270 ESSAY 



during the time that he supposed ice to be form- 

 ing, was only 14 higher than the freezing point 

 of water. I need say nothing of the difference 

 of 18 related by Sir H. Davy, as he does not 

 speak from his own observation, and as he gives 

 no authority for what he advances ; though even 

 this difference is considerably less, than what I 

 have attempted to show must sometimes occur, 

 from the radiation of heat at night, between 

 the temperature of air, a few feet above the 

 earth, and that of bodies placed on its surface. 



It is to be mentioned here also, that, accord- 

 ing to Mr. Leslie*, the power of water to ra- 

 diate heat, exceeds, perhaps, that of all other 

 substances. 



2. Ice is chiefly formed in Bengal during the 

 clearest and calmest nights ; and it is on such 

 nights that the greatest cold, from radiation, is 

 observed on the surface of the earth. In Sir R. 

 Barker's more refined mode of conducting the 

 process, an unusual stillness of the air, in con- 

 tact with the water to be frozen, is procured, 

 by placing the pans containing it a little below 

 the level of the ground ; in which situation, it 

 was formerly shown, bodies must grow colder 

 from radiation to the heavens at night, than in 

 any other. 



* On Heat, p. 80. 



