318 



GEOLOGY OF THE COMSTOCK LODE. 



in which the electromotive force due to the terminals alone was not greater 

 than a few hundredths of a volt Bright steel gads, to the tops of which pieces 

 of thick copper had been firndy fastened, were especially convenient for 

 this purpose, as they could be driven into the vein or again withdrawn from 

 it expeditiously. These gads were from 8 to 10 inches long and about one inch 

 in diameter at the head, from which they tapered gradually to a point. As 

 it would be repeatedly necessary to use them in places where the earth was 

 naturally moist, the question arose whether it might not be desirable in all 

 the experiments to moisten the rock around the gads at once. Accordingly, 

 two sets of experiments, the results of which are contained in Tables I. and 

 II., were made, the former above the surface, the latter below. 



Two suitable positions in rock free from mineral^ matter having been 

 selected, the gads were driven and the circuit completed. Measurements of 

 resistance and electromotive force were then made. The gads were now 

 exchanged and the measurements repeated, and so on. The relative posi- 

 tion of the gads to an observer facing them is indicated in the second column 

 of the tables. Resistance {W) in ohms and electromotive force (e) in volts 

 are given in the third and fourth columns, respectively. The last colunm 

 shows the direction of the current, arbitrarily called " + " when flowing in 

 one way, " — " when flowing in the opposite. 



Table I. — Experiments made on south side of Bullion Ravine. 



[G:uls iliivlin into quartz seams between walls of diorite, about 10 feet apai-t. Seams naturally somewhat moist.] 



' See note, ji.age 313. 



