TRANSACTIONS OF THE SECTIONS. 25 



and at the top the rocks are pegmatites or granites. I first examined a small frag- 

 ment of the rock presented to me by General Cullen, of a greyish-red tint, composed 

 chiefly of felspar and quartz with particles of magnetic iron ore disseminated ; these 

 particles were of about ^ to A inch in diameter, and without any regular form or 

 smooth face (as far as my examination went), when a magnet was presented to one 

 of these particles, detached from the specimen, it showed its polarity by tumbling 

 over, if the homonymous pole was at first nearest the magnet. The specimen 

 alluded to was about 5 inches long, 2| inches broad, and 1^ inch thick, tapering 

 and thinning off to one end (A). On presenting the different extremities to a freely 

 suspended magnet (the declination magnet of the Trevandrum Observatory), the 

 following results were obtained : — 



Sc. div. 



Specimen away. Declination reading O'OO 



End A presented —1*64 



„ B „ +2-13 



Side C „ -1-17 



„ D „ +0-73 



where the negative sign signifies repulsion of the north end of the magnet, and 

 the positive sign attraction of the same pole. The changes of magnetic declination 

 occurring during the experiments were observed by another instrument, and have 

 been subducted. As the line of the magnetic axis of the specimen was evidently 

 towards the direction of its greatest length, the northern end being towards A and C, 

 it was desired to determine whether the same relation would hold true for any 

 fragment ; for this purpose, two ends of the specimen were knocked off, leaving a 

 fragment in the middle with a distance of a (towards A) to b (towards B) of nearly 

 2 inches, while the breadth from C to D was nearly 3 inches ; so that the longest 

 dimension was now nearly at right angles to that of the whole specimen. The cen- 

 tral fragment being placed at the same distance from the suspended magnet as in the 

 previous experiment, the following were the results : — 



Sc. div. 



Fragment away. Declination reading O'OO 



End a presented — - 79 



„ b „ +1-31 



Side C „ —0-57 



„ D „ +111 



The ends and sides show the same polarities as in the whole specimen, but with 

 the north end of the magnetic axis turned more to the side D, for which the deflec- 

 tion has increased. Upon presenting the small fragment constituting the end A of 

 the specimen, the results were as follows : — 



Sc. div. 



Fragment away. Declination reading. . - 00 



End A presented —0*36 



„ a' „ +0-36 



SideC „ +0-14? 



„ D „ +0-14 



Here it will be observed that the broken fragments of the specimen acted exactly as 

 the broken parts of a magnet ; thus the end a in the central fragment gave a repul- 

 sive effect of 0*79 scale divisions, while the end a', the opposite face of the fracture, 

 gave an attractive action of 0'36. 



Several questions of interest presented themselves in connexion with these rocks. 

 Whether the hill as a whole would give results similar to those obtained from this 

 specimen? Whether the lines of magnetic force in it had any relation to the lines 

 of crystallization, or to those of the earth's poles ? Whether any particular direction 

 was most favoured ? or whether the magnetic axes vary from spot to spot, and the 

 magneticules, possessing their present magnetism when tossed up in the liquid mass, 

 had their positions determined by chance ? 



On the 11th of December, 1855, 1 visited the hill, making observations with a 6-inch 



