Si6 POPULAR LECTURES AND ADDRESSES. 



some other external influence, possibly the electric 

 currents in our atmosphere, which Schuster sug- 

 gested as a probable cause. The cause, whatever 

 it may be, for the semidiurnal and higher con- 

 stituents would also probably have a variation in 

 the solar diurnal period on account of the differ- 

 ence of temperature of night and day, and a 

 sidereal and annual period on account of the differ- 

 ence of temperature between winter and summer. 



Even if, what docs not seem very probable, \vc 

 are to be led by the analysis to believe that 

 magnetic force of the sun is directly perceptible 

 here on the earth, we are quite certain that this 

 steady force is vastly less in amount than the 

 abruptly varying force which, from the time of my 

 ancestor in the Presidential Chair, Sir Edward 

 Sabine's discovery, 1 forty years ago, of an apparent 

 connexion between sun-spots and terrestrial mag- 

 netic storms, we have been almost compelled to 

 attribute to disturbing action of some kind at the 

 sun's surface. 



1 Communication to the Royal Society, March iSth, 1852 (Phil, 

 Trans., vol. 162, p. 143). 



