453 



Article XII. 



Memoir on the Nocturnal Cooling of Bodies exposed to a free 

 Atmosphere in calm and serene Weather, and on the resulting 

 Phcenomena near the Earth's surface. By M. Melloxi*. 



[Read to the Royal Academy of Sciences of Naples on the 23rd of February, 

 and yth and 16th of March 1847.] 



V> ILSON was the first who observed the cold produced in 

 bodies exposed during the night, in the open country and under a 

 clear sky and calm atmosphere. His observations were performed 

 towards the end of the year 1 783 by means of two thermometers, 

 one placed on the snow, the other freely suspended at the height 

 of 4 feet. On one of these nights, the lower thermometer, 

 under a perfectly clear sky, marked — 21°'7; the upper ther- 

 mometer — 15°. The difference of six degrees diminished rapidly 

 when clouds appeared on the horizon^ and entirely vanished 

 when the sky was completely covered : the two thermometers 

 had then descended to — 13°-9t- 



Some years later, Six found that a thermometer placed on the 

 grass of a meadow during calm and clear nights continued at 

 several degrees lower than another perfectly similar thermometer 

 suspended at the height of 5 or 6 feet, the difference between the 

 two amounting sometimes to 7°'5!t. 



At the beginning of the present century Wells instituted a 

 long series of experiments analogous to those of Six, but more 

 extended and diversified, by placing thermometers in contact 

 with the ground and leaves of plants, or by enveloping them with 

 wool, cotton, and other substances. These thermometers, placed 

 at a small distance from the earth's surface, in calm and serene 

 weather, gave a fall of 4°*5 and even 7^'8 below a thermometer 

 without any envelope suspended at the height of 4 fcet§. 



All these indications became more nearly equal to each other, 

 and sometimes became absolutely equal when the wind blew, or 



• Translated from the Annales de Chimie et de Physique for February 1848, 

 by Mr. A. W. Ilobson, B..\., St. John's Colh^ge, Cambrid^je. 

 t Edinliurgh Philosophical Transactions, vol. i, p. 153. 

 J Six's Posthumous Works. Canterbury, 1791. 

 § Ann. de Chimie et de Phys. 3rd series, vol. v. p. 183. 



