36 LIGHT SCIENCE FOR LEISURE HOURS. 



in the evening, returns to the west, and attains its great- 

 est westerly excursion at eleven, as at starting. 



Of course, these excursions take place on either side 

 of the mean position of the needle, and as the excursions 

 are small, never exceeding the fifth part of a degree, 

 while the mean position of the needle lies some 20 to 

 the west of north, it is clear that the excursions are 

 only nominally eastern and western, the needle point- 

 ing, throughout, far to the west. 



iNow if we remember that the north end of the nee- 

 dle is that farthest from the sun, it will be easy to trace 

 in M. Arago's results a sort of effort on the part of the 

 needle to turn toward the sun not merely when that 

 luminary is above the horizon, but during his noctur- 

 nal path also. 



"We are prepared, therefore, to expect that a varia- 

 tion, having an annual period, shall appear, on a close 

 observation of our suspended needle. Such a varia- 

 tion has been long since recognized. It is found that in 

 the summer of both hemispheres, the daily variation is 

 exaggerated, while in winter it is diminished. 



But besides the divergence of a magnetized needle 

 from the north pole, there is a divergence from the 

 horizontal position which must now claim our atten- 

 tion. If a non-magnetic needle be carefully suspended 

 so as to rest horizontally, and be then magnetized, it 

 will be found no longer to preserve that position. The 

 northern end dips very sensibly. This happens in our 



