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On the Magnetic Properties of the Rock on the Summit of Ar- 

 thur's Seat. By Mr William G albeaith, M. A. Com- 

 municated by the Author. Plate III- Fig. 3. 



It has been long known that rocks impregnated with iron-ore 

 exert an influence on the magnetic needle, not only from the 

 iron which they contain, but also from portions of the natural 

 magnet imbedded in the mass. Basaltic rocks, in particular, 

 are frequently possessed of this property. One of the oldest in- 

 stances in this country recorded, so far as I know, is the rock 

 on which Dumbarton Castle is built. This circumstance is no- 

 ticed in Buchanan's History of Scotland*. Professor Anderson 

 of the University of Glasgow, made repeated experiments on the 

 magnetism of this rock, and on the direction of its poles. On 

 the south side, near the top of the western peak, a large exposed 

 rock has been pointed out, on which many experiments have 

 been made, and from its situation is probably that alluded to 

 by Buchanan, It has been asserted by Mr Drysdale, the mas- 

 ter-gunner of the Castle, that it extends its influence to the op- 

 posite shore of the Clyde f. 



* In superiore arcis parte ingens est saxum Magnesii quidem lapidis, sed 

 ita catersB rupi coagmentatum et adhjerens ut commissura omnino non ap- 

 pareat. Liber xx. sectio 28. 



+ The first distinct observations concerning the magnetic polarity of 

 rocks, were made by Baron Humboldt in 1796. He noticed it in a serpen- 

 tine rock on the Haidberg, near Celle, in the country of Baireuth. It was 

 afterwards observed in many other rocks, such as hornblende-slate, porphyry, 

 trachyte, basalt, &c. It is apparently confined to mountains containing mag- 

 netic ironstones, although the quantity of this admixture in itself does not 

 limit the intensity of the property ; as, indeed, it shows itself with different 

 purely magnetic ironstones, in the greatest variety of degrees of strength, 

 and there are some of these which show no magneto-polar action. Neither 

 is there any regularity in the position of the axes either in one and the same 

 mass of rock in general, or a fixed correspondence in the position of these 

 axes with the direction of the strata of the rocks. Bergmeister Schulze of 

 Duren, in an excursion in the Eifel, a region of greywacke and basalt, ob- 

 served from the top of the Nurburg Mountain (a basaltic cone 2000 Prussian 

 feet above the level of the Rhine), on an elevation in an eastern direction, 

 something resembling the ruins of a building. Instead of ruins, however, he 

 found it to be two small rocks, about three feet distant from each other in 

 their diagonals, about six feet high, with bases not far from three feet square; 

 ')nc of them was six long and three feet broad; the other was a little shorter 



