MAGNETIC ROCKS OF CORNWALL. 



283 



No. 2 slice, immediately after being sawed, acquired the 

 intense affinity for the magnet of 45-6 per cent, of its weight, 

 and 48 hours later it relapsed to 6-3 pe]* cent. ; in 28 days after, 

 being reduced in size, it acquired the power of 13 per cent., 

 which I take to be its normal affinity. 



These figures show how much the magnetic powers of such 

 rocks may be intensified by friction ; such being the case in a small 

 portion, I consider in the ease of miles of basic beach and cliffs, 

 with its thousands of tons of boulders and shingles of the same 

 nature brought into motion by such storms as oftentimes visit our 

 coast, that the magnetic power would be increased to an almost 

 incalculable degree, to which water would be no barrier, for we 

 all know its conducting properties for magnetism. This power 

 it is dangerous to despise, as it may greatly imperil any misguided 

 ship that unfortunately drift within its influence. 



From the observations of Messrs. Eiiker and Thorp, the 

 influence on the magnet of the Canna basalt cap scarcely reaches 

 down to the sea, but such is not the case with some of the 

 Cornish rocks, for they extend far beneath the sea, viz. : at 

 Botallack, St. Just, and in the Lizard district. 



