142 Essay on the Indian Summer. • 



always be regarded as one of the most powerful causes by which the 

 transparency of the air is affected ; and as before observed, a north 

 westerly wind (from obvious causes) brings with it a smaller supply 

 of moisture than belongs to the mean hygrometric condition of the air 

 in this region, and is hence always attended with a transparent at- 

 mosphere, so the very reverse is occasioned by easterly and south- 

 erly winds which obscure and thicken the air. The humid effects 

 of particular winds are however greatly modified by local circum- 

 stances — a celebrated naturalist remarks, " that the sky of Xalappa 

 in New Spain, which is beautiful and serene in summer, assumes a 

 gloomy appearance from the month of December to the month of 

 February ; whenever the north winds blow at Vera Cruz, the inhab- 

 itants of Xalappa are enveloped in a dense fog, and the thermometer 

 then descends to 45 or 50 degrees ;" (on our coast the haze is pro- 

 duced by a warm current from the south, hence the thermometer ri- 

 ses with us instead of falling.) So at Lima, in Peru, the cloudy state 

 of the air begins about the middle of July, and continues to the end 

 of November, the wind blowing chiefly from the south and south east* 

 The third cause which we shall notice as concerned in the production 

 of the Indian summer is the elevation and depression of atmospheri- 

 cal strata ; this effect is sometimes produced by electrical agencies, 

 by elevation and conformation of country, but more generally and ex- 

 tensively than either by changes of temperature at the earth's sur- 

 face, for it is a well known fact, that the air, being a diaphanous 

 body, can receive no direct heat from the solar rays, but becomes 

 warmed only by the contact of its lower stratum with the earth's sur- 

 face, which portion when rarified ascends and gives place to a cooler 

 descending one : this remains below, until its thermal condition is 

 again altered, when it reascends, thus establishing a constant circu- 

 lation and perfect admixture of the different strata of air. That elec- 

 trical agencies are concerned in the elevation and depression of at- 

 mospherical strata, there can be no doubt, but we are aware that the 

 immediate and more extensive operation of this cause, is in tropical 

 latitudes. Thus Humboldt, in his personal narrative, remarks, "that 

 the rainy season takes place within the tropical regions, when the 

 causes which concur to produce a mixture of the atmospherical strata 

 operate with the fullest effect ; for instance, when the sun approaches 

 the zenith of any particular parallel, the trade winds become less 

 regular, the temperature increases, and the causes which contribute 

 to the humidity of the atmosphere act with fullest vigor. The su- 



