264 ESSAY 



prevents the freezing altogether. Sir R. Barker 

 admits, that the excavations in the earth are 

 made to increase the stillness of the air in con- 

 tact with the water in the pans ; but, with the 

 view to explain the utility of this stillness he 

 supposes, in opposition to all experience, that 

 water kept very quiet freezes more readily, 

 when other circumstances are the same, than if 

 it were a little agitated. 



2. No proof is given, that evaporation from 

 the pans actually does occur, at the times which 

 are the most favourable for the appearance of 

 ice. At any rate it cannot be considerable ; 

 since, agreeably to what is mentioned by Sir R. 

 Barker, dew forms in a greater or less degree 

 during the whole of the nights, the most pro- 

 ductive of ice ; and it is not to be thought, as 

 was said upon a former occasion, that one por- 

 tion of air will be depositing moisture, from 

 possessing a superabundance of it, while another 

 in the immediate vicinity is receiving moisture 

 in great quantity, in the state of pellucid vapour; 

 as the latter fact can exist only when the air 

 is far removed from a state of repletion with 

 water. 



3. If evaporation produced the cold under 

 consideration, the wetting of the straw or other 

 matter, upon which the pans are placed, would 

 tend to increase it; and, accordingly, Sir H. 



