6 LIGHT SCIENCE FOR LEISURE HOURS. 



But I must pass on to consider further the circum- 

 stances which give interest and significance to the 

 strange discovery which is the subject of this paper. 



Could we only associate auroras with terrestrial mag- 

 netism, we should still have done much to enhance the 

 interest which the beautiful phenomenon is calculated 

 to excite. But when once this association has been 

 established, others of even greater interest are brought 

 into recognition. For terrestrial magnetism has been 

 clearly shown to be influenced directly by the action 

 of the sun. The needle in its daily vibration follows 

 the sun, not indeed through a complete revolution, but 

 as far as the influence of other forces will permit. 

 This has been abundantly confirmed, and is a fact of 

 extreme importance in the theory of terrestrial mag- 

 netism. Wherever the sun may be, either on the visible 

 heavens or on that half of the celestial sphere which 

 is at the moment beneath the horizon, the end of the 

 needle nearest to the sun makes an effort (so to speak) 

 to point more directly towards the great ruling centre 

 of the planetary scheme. Seeing, then, that the daily 

 vibration of the needle is thus caused, we recognise the 

 fact that the disturbances of the daily vibration may be 

 referred to some peculiarity of the solar action. 



It was not, therefore, so surprising as many have 

 supposed, that the increase and diminution of these 

 disturbances, in a period of about eleven years, should 

 be found to correspond with the increase and diminu- 

 tion of the number of solar spots in a period of equal 

 length. 



