CLIMATE &C, NEAR THE MISSISSIPPI. 51 



Before the close of November we are reminded of the ap- 

 proach of winter by a few cold mornings and evenings and 

 sometimes nipping frosts, which exhibit their destructive 

 power, first, in the vallies by killing tender plants, while those 

 on the adjoining hills retain sometime longer their bloom and 

 verdure. This effect is to be accounted for by the greater 

 specific gravity of the condensed freezing air, which runs off 

 on all sides "from elevated situations into the nearest vallies, 

 there forming a mass of great extent, while the hills are sup- 

 plied with air less dense and warmer from a superior stratum 

 of the atmosphere. The influence of this cause is so great at 

 the first approaches of winter that a difference of 10°. of 

 Farenheit's scale has been noted at the short interval of 3 miles 

 in the direction of East and West; one position overlooking 

 the great valley of the Mississippi 30 miles wide, while the 

 other was in the interior, environed by forests. On the morn- 

 ing of the 13th, November 1799 the thermometer stoOd in the 

 first situation at 42°. and in the latter at 32°. 



We are told that at Benares in the East Indies lat. 231°. ice 

 is produced by natural agents artificially brought together, 

 sufficient for the purposes of luxury. Large excavations are 

 dug in an extensive plain, into which the condensed freezing 

 air is collected in a considerable mass, but which probably 

 might have formed upon the plain a stratum of a few inches 

 only, and consequently must have been speedily mollified by 

 the transpiration of the earth, without producing any effect; 

 but being thus accumulated into a body of considerable mag- 

 nitude it is found that in the stillness of a fine night, water 

 contained in shallow unglazed vessels, placed upon a stratum 

 of about a foot in thickness of some imperfect conductor of 

 heat, such as the stalks of Maize &c. on the bottom of those 

 excavations, is partially or totally converted into ice, according 

 to the degree of temperature of the atmosphere, while per- 

 haps the slightest hoar-frost is not perceptible on the natural 

 surface — perfect calmness is essential to the success of this 

 curious experiment: a moderate circulation of air counteracts 

 the laws of specific gravity, and restores the equilibrium of the 

 caloric in the adjacent strata of the atmosphere. 



