34 LIGHT SCIENCE FOR LEISURE HOURS. 



singular that, althongli at Paris the retrograde motion 

 thus presented itself five years earlier than in London, 

 the needle pointed due north at Paris six years later 

 than in London, viz., in 1663. Perhaps the greater 

 amplitude of the needle's London digression may ex- 

 plain this peculiarity. 



" It was already sufficiently difficult," says Arago, 

 " to imagine what could be the kind of change in the 

 constitution of the globe which could act during one 

 hundred and fifty-three years in gradually transferring 

 the direction of the magnetic needle from due north to 

 23 west of north. We see that it is now necessary to 

 explain, moreover, how it has happened that this grad- 

 ual change has ceased, and has given place to a return 

 toward the preceding state of the globe." " How is 

 it," he pertinently asks, " that the directive action of 

 the globe, which clearly must result from the action 

 of molecules of which the globe is composed, can be 

 thus variable, while the number, position, and tempera- 

 ture of these molecules, and, as far as we know, all 

 their other physical properties, remain constant ? " 



But we have considered only a single region of the 

 earth's surface. Arago's opinion will seem still more 

 just when we examine the change which has taken 

 place in what we may terrn the " magnetic aspect " of 

 the whole globe. The line which separates the region 

 of westerly magnets from the region of easterly 

 magnets now runs, as we have said, across Canada and 



