TERRESTRIAL MAGNETISM. 



135 



leaders. On these and many other grounds, he (Mr. Maunder) 

 begged to second the resolution. 

 The resolution was duly carried. 



Several members asked questions of the lecturer, Colonel Alves 

 inquiring " Is there any idea as to the approximate depth of the 

 magnetic poles beneath the surface of the earth ? As far as is 

 known are there, as supposed by Dr. Halley, two magnetic poles 

 in each hemisphere — four in all — or only one ? " Mr. W. Hoste 

 asked " How is it accounted for that the northern and southern 

 magnetic poles are not exactly opposite, but are actually in the 

 same hemisphere, seeing that the earth is of regular shape ? Is 

 there a tendency for this relative position to vary, and for the two 

 poles to become opposite eventually ? " 



Lecturer's Keply. 



The Lecturer replied: "The term 'magnetic pole' usually 

 denotes a point on the earth's surface at which the dip needle 

 becomes vertical. The poles roughly located by Ross and other 

 polar explorers w^ere of this kind. By definition, these lie at no 

 depth, but on the surface. The word " pole " is also used in reference 

 to the " foci " or points of maximum magnetic force, such as that 

 in Siberia. It cannot at present be stated at what depth these 

 local irregularities in the earth's magnetism have their origin. 



" Halley's conception of the earth's magnetism, as resulting from 

 the combination of magnetizations in two directions resulting in there 

 being two magnetic poles in each hemisphere, is now known to be 

 too simple to represent the actual facts. It is indeed hardly correct 

 to speak of the earth as possessing a definite magnetic axis and 

 magnetic poles at all, except in the sense above defined. 



" The precise positions of the northern and southern magnetic 

 poles are not of fundamental importance in the theory of the earth's 

 magnetism, as they are determined partly by local irregularities 

 in the earth's field. Why the main direction of magnetization of 

 the earth is inclined to the geogTaphical axis, however, and why 

 there are such pronounced irregularities in the surface magnetic 

 force, cannot yet be explained. Similarly it is at present impossible 

 to say whether the poles will in time become opposite to one another. 



