(J6 Peculiar Aspect of the Air, in the Indian Summer. 



Art. XI. — On the cause of the peculiar aspect of the air, in the 

 Indian Summer. 



TO PROFESSOR SILLIMAff. 



You requested in one of the numbers of your Journal, a long 

 time ago, a communication on the Indian summer. You of course 

 are acquainted with the explanation of the phenomenon, which refers 

 it to the smoke, arising from the combustion of the dead vegetable 

 substances, that are strewed over the surface of the earth in autumn. 

 This account of the matter seems to have arisen from the fact, that 

 there are frequently, from accident or design, during the the Indian 

 Summer, great conflagrations in the prairies and mountains; but these 

 appear rather to follow the season, as the effect, than to precede it as 

 the cause. The warm dry weather fits every thing, in the decay of 

 vegetable life, for inflaming at the slightest touch of fire ; and of 

 this, numerous conflagrations are a natural consequence. Besides, 

 smoke does cause a similar appearance in the atmosphere, but of a 

 deeper shade ; and although its peculiar sensation is not felt at the 

 beginning of the Indian Summer, yet soon after the extensive fires 

 have commenced, the darkness of the atmosphere is increased ; and 

 the smoke becomes painful to the organs of vision. It is not strange 

 therefore that the bluish atmospheric appearance should have been 

 attributed to what usually accompanies it. Some imagine that the 

 phenomenon, of which we are speaking, is connected with the decay 

 of vegetation in itself considered ; but how this is so, they have not 

 been able to explain. 



If we inquire for the cause of the bluish appearance in the air, we 

 have no difficulty in perceiving, that it must be ascribed to the reflec- 

 tion of the darker part of the solar rays. The reflecting power in 

 the atmosphere therefore must be greatly increased during the In- 

 dian Summer : since only the less reflexible portion of the rays of 

 light is transmitted. The whole inquiry then is resolved into this : 

 Whence arises the increased reflecting, or (which is the same thing) 

 the increased refracting power in the atmosphere ? 



It is evident at once that this must be owing to foreign substances 

 intermingled with the atmosphere, rather than to the latter itself. We 

 have therefore further to examine what substances the change of sea- 

 sons, from heat to cold, and the reverse, would be likely to produce ; 

 and which might be intermingled with the atmosphere. 



