34 LIGHT SCIENCE FOR LEISURE HOURS. 



weeks the most perfect uniformity of oscillation. Day 

 after day, the careful microscopic 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 ocean. 

 Suddenly a change is noted ; irregular jerking move- 

 ments are perceptible, totally distinct from the regular 

 periodic oscillations. A magnetic storm is in progress. 

 But where is the centre of disturbance, and what are 

 the limits of the storm ? The answer is remarkable. 

 If the jerking movements observed in places spread 

 over very large regions of the earth and in some well- 

 authenticated cases over the whole earth be compared 

 with the local time, it is found that (allowance being 

 made for difference of longitude) they occur precisely 

 at the same instant. The magnetic vibrations thrill in 

 one moment through the whole frame of our earth ! 



But a very singular circumstance is observed to 

 characterise these magnetic storms. They are nearly 

 always observed to be accompanied by the exhibition 

 of the aurora in high latitudes, northern and southern. 

 Probably they never happen without such a display ; 

 but numbers of auroras escape our notice. The con- 

 verse proposition, however, has been established as an 

 universal one. No great display of the aurora ever 

 occurs without a strongly marked magnetic storm. 



Magnetic storms sometimes last for several hours or 

 even days. 



Remembering the influence which the sun has been 

 found to exercise upon the magnetic needle, the ques- 



