TilE CENTURY'S PROGRESS IN METEOROLOGY 



the spectroscope has been baffled, for the line it sifts 

 from the aurora is not matched by that of any recog- 

 nized substance. A like line is found in the zodiacal 

 light, it is true, but this is of little aid, for the zodiacal 

 light, though thought by some astronomers to be due to 

 meteor swarms about the sun, is held to be, on the 

 whole, as mysterious as the aurora itself. 



Whatever the exact nature of the aurora, it has long 

 been known to be intimately associated with the phe- 

 nomena of terrestrial magnetism. Whenever a brilliant 



CUMULUS CLOUDS 



aurora is visible, the world is sure to be visited with 

 what Humboldt called a magnetic storm a " storm " 

 which manifests itself to human senses in no way what- 

 soever except by deflecting the magnetic needle and 



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