GALVANIC EXPERIMENTS. 25 



in contact. The experiment was performed in an adit or level, 

 driven partly in each of these strata, at a mine called White Well, 

 near Alston, and the spot selected was at a distance from any 

 vein. We proceeded by fixing, about twelve inches above the 

 under surface of the limestone, in holes bored for the purpose, 

 twelve hollow copper cylinders, each five inches long by one 

 inch diameter, which were firmly secured in the holes and forced 

 strongly into contact with the rock by means of wooden plugs 

 driven within the cylinders. With these cylinders a copper wire 

 one twentieth of an inch in diameter was connected, and one end 

 of this wire was attached to one end of the wire of a delicate 

 galvanometer. In the same way twelve copper cylinders were 

 inserted in holes made in the sandstone, about a foot from its 

 upper surface, and to these were attached a similar wire, which 

 was brought in contact with the other end of the galvanometer 

 wire, so that any current between the two strata would be im- 

 mediately perceived by its action upon the needle. On making 

 the contacts, no motion whatever was produced on the needle, 

 and every expedient for increasing the eft'ect of a feeble action 

 was resorted to, such as making contact at short and equal in- 

 tervals, the interval being the time required for one vibration of 

 the needle, which was previously determined. At the same time 

 the sensibility of the instrument was such, that a plate of zinc 

 and a plate of copper, each one eighth of an inch square, in 

 pure water produced a most distinct action. The galvanic 

 action, if any existed between this limestone and sandstone 

 stratum when the experiment was performed, was, as col- 

 lected by the metallic surfaces in contact with each, decidedly 

 less than that of a pair of zinc and copper plates, each one 

 eighth of an inch square, in pure water. 



The next experiment was upon a stratum of limestone and a 

 stratum of hazle, having a plate bed between them, and the 

 situation selected was a shaft near Alston, called Water Greens 

 shaft, sunk by the Commissioners of Greenwich Hospital some 

 years ago into Nent-force level. The strata sunk through in 

 this shaft, as shown in the Hospital books, are as below; it is in 

 the lower part of the series just given from Forster's section, 

 the beds differing a little from the thicknesses stated by him, 

 as they almost always do when measured at different places. 



yds. ft. in. yds. ft. in. 



Clay 10 10 



Part of six fathoms hazle . . 7 2 



Hard plate 100 



Five yards limestone 3 1 6 



Hard plate 1 6 



Hazle 800 



Carried forward .... 3100 



Brought forward. . . . 

 Plate 



31 

 ... 300 



Scar limestone 



Hazle 



... 18 6 

 . . . 2 



Plate 



Hazle 



... G 1 6 

 ... 200 







61 1 



