ELECTRICAL ACTIVITY OF ORE BODIES. 321 



important; that, finally, the time during which exposure to high tempera- 

 tures can be endured with safety is itself necessarily limited. A simple 

 method, analogous to one of consecutive substitution of two elements in 

 the same circuit of large resistance, was therefore adopted. If e and E denote 

 the lode electromotive force and the electromotive force of a normal element, 

 respectively, i and / the intensities due to the action of e and J^dbe in the 

 same circuit, we shall have, approximately,' 



e i jj, i 



■=. -, ov e-^Mj 



E±,e I I^i 



Intensities were measured by the aid of the galvanometer above de- 

 scribed, the instrument having been carefully calibrated at the outstart — an 

 operation which was frequently repeated during the course of the experi- 

 ments, i and / could both be determined in the same circuit without 

 inserting auxiliary resistances. 



Results. — By way of example, some of the results obtained in the mine.s of 

 the CoMSTOCK will now be cited. The plan has been indicated in a forego- 

 ing paragraph (page 3 1 6-7). It will be remembered that a permanent contact 

 placed conveniently in one end of the network of drifts, is successively con- 

 nected with points in positions of sufficient interest to justify measurement. 

 In the tables, unless otherwise stated, P. C. is to be understood as coinciding 

 with point I. The second column contains the distance, in feet, of the 

 points tapped below the level of the mouth of the shaft as a datum. "Dis- 

 tance" and "bearing" refer to the imaginary lines connecting P. C. (I) 

 with the remaining points of the series. An exception is, however, made in 

 Table VI., where the data contained in corresponding columns give the 

 horizontal distance and bearing of the lines joining consecutive points e. 

 the lode electromotive force, is expressed in volts, and is taken as positive 

 when it acts in the direction P. C. > Earth > T. C. * 



'Approximately, because, in the case when the lode electromotive force acts alone, we have not a 

 true circuit, in the ordinary sense. Between the holes, both in the earth and in the wire, the direction 

 of the current is the same. But since the resistance of the rock, passing from the hole into the earth, 

 diminishes rapidly (see pnge SiiO), the former may be considered, with a degree of accuracy sufficient lor 

 the purpose, as acting through the same resistance as does the normal element, subsequently inserted. 



21 O L 



