llf EVAPORATION, Sec. [Lesson xix. 



of Fogs, the reason of this will appear evident. 

 There is, besides the evaporation from the seas, 

 lakes, and rivers, a constant and very large exha- 

 lation from the surface of the earth, at all seasons, 

 of water in the form of vapour ; and the warmer 

 the ground the greater will be the evaporation. 

 When the air is waum-r, or even but a little colder 

 than the earth, the ascent of vapour is not per- 

 ceptible to the eye : but when the temperature of 

 the air is considerably lower, the vapour as soon 

 as it rises is deprived of part of its heat, the 

 watery particles are brought more into union, and 

 they become visible in the form of steam. It is 

 also essential to the formation of Fog, that there 

 should be little or no wind stirring, in order that 

 the rising exhalations may have full opportunity 

 to condense. The heat of the middle of the days 

 in autumn is stiil sufficient to warm the earth and 

 cause a large ascent of vapour, which .the chill- 

 ing frosty nights, which are also generally very 

 calm, condense into Fogs. 



When vapour is condensed into small drops upon 

 the surface of bodies on the ground, it is called 

 Dew ; the most apparent difference between Dew 

 and Rain is, that the condensation of vapour, in 

 the one case, is made at or near the surface of the 

 body receiving it, and in the other the drops fall 

 a considerable space before thev reach the earth : 

 the cause is the same in both cases, namely, cold 

 operating upon vapoury air. At first view, 4t will 

 appear improbable that a condensation of vapour 

 should take place in the air resting upon the earth's 



surface, 



