THE EARTH A MAGNET. 33 



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 (we believe) to point out 

 that " the progressive movement of the magnetic needle 

 toward 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 defini- 

 tively 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 con- 

 tinued. But he had omitted to take notice of one very 

 simple fact, viz., that London and Paris are two differ- 

 ent places. A few years later the retrograde motion 

 became perceptible at London also, and it has now been 

 established by the observations of forty years. It ap- 

 pears, from a careful comparison of Beaufoy's observa- 

 tions, that the needle reached the limit of its western 

 digression (at Greenwich) in March, 1819, at which 

 time the declination was very nearly 25. In Paris, 

 on the contrary, the needle had reached its greatest 

 western digression (about 22-) in 1814, It is rather 



