[297 I 



when the water is the colder of the two. For Richman's experi- 

 ments fhew, that other circumftances being fimilar, evaporation 

 in equal times (at leaft in portions of time not exceeding feveri 

 hours) is fo much the greater, as the difference between the tern-- 

 perature of the air and that of the water is greater. Or, in other 

 words, the evaporation is as the differences between the colder air and 

 the warmer water. Thus, if a difference of 92° produces an eva- 

 poration as 13,5, a difference of 68*^ will produce an evaporation 

 of 9,4-6. This law, however, refts on the fuppofition, that the 

 temperature remains unaltered (otherwife a new calculation would 

 be requifite), which may be fuppofed nearly true, when large 

 maffes of water in contact with the air- are coole^i by it; theydefcend 

 and give place to the lower and warmer water, while the air, 

 warmed by conta£f with the Water, rifcs, and gives place to the 

 colder which defcends : but fuch retrogade movements, whether of 

 the water or of the air, are imperceptible, unlefs the differences of 

 temperature be confiderable in the different ftrata of each ; and 

 with refped to air ia particular, unlefs the fpecific gravity of the 

 lower ftrata be inferior to that of the upper in a notable degree. 

 The previous faturability of air, (that is, its diftance from the 

 point of faturation) muft occafion confiderable differences in the 

 evaporation of the water expofed to it. 



Lastly, if we fuppofe the temperature of the water conflant, 

 and the temperature of the air in one cafe fome degrees hotter, 



and 



