144 Essay on the Indian Summef. 



further, that this visible aheration, or increased density, is conBned 

 chiefly or wholly to the lower stratum of air, and is immediately re- 

 ferrible to partial and slow currents of moist winds from a southerly 

 or south-easterly direction, (as meteorological records confirm,) and 

 that these moist winds usually follow cool dry westerly or northerly 

 ones, which having prevailed extensively both in the higher and low- 

 er regions of the atmosphere, have lowered its temperature and re^- 

 duced its absolute quantity of moisture. Now, if we suppose this 

 moist and warm current of air (extending perhaps in height some 

 hundreds or even thousands of feet above the earth's surface) to come 

 into contact with a cool dry northerly one, the obvious result would 

 be a reduction of temperature, attended with a hazy or cloudy for- 

 mation ; while the superior stratum of air being altogether westerly 

 or northerly, would remain dry and cloudless, which is in fact the 

 precise condition of things during the Indian summer. 



It might be asked whether those partial currents do not occur in 

 the spring and summer season, and if so, why not give rise to the 

 same phenomena ? The reply would be, that the general tendency 

 to foggy or cloudy weather is obviously less at those seasons than in 

 the fall, from the existence of a cause which we mentioned in the 

 early part of this paper, viz. that the mean temperature of the air be- 

 ing on the increase during the spring and summer seasons, (from the 

 more direct action of the solar rays on the earth's surface,) its capa- 

 city to receive moisture and contain it invisibly is also augmented. 

 Hence, an easterly wind will not produce rain, nor indeed sensibly af- 

 fect the transparency of the air until it has continued for such a length 

 of time as to bring a great excess of moisture ; when the solar heat 

 becoming also obstructed, a further reduction of temperature, and 

 consequent precipitation occurs. And again two other very impor- 

 tant agents concur to prevent these phenomena from appearing in 

 warm weather, we allude to the excessive heat at the earth's surface, 

 acting on the lower stratum of air, and the vigorous movement of 

 electrical agencies on the higher regions, both of which causes con- 

 cur to produce a constant and steady admixture of the atmospherical 

 strata ; the former, by rarifying the lower stratum and causing it to 

 ascend; the latter by exploding and destroying large portions 

 above, and reducing the temperatures of others, which as rapidly de- 

 scend, and it is this activity of circulation among the currents of air 

 during warm weather, that prevents the formation and continuance of 

 a hazy or foggy atmosphere. 



