Barrett — On the Magnetic Properties of Columnar Basalt. 383 



-i-r 3 tan OH, where r is the distance of the centre of the block from 

 the magnetometer, the angle of deflection made by one pole 

 (that is, half the total scale reading), and if the horizontal intensity 

 of the earth's magnetism in c. g. s. units. The total moment of 

 each block, being divided by its weight in grammes, gives the 

 moment per gramme of the basalt. The centre of the blocks was 

 in each case 60 centims. from the magnetometer, the scale of 

 which was 100 centims. distant from the mirror ; the total 

 deflection produced by A was 1*2 centim. ; by B was 1 centim. ; 

 and by C was 1*56 centim. The value of H, determined for the 

 place of observation, was found to be 0-16 c. g. s. ; whence we have 

 the moment per gramme of — 



Block A = 0-00188 c. g. s. units, 

 „ B = 0-00179 

 „ C = 0-00182 



or a mean of 0-00186. 



A small, feebly magnetized steel bar magnet at the same dis- 

 tance gave a moment per gramme of 10 c. g. s. units. Highly 

 magnetized steel runs up to 95 units per gramme, a moment 50,000 

 times greater than that of the basalt. Dividing the total moment 

 by the volume in c. c, instead of by the weight, gives the intensity 

 of magnetization. In the case of the basalt this is - 0054. Now, 

 if the earth be regarded as a uniformly magnetized body its inten- 

 sity of magnetization is 0-079', or nearly fifteen times greater 

 than the basalt. Hence, although basaltic masses must cause a 

 considerable local deviation of the compass over a wide area, they 

 cannot be regarded, even if they encompassed the earth, as the 

 source of terrestrial magnetism. 



1 Everett's "Physical Constants." 



