26 LIGHT SCIENCE FOR LEISURE HOURS. 



the extreme delicacy of the variation renders the deter- 

 mination of its amount very difficult, enough has been 

 done to show that in all probability the sun's influence 

 varies according to the same law as gravity that is, 

 inversely as the square of the distance. 



That the sun, the source of light and heat, and the 

 great gravitating centre of the solar system, should 

 exercise a magnetic influence upon the earth, and that 

 this influence should vary according to the same law as 

 gravity, or as the distribution of light and heat, will 

 not appear perhaps very surprising. But the dis- 

 covery by Sabine that the moon exercises a distinctly 

 traceable effect upon the magnetic needle seems to me 

 a very remarkable one. We receive very little light 

 from the moon, much less (in comparison with the 

 sun's light) than most persons would suppose, and we 

 get absolutely no perceptible heat from her. There- 

 fore it would seem rather to the influence of mass and 

 proximity that the magnetic disturbances caused by the 

 moon must be ascribed. But if the moon exercises 

 an influence in this way, why should not the planets ? 

 We shall see that there is evidence of some such in- 

 fluence being exerted by these bodies. 



More mysterious, if possible, than any of the facts 

 I have discussed is the phenomenon of magnetic storms. 

 The needle has been exhibiting for several 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 



