358 GEOLOGY OF THE COMSTOCK LODE. 



Of course all these experiments are only intended to furnish estimates 

 as to the probable magnitude of disturbances of an analogous kind, which 

 may possibly have influenced the data given above. 



Mr. E. Kittler^ has recently commenced a new study of the question of 

 potential diff"erence due to the contact of liquids. From a large number of 

 careful experiments he finds electromotive forces between them far exceed- 

 ing, as a rule, those met with in the measurements of earth currents here 

 described. These forces, however, obey the law of Volta's potential series. 



From all these considerations, it seems to follow that in the present 

 investigation the discrepancies due to the presence of different liquids in 

 the rocks have been eliminated to a great extent. Certainly their effect 

 can hardly be estimated as much greater than a few thousandths of a volt. 

 It is obvious, moi'eover, that the use of simyjle metallic contacts (plates and 

 gads) is under all conditions unsafe. To this is to be added the fact that 

 metallic plates are never identical in their electrical properties, and that their 

 difference (as large effects of polarization are also included therein) cannot 

 be eliminated by such a process of commutation as was employed. 



The phenomenon of conduction of rocks being essentially hydro-electric, 

 the determination of the thermo-electric power eai'thcoppei", for which it 

 was at first thought the high temperature on the lower levels of the Com- 

 STOCK Lode, in comparison with those at the surface, would offer excellent 

 facilities, has no further interest. No attempt of this kind was therefore 

 made. 



If the hole drilled for the reception of the terminals be regarded as 

 a cylinder with a hemispherical base, the directrix of the former as tangent 

 to the sphere corresponding to the latter, its axis as norma! to the plane 

 face of the di-ift, approximate values may be derived for the specific resist- 

 ance of the rock met with. Let h be the height of the cylinder, a the common 

 radius of both the latter and the hemisphere. Let r be the radius of any 

 similar figure, the axis of whose cylinder and center of hemisphere coincide 

 with those of the hole. Finally, let G be the specific resistance of the rock, 

 or the resistance in ohms between opposite faces of a cubic centimeter. 



'E. Kittler : " Ueber Spanniiugsdifierenzeii, etc." Wied. Ann., Xll.. j). .iTi et seq., 1881. 



