TRANSACTIONS OF THE SECTIONS. 



27 



The north end of the magnetic axis was here evidently nearly in the direction 

 N. by E. £ E. Upon raising this point of the stone, presented to the centre of the 

 magnet, the deflection diminished ; on lowering it, the deflection increased to 10° ; so 

 that the north end of the axis here inclined 10° above the horizon to N. by E. A E. 

 This result also agrees with the increase of dip found at B. 



In a third specimen examined, which was weakly magnetic, the north end of the 

 axis made an angle of 80° above the N. W. by W. point of the horizon. 



From these results it is evident that though the direction of the magnetic axis 

 may not vary much in small specimens, it does so in parts of the rock separated by 

 a few feet only from each other ; and it appears probable that it may be considerable 

 for smaller distances than those under experiment. Neither do the directions of the 

 axis seem to have any relation to the lines of crystallization. 



Another question was examined by me, namely, whether the magnetic intensity of 

 the rock varied with the temperature. For this question I chose a specimen of 

 about 6 inches long by 4 broad and 3 thick, taken from near the middle of the ascent 

 of the hill. The observations were made in the same manner as for the temperature 

 coefficient of a magnet. The specimen was placed in a wooden trough, into which 

 water of different temperatures was poured : the deflections of the declination mag- 

 net by the specimen at different temperatures were noted ; the variations of declina- 

 tion during the experiments were eliminated by means of another instrument. The 

 results are contained in the following Table : — 



The result is that the magnetic rock, like a steel magnet, loses force by an increase 

 of temperature ; and, using the notation employed for steel magnets, the temperature 

 coefficient is approximately 



9=0-000214, 

 nearly the value obtained for steel magnets used in the British and Colonial Obser- 

 vatories. 



The following may be considered as the conclusions at which I have arrived : — 



1st. The rock fragments have determinate magnetic axes. 



2nd. Broken fragments resemble broken magnets, showing opposite polarities at 

 the two surfaces of fracture. 



3rd. The magnetic axis varies from place to place within small distances. 



4th. The action of the whole hill on magnets freely suspended at moderate di- 

 stances is nearly imperceptible ; the opposite directions of the magnetic axis in the 

 rocks rendering the total action nearly zero. 



5th. As in some cases the north end of the magnetic axis was found to the south- 

 ward (as with specimen B), we cannot suppose that the magnetism of the small 

 magnets has been due to the inducing action of the earth in their present position or 

 since the rock mass became solid. 



6th. The directions of the magnetic axis have no relation to the lines of division 

 of the rock masses. 



7th. The magnetic force of the rock masses varies with temperature like that of 

 steel magnets. 



On a Magnetic Survey of the West Coast of India. 

 By John Allan Broun, F.R.S. 



This survey was undertaken at the expense of His Highness the Rajah of Travan- 



