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PAPER AND SKETCH MAP OF CORNWALL, SHEWING THE 



LOCALITY OF VARIOUS ROCKS POSSESSING POWER 



TO DEFLECT THE MAGNETIC NEEDLE. 



By THOMAS CLARK, Teuko. 



During the past summer I have devoted my spare time to 

 testing and mapping the basic rocks of Cornwall, believing this 

 subject to be worthy the attention of the miner, surveyor, and 

 mariner. 



I am not aware that anyone, up to the present time, has 

 ever attempted to ascertain the percentage of affinity or power 

 of such rocks over the bar magnet or needle, by scales and 

 weights. 



The short notes I made for a former Journal respecting the 

 magnetic power of the Lizard rocks, has called for a far more 

 extensive research of our coast line, which is about 180 miles in 

 extent, and jutting far out into the great marine highway of 

 nations. This work I commenced in earnest, and obtained 

 materials from almost every available point, not being desirous 

 of pursuing the course of the mistletoe for either matter or 

 shelter, but with a full intent to record faithfully my own work 

 on the subject. 



I am indebted to Mr. Howard Fox, of Falmouth, for the 

 sample of Canna basalt, the high power of which I have used as a 

 standard for other rocks. This sample was a portion of the 

 rock known as Compass rock, situated on the apex of the Island 

 of Canna in the Hebrides. It acquired its name from the fact 

 that when a mariner's compass was taken round it, the end of 

 the needle mark N. would point to the rock, whether the observer 

 placed the compass E., W., N., or S., of it. This extreme power 

 of the rock over the needle only extended a short distance, for 

 at 80 yards the needle is recorded to nearly assume its normal 

 position, only deflecting 1 deg. -^ ; but I have found for this 

 hitherto basic champion of the British Isles, rivals in Cornwall, 



