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Electro-magnetic properties in the mines of Cornwall. 137 
the information on these points which may be found in the pages of 
the American Journal, it is to be hoped that some of its readers may 
contribute something more specific, and accompany their statements 
with the result of experimental engineers, similar to those detailed in 
the following abstract. J. G: 
_The previous experimental researches of the author on the pro 
gressive increase of temperature in mines, suggested to him the opin- 
ion, that this internal heat, which he has so satisfactorily proved to be 
augmented with the depth, might be connected with electrical action: 
fact of such an action his experiments have clearly established. 
His apparatus consisted of small plates of sheet copper, which 
were fixed in contact with ore in the veins by copper nails, or press- 
ed closely against it by wooden props, stretched across the “levels” 
or galleries. Between two of these plates at different stations, and 
. galvanometer, a communication was made by means of copper 
wit; one twentieth of an inch in diameter, which was at first coated 
With sealing wax ; but this prevention was afterwards dispensed withs 
The galvanometer, used for detecting the electric action, consisted 
simply of a magnetic needle three and a quarter inches long, one 
eighth of an inch wide, and one twentieth of an inch thick. It was 
closed in a box four inches square and one inch in depth, having a 
a plated Copper wire, one fiftieth of an inch in diameter, coiled round 
twenty five times. No magnet was used to neutralize the terres- 
tnal Polarity. In some instances nearly three hundred fathoms of 
Copper wire were em loyed. ee 
The intensity of the electro-magnetic action differed greatly in 
_ tent places; in some cases the deviation of the needle was in- 
“siderable, in others it went completely round the circle. In gen- 
eral it was greater, ceteris paribus, in proportion to the greater abun- 
dance of Copper ore in the veins, and in some degree perhaps to the 
“epth of the stations ; and where there was little or no ore, there 
— er no action. Hence it seems likely, that electro-magnet- 
aac rs become useful to the practical miner in determining with 
va gree of probability, at least, the relative quantity of ore in 
"S; and the directions in which it most abounds. When the dis- 
: ne . the plates from each other in a horizontal direction was only 
ao and the copper ore between them was plentiful, and 
se Tupted by non-conducting substances, or the workings in the 
v, nO action occurred, owing no doubt to the good conducting 
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OL. XX.—No. A 
