant vertical and horizontal 

 forces due to the opposite magnetisation of the upper and lower 

 surfaces. 



The minor plateaux are supposed to consist of two such plates, the 

 principal plateaux of three plates side by side. 



The resultant force at any point is obtained by adding, algebraically, 

 the components due to each mass. 



Fig. 3 represents the two cases which have been submitted to 

 calculation. They differ only in that the magnetic matter outside 

 the central plateau is supposed, in the one case, to be limited, and in 

 the other, to be unlimited, in an east and west direction. 



The forces are determined for points on the surface 10 kilom. apart. 



