THE CENTURY'S PROGRESS IN METEOROLOGY 



ning, all spectacular meteors came to be regarded as 

 natural phenomena, the aurora among the rest. Frank- 

 lin explained the aurora which was seen commonly 

 enough in the eighteenth century, though only recorded 



CIRRUS CLOUDS 



once in the seventeenth as due to the accumulation of 

 electricity on the surface of polar snows, and its dis- 

 charge to the equator through the upper atmosphere. 

 Erasmus Darwin suggested that the luminosity might be 

 due to the ignition of hydrogen, which was supposed by 

 many philosophers to form the upper atmosphere. Dai- 

 ton, who first measured the height of the aurora, esti- 

 mating it at about one hundred miles, thought the phe- 

 nomenon due to magnetism acting on ferruginous 

 particles in the air, and his explanation was perhaps 

 the most popular one at the beginning of the century. 



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