190 ESSAY 



climates, upon the naked parts of a living and 

 healthy human body, during the night ; since 

 their heat is never less in this season, in such 

 climates, than that of the atmosphere. I have, 

 in fact, never perceived dew on any naked part 

 of my own body at night, though my attention 

 was much occupied, for three years, with every 

 thing relative to this fluid, and though I had 

 been, during that period, much exposed to the 

 night air. On the other hand, in very* hot 

 countries, the uncovered parts of a human body 

 may sometimes, from being considerably colder 

 than the air, condense the watery vapour of 

 the atmosphere, and hence be covered with a 

 real dew, even in the day-time. 



VI. Hygrometers formed of animal or vegeta- 

 ble substances, when exposed to a clear sky at 

 night, will become colder than the atmosphere ; 

 and hence, by attracting dew, or, according to 

 an observation of Saussure*, by merely cooling 

 the air contiguous to them, mark a degree of 

 moisture, beyond what the atmosphere actually 

 contains. This serves to explain an observation 

 made by Mr. De Lucf, that in serene and 

 calm weather, the humidity of the air, as de- 

 termined by an hygrometer, increases about, 



* Hygronometrie, p. 25. 



f Introduction a la Physique Terrestre, II, 491. 



