ON DEW, &c. 



and after sunset, with a greater rapidity, than 

 can be attributed to a diminution of the general 

 heat of the atmosphere. 



These examples are sufficient to show the 

 value of the fact, that bodies become colder than 

 the neighbouring air, before they are dewed, 

 in explaining many atmospherical appearances. 

 To this point, the investigation of the cause of 

 dew might have been carried at any time, since 

 the invention of thermometers ; but its com- 

 plete theory could not possibly, in my opinion, 

 have been attained, before the discoveries on 

 heat were made, which are contained in the 

 works of Mr. Leslie and Count Rumford. 



The experience of most persons, respecting 

 the communication of heat among bodies in the 

 open air, is confined to what happens during 

 the day; at which time, those that are situated 

 near to one another are always found to possess 

 the same temperature, unless some very evident 

 reason for the contrary should exist. To 

 many, therefore, it may appear incredible, that 

 a perfectly dry body, placed in contact, on all 

 sides, with other bodies of the same tempera- 

 ture with itself, shall afterwards, without un- 

 dergoing any chemical change, become much 

 colder than they are, and shall remain so for 



