ELECTRICAL ACTIVITY OP ORE BODIES. 317 



be near the ore body, whereas a, /?,;/.. . and g, t,v . . are remote from it. 

 As I shall frequently have occasion to refer to the point a in contradistinc- 

 tion to the remaining points /?, y,S, . . . g,t,v . . ,1 will throughout this chap- 

 ter refer to the former under the name permanent contact (P. C), while to 

 any of the others the name temporary contact (T. C.) will be applied. Then 

 will the electromotive force (e) between P. C. and any T. C. in general vary 

 with the distance {x) between these points. This relation will usually be so 

 complex as not to be easily expressible by mathematical means, but it can 

 nevertheless be indicated symbolically by 



e=f{x). 



If, however, x is supposed to increase from zero (in which case P. C. 

 and T. C. coincide) to the value it has for some remote point, v, then as a 

 field of electrical activity is encountered in the neighborhood of n, v, ^, 

 f{x) must pass through a single maximum or minimum, or a number of them. 

 It is therefore toward a characteristic variation of this kind that we must 

 look in endeavoring to define a position of greatest proximity to the ore 

 body. Analogously, though less generally, it may be stated that the incre- 

 ment of potential due to successive increments of distance «/?,/? y, y S, etc., 

 will be small except in the neighborhood of the ore body. This is probably 

 the idea which Reich had in mind, and which he must have come upon had 

 he followed out the line of his argument to its consequences. 



I will add here that local difiiculties did not permit me actually to pass 

 linearly through an ore-region. I had to content myself, therefore, with a 

 progress from the latter into barren rock. 



EXPERIMENTS MADE IN SOME OF THE MINES ON THE COMSTOCK. 

 Method. — Experiments were commenced in the Consolidated Virginia, Cal- 

 ifornia and Ophir mines, the line at times extending into Union and Mexican 

 ground. 



From the work of previous investigators I was naturally led to expect 

 currents due to electromotive forces of considerable magnitude, and as a 

 consequence, was satisfied with a method of obtaining contact with the vein 



