Magnetism of Basalt. 573 



basaltic rock, and on bars of magnetite, ilmenite, and 

 haematite ore. 



It is proposed to give here the particulars obtained for 

 basalt. Five new bars were cut from specimens obtained in 

 the following localities: — 



1. Top of Sgurr nan Grillean, Skye. 



2. The Storr Mountain, Skye. 



3. Dunvegan district, Skye. 



4. Stacks of Netherton, a basaltic * dyke in the sea near 



Stromness, Orkney. 



5. Faule Ader, Dattenberg, a vein of non-columnar basalt 



which passes roughly across the middle of the Dat- 

 tenberg quarry, near Linz, on the Rhine. 



Metlwd. 



As in the previous research, the magnetometric method 

 was employed, certain modifications having been introduced 

 from time to time. The quartz-fibre suspension was replaced 

 by a single silk fibre which has been found to work very 

 satisfactorily. The length of the needle was reduced from 

 1 cm. to '2b cm. A platinum thermometer was employed in 

 the measurement of temperature at first, but was replaced 

 during the course of the experiments by a thermo-electric 

 couple which was standardized in ice, steam, mercury- vapour, 

 and molten aluminium. 



Method of Compensation. 



A few remarks may be made here on the compensation of 

 the magnetizing coil. In making magnetometer observations 

 with weakly magnetic material, such as basalt or Heusler's 

 alloy, the specimen requires to be near the needle in order 

 to cause a suitable deflexion. Erhardf has pointed out that 

 with such a disposition, any slight deviation of the axes of 

 the magnetizing and compensating coils from the magnetic 

 east and west line gives rise to a component along the 

 meridian which, for one direction of the magnetizing current, 

 increases the directive force H, and for the opposite direction 

 diminishes it. Erhard has also shown that the presence of 

 this effect may be tested by first compensating for the zero 

 position, and then deflecting the needle with a bar-magnet 



* The geological nature of this rock is described by J. S. Flett in 

 " The Trap Dykes of the Orkneys/' Trans. Roy. Soc. Edin. vol. xxxix. 

 p. 865. 



f T. Erhard, Ann. d. Physik [41 vol. ix. p. 724 (1902). 



