ON DEW, &c. 249 



observed the inside of any pane to be more than 

 a little damped, though it might be from 8 to 

 12 colder than the general mass of the air in 

 the room ; while, in the open air, I had often 

 found a great dew to form on substances, only 

 3 or ,4 colder than the atmosphere. This at 

 first surprised me ; but the cause now seems 

 plain. The air of the chamber had once been 

 a portion of the external atmosphere, and had 

 afterwards been heated, when it could receive 

 little accession to its original moisture. It con- 

 sequently required being cooled considerably, 

 before it was even brought back to its former 

 nearness to repletion with water ; whereas the 

 whole external air is commonly, at night, nearly 

 replete with moisture, and therefore readily 

 precipitates dew, on bodies only a little colder 

 than itself. 



When the air of a room is warmer than the 

 external atmosphere, the effect of an outside 

 shutter, on the temperature of the glass of the 

 window, will be directly opposite to what has 

 been just stated; since it must prevent the 

 radiation, into the atmosphere, of the heat of 

 the chamber transmitted through the glass. 



II. Count Rumford* appears to have rightly 

 conjectured, that the inhabitants of certain hot 



* Phil. Trans. 1804. p. 182. 



