330 GEOLOGY OF THE COMSTOCK LODE. 



T. C. and finishing with the one nearest P. C. The two sets, therefore, form 

 a symmetrical series, and from the means of all the values corresponding 

 to any particular T. C. any change which may have taken place in the hole 

 P. C. (see page 360), as well as in the electromotive force of the Daniell, 

 may be regarded as practically eliminated. A comparison of the two sets, 

 moreover, affords a good criterion of the constancy of the currents as well 

 as of the trustworthiness of the results obtained in general. 



Resistance. — Bcsidcs the clectromotive force, the resistance of the differ- 

 ent circuits was also measured, being an item of interest. The values usu- 

 ally ranged between 2,000 and 3,000 ohms, though at times they went as 

 high as 20,000, or as low as 700 ohms. Almost the whole resistance of the 

 circuit is encountered by the current in passing fi'om the wire into the rock, 

 and from the latter back again into the former. In other woi-ds, the resist- 

 ance of the layers of rock immediately surrounding P. C. and T. C. is so 

 large that in comparison with it that of the rest of the circuit (never greater 

 than 20 ohms) can be completely neglected The total resistance is, there- 

 fore, essentially the sum of two terms, corresponding to the holes, respect- 

 ively. Suppose now that in a circuit P. C. {T. C.) these partial resistances 

 are iv and r, respectively; in a circuit P. C. (T. 0.)', tv and r', respectively; 

 if it is found, experimentally, that 



w-\- r'zn h, \ , and if s = fl -f- ^ + ^7 ^^^n /»■'— '^ — a, 

 r + r'zr c, ) 



s 

 tvz=- — c. 



These points have been described in considerable detail, being of such 

 importance that without them the results reached would be illusory. I was 

 twice obliged to discard whole sets of experiments because one or the other 

 of the disturbances set forth had found their way into the results in the most 

 insidious manner. It is true that Fox actually used uncovered wire; but 

 it must be remembered that the currents obtained by him were abnormally 

 large. Moreover, I am convinced that the currents found by Fox, when 

 connecting two different points in rock, were entirely due to, and that those 



