Philosophical Transactions. 345 



On the evening of the 25th of May, when there was no moon- 

 light, and the night dark, with occasional showers, the appearance 

 of the light viewed from Blackwall, a distance of ten miles, was 

 described as being very splendid. Distinct shadows were discern- 

 ible, even on a dark brick wall, though no trace of such shadows 

 could be perceived when the other lights, consisting of seven re- 

 flectors with Argand lamps, and the French lens, were directed on 

 the same spot. Another striking and beautiful effect peculiar to 

 this light was discernible when the reflector was turned, so as to be 

 itself invisible to the spectator. A long stream of rays was seen 

 issuing from the spot where the light was known to be placed, and 

 illuminating the horizon to a great distance. As the reflector re- 

 volved, this immense luminous cone swept the horizon, and indi- 

 cated the approach of the light long before it could itself be seen 

 from the position of the reflector. 



These singular effects must not, however, be understood as con- 

 stant accompaniments of this light, for on a moonlight night, or 

 when the weather is very hazy, they cease to appear. 



9. On the Electro- Magnetic properties of Metalliferous Veins in the 

 Mines of Cornwall. By Robert Ware Fox. [Read June 10, 

 1830.] 



THE author having been led, from theory, to entertain the belief that 

 a connection existed between electric action in the interior of the 

 earth, and the arrangement of metalliferous veins, and also the pro- 

 gressive increase of temperature in the strata of the earth as we 

 descen&from the surface, proceeded to the verification of this opinion 

 by experiment. His first trial wasfcunsuccessful, but in the second, 

 he obtained decisive evidence of considerable electrical action in 

 the mine of Huel Servel, in Cornwall. His apparatus consisted of 

 small plates of sheet copper, nailed, or else wedged closely, against 

 the wooden props stretched across the galleries. Between two 

 of these plates of different stations, a communication was made, 

 by means of copper wire, one twentieth of an inch in diameter, which 

 included a galvanometer in its circuit. In some instances three 

 hundred fathoms of copper wire was employed. 



The intensity of the electric currents was found to differ consi- 

 derably in different places ; it was generally greater in proportion 

 to the greater abundance of copper ore in the veins ; and in some 

 degree also to the depth of the station. Hence the discovery of the 

 author seems likely to be of practical utility to the miner in disco- 

 vering the relative quantity of ore in veins, and the directions in 

 which it most abounds. The electricity thus perpetually in action 

 in mines, does not appear to be influenced by the presence of the 

 workmen and candles, or even by the explosion of gunpowder in 

 blasting. 



The author's experiments enable him to give a table of the rela- 

 tive powers of conducting galvanic electricity possessed by various 

 metalliferous minerals, This power, he remarks, appears to bear no 



