THE NON-LIVING WORLD 171 



disappeared such would be the solid matter filling up 

 the hollows formed by the hailstones and so replacing 

 them ; (5) casts of hollow structures, i.e., mineral matter 

 which has filled cavities in the interior of fossils and so 

 formed internal casts (as " moulds " are external casts) 

 of fossils. For further information about the earth's 

 crust the reader is referred to special works on geology. 



Having now considered the effects of heat, motion, and 

 chemical changes upon the globe as a whole, it remains to 

 speak of the influence exercised upon it by electricity and 

 magnetism, as well as of the external sources of its light 

 and heat, and the effects produced upon it by them 

 through gravity. 



We have seen * that the motion of either electricity or 

 magnetism, circulating round an axis, develops the 

 other force along that axis. Now there is a constant 

 flow of electric currents around the earth from east to 

 west, and, besides this, the unequal heating of the 

 surface of the globe in each twenty-four hours has its 

 necessary electrical consequences.! 



The result of the circulation of electricity round the 

 globe is to make the world itself a huge magnet with 

 two opposite magnetic poles. These poles are not far 

 from the poles of the earth's rotation the north and 

 south poles. The northern magnetic pole is near 

 Hudson's Bay, and the opposite one is amidst the 

 Antarctic ice ; but their positions slowly change and 

 revolve round the earth from east to west. 



It is the presence of these poles which has enabled 

 navigation to be aided by the magnetised needle of "the 

 mariner's compass." This magnetic needle is so placed 

 on a pivot that it can turn freely in any direction, and 



* See ante, p. 127. t See ante, p. 124. 



