i 5 6 



THE POPULAR SCIENCE MONTHLY. 



of the year, but with that of the lunar month. The daily oscillation also varies 

 at times in a sudden and irregular manner. The needle has been exhibiting, for 

 several weeks, the most perfect uniformity of oscillation. Day after day the 

 careful observation of the needle's progress has revealed a steady swaying to and 

 fro, such as may be seen in the masts of a stately ship at anchor on the scarce- 

 heaving breast of the ocean. Suddenly a change is noted ; irregular movements 



Fig. 7. 



! . 



\ \ 



- .. ..- ' 



-J 



r r 



<jr 



i&* -J 



i 



1 



%>* 





J a 



.,' 



. -".1. 



Changes of Appearance of the Great Sun-Spot of 1S65. 



become perceptible which are totally distinct from the regular periodic oscilla- 

 tions. A magnetic storm is in progress, and its progress does not affect only 

 the place of observation, but widely-extended regions of tlie earth ; and in some 

 well- authenticated instances these magnetic vibrations thrill in one moment 

 the whole frame of the earth." 



