264 ELEMENTARY LESSONS ON [CHAP. iv. 



years, which agrees pretty well with the cycles of 

 maximum of magnetic storms (see Art. 144) and of 

 sun-spots. 



The spectroscope shows the auroral light to be due 

 to gaseous matter, its spectrum consisting of a few 

 bright lines not referable with certainty to any known 

 terrestrial substance, but having a general resemblance 

 to those seen in the spectrum of the electric discharge 

 through rarefied dry air. 



The most probable theory of the aurora is that origin- 

 ally due to Franklin, namely, that it is due to electric 

 discharges in the upper air, in consequence of the differ- 

 ing electrical conditions between the cold air of the polar 

 regions and the warmer streams of air and vapour raised 

 from the level of the ocean in tropical regions by the 

 heat of the sun. 



