112 PRESSURE OF VAPOR IN THE ATR. 



63. Formation of Dew, the Dew Point. Dew is 



the name given to those drops of water which are seen in 

 the morning on the leaves of plants, and are especially 

 noticeable in the spring and autumn. If any portion of the 

 space occupied by the atmosphere be saturated with vapor, 

 , e., if the density of the vapor be as great as it can be for 

 the temperature, then the slightest fall of temperature will 

 produce condensation of some portion of the vapor ; but if 

 $he density of the vapor be not at its maximum for that 

 temperature, no condensation will take place until the tem- 

 perature is lowered below the point corresponding to the 

 saturation of the space. 



If any body in contact with the atmosphere be cooled 

 down until its temperature is below that which corresponds 

 to the saturation of the air around it, condensation of the 

 vapor will take place, and the condensed vapor will be 

 deposited in the form of dew upon the surface of the body. 

 Heat radiates from the ground, and from the bodies upon 

 it, and unless there are clouds from which the heat would 

 be radiated back, the surfaces are cooled, and the vapor in 

 the adjacent stratum of the atmosphere condenses and falls 

 in small drops of water on the surface. The formation of dew 

 on the ground depends therefore on the cooling of its surface, 

 and this is in general greater and more quickly effected when 

 the sky is free from clouds. This accounts for the dew 

 with which the ground is covered after a clear night. A 

 covering of any kind will diminish the formation of dew 

 beneath ; for instance, but very little dew will be formed 

 under the shade of large trees. 



The detv-point is the temperature at which vapor begins 

 to be deposited in the form of dew, and it must be deter- 

 mined by actual observation. 



64. Pressure of Vapor in the Air. Tables* have 



* Besant'e Hydrostatics, p. 143. 



