1 8 LIGHT SCIENCE FOE LEISURE HOURS. 



nation. This gradually diminished, so that in about 

 the year 1657 the needle pointed due north. After 

 this the needle pointed towards the west, and con- 

 tinually more and more, so that scientific men, having 

 had experience only of a continual shifting of the 

 needle in one direction, began to form the opinion that 

 this change would continue, so that the needle would 

 pass, through north-west and west, to the south. In 

 fact, it was imagined that the motion of the needle 

 would resemble that of the hands of a watch, only in 

 a reversed direction. But before long observant men 

 detected a gradual diminution in the needle's westerly 

 motion. Arago, the distinguished French astronomer 

 and physicist, was the first (I believe) to point out 

 that ' the progressive movement of the magnetic needle 

 towards the west appeared to have become continually 

 slower of late years ' (he wrote in 1814), which seemed 

 to indicate that after some little time longer it might 

 become retrograde.' Three years later, namely, on the 

 10th of February, 1817, Arago asserted definitively 

 that the retrograde movement of the magnetic needle 

 had commenced to be perceptible. Colonel Beaufoy 

 at first oppugned Arago's conclusion, for he found from 

 observations made in London, during the years 1817- 

 1819, that the westerly motion still continued. But 

 he had omitted to take notice of the circumstance, 

 that London and Paris are two different places. A 

 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 



