476 THE POPULAR SCIENCE MONTHLY, 



cumulate again. It was thought that years of dry weather were years 

 of maxima of auroras, and it seemed natural to suppose that moisture 

 would hinder exhalations. Extensive efforts were made, without suc- 

 cess, by studying the properties of the recently discovered phospho- 

 rescent substances, to determine the nature of the stuff that thus shone 

 in space. Previous to this, an explanation of the phenomenon had 

 been suggested by supposing a fermentation of gross exhalations from 

 the earth's surface which were driven toward the pole and there 

 took fire. 



Quite different from this was Mairan's theory ; and the reading of 

 his book, " Traite physique et historique de l'aurore boreale " (" A 

 Physical and Historical Treatise on the Aurora Borealis "), which ap- 

 peared in 1733, is still indispensable, after a hundred and fifty years, 

 to any person wishing to study the meteor to-day. Rejecting the 

 ideas outlined above, and another curious hypothesis, that the rays of 

 the sun were reflected from the polar ice, and sent back to the observer 

 from the concave surface of the upper strata of the atmosphere, he 

 had recourse to the zodiacal light which had been observed by Cassini 

 some fifty years before. While some explained this phenomenon by 

 supposing a ring of light concentric with the sun, and surrounding it 

 without touching it, others, and Mairan among the number, considered 

 it a prolongation of the solar atmosphere, accumulated chiefly in the 

 plane of the ecliptic or of the solar equator, and extending beyond the 

 orbit of Venus. Emanations from the sun, or rather the corona that 

 surrounds it, according to Mairan, strike our atmosphere and illumi- 

 nate our globe. Then, must we suppose that the zodiacal light shines 

 of itself ? That is not necessary, says Mairan. A chemical combina- 

 tion, an essentially luminous precipitate, results from the mixture that 

 takes place in the upper regions of the atmosphere.* This supposition 

 is hazardous, and Mairan seems to be a little too fast. It is, however, 

 indisputable that then, as now, auroras were more frequent in March 

 and September, or the months when the zodiacal light is brightest. 

 It is also worthy of remark that Angstrom, in 1867, and Respighi, 

 in 1872, found in the spectrum of the zodiacal light a green ray 

 identical with a line of the same color characteristic of the aurora 

 borealis. 



Mairan found a redoubtable antagonist in the celebrated mathe- 

 matician Euler, who did not admit the hypothesis of an immense solar 

 atmosphere, and believed only in the existence of a ring. He invented, 

 in explanation of the meteor, a somewhat obscure theory, according to 

 which the subtile and rarefied portions of the air were driven away 

 from the surface of the globe, and the particles, having become lumi- 



* Mairan observes that, the centrifugal force being less toward the poles than at 

 the equator, the parts of the globe at the tropics will repel the foreign matter, and it will 

 accumulate toward the high latitudes. Hence there will be few auroras except in the 

 frigid and temperate zones ; and this is the case. 



