138  Llectro-magnetic properties in the mines of Cornwall. 
power of the vein; but where a cross vein of quartz or clay hap- 
pened to be between the plates under similar circumstances, the ac- 
tion was unusually great. 
hen the communication was established between two plates at 
different depths on the same vein, or between different veins, whether 
at the same level or otherwise, the electrical action was in general 
the most decisive. In fact, veins, which in some instances were al- 
most destitute of ore, and did not affect the needle, per se, did so, 
though perliaps in a slight degree, when electrical communications 
were made between them. 
The direction of the positive electricity was in some cases from 
east to west, and in others: from west to east; and when parallel 
veins were compared, its general tendency was, the author thinks, 
from north to south, though in several instances it was the reverse. 
In veins having an underlie towards the north, the east was commonly 
positive with respect to the west; but in veins dipping towards the 
south, the contrary was observed, with one exception only, and that 
under rather unusual circumstances. In comparing the relative 
states of veins at different depths, the lower stations appeared to be 
negative to the upper; but exceptions sometimes occurred when 4 
cross vein of quartz or clay intervened between the plates, and the 
higher one was on the negative side with respect to the horizontal 
currents. 
In such cases it may be supposed that there is an accumulation of 
electricity in different states, on the opposite sides of the non-cot- 
ducting vein. Such intersections of ore veins, and their being often 
very rich to a great depth in one direction and not in another, add 
to their varying underlie at different depths, which is not unfre- 
quently reversed, may tend to produce apparent anomalies in expel” 
iments of this nature. 
At Huel Jewel mine, the author obtained results between a heap 
of copper ore at the surface, anda plate fixed at different depths 
against the ore in the vein; the latter becoming negative, in propo! 
tion to the depth at which it was placed: Piles of copper ore 
the surface did not act on the needle when tried together, independ- 
ently of veins, nor was it to be anticipated that they would. 
It is not improbable that the progressive increase of negative elec 
tricity observed in descending into mines, if hereafter confirmed, 
may be found to be connected with the progressive increase of te™- 
perature. The author has not discovered any distinct eonnecto® 
he 
