146 Essay on the Indian Summer. 



the direct action of the solar heat on our continent during the sam- 

 mer, and early part of the fall,] comes to act again with renewed 

 force, during the first cold weather of autumn, and continues at inter- 

 vals until it has, as it were, more than fulfilled its intention, and is 

 hence usually succeeded again in November and the early part of 

 December, by what may be termed a southern reaction — at which 

 period the phenomenon of the Indian summer occurs, being at the 

 precise time when the general predisposition of the air to form haze 

 or fogs, is at its maximum. 



It appears to us also, that the existence and duration of the Indian 

 summer in this country, has an important connection with the exten- 

 sive forests and uncultivated lands, peculiar to America, and it is 

 worthy of remark, that according to the recollection of our older in- 

 habitants, its former duration was often three or four weeks, whereas 

 its present continuance is short and uncertain, seldom exceeding ten 

 or fifteen days. It appears further, that this decline has been some- 

 what regular, keeping pace with, and evidently influenced by, the 

 gradual uncovering of the country. Humboldt states, that plains 

 abounding with trees, are usually characterized by a foggy atmos- 

 phere. This is particularly the case with Brazil and Guiana, and the 

 great central basin of South America, which receives the waters of 

 the Amazon. In the middle of a continent overspread with forests 

 and watered by equatorial rains, the humidity is nearly the same as 

 on the ocean. 



The temperature of the atmosphere, diminishes at an average of 

 one degree for every three hundred feet, hence the humidity is great- 

 est in the lower regions. The hygrometric condition of the air also 

 varies (of course) with different elevations of country, the more ele- 

 vated, possessing the drier atmosphere. Again in tropical latitudes 

 the clouds form at a greater height above the earth's surface, because 

 the point of condensation or congelation, is confined to those cold el- 

 vated regions, whereas in the temperate zones, (and especially in the 

 fall season, when the mean temperature of the air is but a few de- 

 grees above the point of deposition,) fogs and clouds form near the 

 earth's surface — all these facts, if closely examined, corroborate the 

 views we have been trying to establish — but which we confess are too- 

 imperfectly and theoretically founded. 



One of the most remarkable phenomena of the Indian summer is 

 the peculiar redness of the sky, which in our opinion is explained on 

 the principle that the white beam of light being unfolded in its pass- 

 age through the foggy stratum near the earth's surface, its more del- 



