OF METALLIFEROUS VEINS IN THE MINES OF CORNWALL. 
401 
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. (See Fig. 2 7-) In compa- 
ring the relative states of veins at different depths, the lower stations appeared 
to be negative to the upper ; but exceptions sometimes occurred when a 
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. (See 
Fig. 8. & 12.) 
In such cases it may be supposed that there is an accumulation of electricity 
in different states, on the opposite sides of the non-conducting vein. Such in- 
tersections of ore veins, and their being often very rich to a great depth in one 
direction and not in another, added to their varying underlie at different 
depths, which is not unfrequently reversed, may tend to produce apparent 
anomalies in experiments of this nature. 
At Huel Jewel mine, I obtained results between a heap of copper ore at 
the surface, and a plate fixed at different depths against the ore in the vein ; 
the latter becoming more negative, in proportion to the depth at which it 
was placed. Piles of copper ore at the surface did not act op the needle when 
tried together, independently of veins, nor was it to be anticipated that they 
would. 
It is not improbable that the progressive increase of negative electricity ob- 
served in descending into our mines, if hereafter confirmed, may be found to 
be connected with the progressive increase of temperature. I have not, how- 
ever, discovered any distinct connection between them at the same level, but 
then the differences of temperature are comparatively small. Nor does the 
electricity appear to be influenced by the presence of the workmen and candles, 
or by the explosion of gunpowder, although some veins of copper ore were 
blasted on different occasions in the immediate vicinity of the copper plates. 
And at a very productive copper vein in Great St. George Mine, the ground is 
so soft that gunpowder is not used ; yet the needle was powerfully acted upon 
by the electricity it contained. On this occasion, as well as on some others, 
I remained with the galvanometer at the surface, letting the wires down through 
the shafts ; and in this manner I have sometimes found the electricity act with 
3 f 2 
