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XL. 



NOTE ON THE MAGNETIC PEOPEETIES OF COLUMNAR 

 BASALT. By PROFESSOR W. F. BARRETT. 



[Eead December 18, 1889.] 



Professor Hull having brought under my notice the interesting 

 fact that blocks of basalt from the Giant's Causeway were not only 

 magnetic but magnetized in a definite direction, I thought it would 

 be of interest to submit them to a more careful examination by 

 means of a mirror' magnetometer, and at the same time to 

 determine their intensity of magnetization in absolute measure. 

 Examination showed that the blocks had, in general, a strongly 

 marked north and south pole, the magnetic axis running 

 diagonally through the block. The magnetic distribution was, 

 in fact, similar to that of all magnetic bodies submitted to the 

 earth's magnetism in northern latitudes ; the magnetic axis being- 

 inclined to the horizon, approximately at the angle of dip. The 

 concave or less convex side of the blocks was, in each of those 

 examined, a north pole, i. e. it repelled a north-seeking pole, and 

 must, therefore, have been downwards when in situ at the Cause- 

 way : this is, assuming the blocks to have been magnetized by the 

 action of the earth, as was probably the case. 



Three blocks, A, B, and C, were now weighed, their respective 

 weights being 27,600, 24,100, and 37,000 grammes. The specific 

 gravity of a fragment of one of the blocks was found to be 2*86. 

 The magnetic moment of each of the blocks was determined by 

 placing it on a turn-table at a given distance from the magne- 

 tometer. The deflection produced in one direction having been 

 read, the block was turned round by means of the turn-table, 

 the distance of its centre from the magnetometer remaining 

 unchanged. The deflection in the opposite direction was now 

 read. The moment was then calculated from the formula 1 



1 This formula can only be regarded as approximately true of the large masses, 

 with feeble subsidiary poles, with which we are here dealing. 



