Curiosities of Science. 207 



apparatus consisted of small plates of sheet-copper, which were 

 fixed in contact with a plate in the veins with copper nails, or 

 else wedged closely against them with wooden props stretched 

 across the galleries. Between two of these plates, at different 

 stations, a communication was made by means of a copper 

 wire TjVth of an inch in diameter, which included a galvanometer 

 in its circuit. In some instances 300 fathoms of copper wire 

 were employed. It was then found that the intensity of the 

 voltaic current was generally greater in proportion to the 

 greater abundance of copper ore in the veins, and in some de- 

 gree to the depth of the stations. Hence Mr. Fox's discovery 

 promised to be of practical utility to the miner in discovering 

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

 which it most abounds. 



The result of extended experiments, mostly made by Mr. 

 Robert Hunt, has not, however, confirmed Mr. Fox's views. 

 It has been found that the voltaic currents detected in the lodes 

 are due to the chemical decomposition going on there ; and the 

 more completely this process of decomposition is established, 

 the more powerful are the voltaic currents. Meanwhile these 

 have nothing whatever to do with the increase of temperature 

 with depth. Recent observations, made in the deep mines of 

 Cornwall under the direction of Mr. Fox, do not appear consis- 

 tent with the law of thermic increase as formerly established, 

 the shallow mines giving a higher ratio of increase than the 

 deeper ones. 



GEKMS OF ELECTRIC KNOWLEDGE. 



Two centuries and a half ago, Gilbert recognised that the 

 property of attracting light substances when rubbed, be their 

 nature what it may, is not peculiar to amber, which is a con- 

 densed earthy juice cast up by the waves of the sea, and in 

 which flying insects, ants, and worms lie entombed as in eter- 

 nal sepulchres. The force of attraction (Gilbert continues) be- 

 longs to a whole class of very different substances, as glass, 

 sulphur, sealing-wax, and all resinous substances rock crystal 

 and all precious stones, alum and rock-salt. Gilbert measured 

 the strength of the excited electricity by means of a small 

 needle not made of iron which moved freely on a pivot, and 

 perfectly similar to the apparatus used by Haiiy and Brewster 

 in testing the electricity excited in minerals by heat and fric- 

 tion. "Friction," says Gilbert further, "is productive of a 

 stronger effect in dry than in humid air ; and rubbing with 

 silk cloths is most advantageous." 



Otto von Guerike, the inventor of the air-pump, was the 

 first who observed any thing more than mere phenomena of 

 attraction. In his experiments with a rubbed piece of sulphur 



