ELECTRICAL ACTIVITY OF ORE BODIES. 35<) 



The elementary resistance of a shell at the distance r from the axis and 

 of the thickness dr, is then 



, _ <T dr 



^^~2 7r r(r+fi)' 



and, therefore, the resistance of the layer of rock between coaxial and 

 concentric figures, the inner radius being a, the outer r, is 



["]; 



a j(a-\-h)r 



the symbol w being used to express the resistance of the layer of rock 



between the similar surfaces just defined. If r is allowed to increase to 

 infinity, approximate values for 6 can be determined from the data given 

 al)ove for the resistances of the circuits, and the known dimensions of the 

 holes. In this way it appears that the mean value of this quantity was about 



a = 40,000, 

 whereas values as high as 500,000 and as low as 20,000 were met with. 

 From the invariable presence of moisture, however, these figures possess 

 only minor interest. 



If the resistance of layers of rock between consecutive similar sur- 

 faces be compared, the same notation being again employed, in round num- 

 bers, 



\-f, = 0.6; LJ- ^0.07; L Jjooo ^ ^ ^^^^ ^^^^ 



ri H„ V'l 



all dimensions being expressed in centimeters, and a being 1.2 cm.; whence 

 it follows that the resistance of coaxial and concentric layers decreases, 

 though hardly as rapidly as might be desirable. In point of fact, however, 

 the convergence is more rapid than this approximate calculation indicates. 

 A drift may with greater accuracy be regarded as a cyhndrical tunnel, Into 

 the sides of which the contact holes have been drilled, with their axes at 

 right angles to that of the drift. Now, it is obvious that as r (in the former 

 signification) increases, the values of dw will in this case decrease more 

 rapidly than in the previous one; this because the superficial area of the 

 infinitesimally thin shell increases much more rapidly. The actual analysis, 



