360 GEOLOGY OF THE COMSTOOK LODE. 



however, id unnecessary here The points of greatest interest have already 

 been sufficiently illustrated by what precedes. 



Earth-currents. — A secoud important consideration relative to the causes 

 which might have produced discrepancies in the present investigation is the 

 effect to be ascribed to earth-currents. Although numbers of experiments 

 have been made in different parts of the world as to the magnitude and 

 direction of such currents, I am unable to estimate their effect in this case, 

 especially as the constants for the currents probably vary with the position 

 of the field of observation on the surface of the earth. Most observers 

 have availed themselves of telegraphic connections between points very 

 many miles apart. Matteuci,^ I believe, was the only one who laid a care- 

 fully insulated line especially for this purpose, and it is to his investigation 

 that we can with greatest advantage refer. Yet, though his points were at a 

 distance of six kilometers apart, the currents obtained, so far as can be seen, 

 were certainly not much larger than those here recorded If, however, the 

 variation of potential in the above experiments were due to some normal, 

 non-local cause, it would be fair to assume a linear change of potential 

 with distance throughout the comparatively small area in which the experi- 

 ments were made. Such is by no means the case. In fact, some of the largest 

 variations observed occur within distances of a few hundred feet, while 

 elsewhere a range of 1,000 feet is without marked alteration of potential. 

 It is probable, therefore, that earth-currents have not perceptibly affected 

 the results." 



Drill-holes. — The auguhir and somewhat irregular curves (Figs. 28, 30, 

 32) might give rise to a suspicion that the difference of potential observed 

 is in some way to be ascribed to the accidental condition of the holes them- 

 selves. A priori, therefore, the presence of little pieces of steel, worn or 

 bi'oken off from the drill, crystals of iron pyrites, particles of ore, etc., in 

 the walls of the hole should not be disregarded. That such material is, 

 however, entirely without disturbing effect will be seen from the following 

 experiments : 



' Ch. Matteaci : " Snr les courants ^loctriques de la terre." Ann. d. Chim. et de Phys., [4"], T. IV., 

 I>. 177, lH(i5; ibid. [4], T. X., p. 148, 1867. 



-Temporary (li.stnrbances, such, for instance, as are due to atmospheric induction, are obviously 

 without influence in the present case. luductive action, nicreover, is hardly to be expected from the 

 clear, dry air of Nevad.i. 



