26 LIGHT SCIENCE FOR LEISURE HOURS. 



few years later the retrograde motion became percep- 

 tible at London also, and it has now been established 

 by the observations of forty years. It appears, from a 

 careful comparison of Beaufoy's observations, that the 

 needle reached the limit of its western digression (at 

 Greenwich) in March 1819, at which time the declina- 

 tion was very nearly 25. In Paris, on the contrary, 

 the needle had reached its greatest western digression 

 (about 22^) in 1814. It is rather singular that 

 although 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 explain this 

 peculiarity. 



6 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 

 gradual change has ceased, and has given place to a 

 return towards 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 tem- 

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

 their other physical properties, remain constant ? ' 



