42 COMMON PHENOMENA DUE TO HEAT 



of the cooling effect of evaporation to lower the temperature 

 of the air in summer; streets are sprinkled not only to lay 

 the dust, but also to cool the surrounding air by the evapora- 

 tion of the water. 



Some thrifty housewives economize by utilizing the cooling 

 effects of evaporation. Butter, cheese, and other foods sensi- 

 tive to heat are placed in porous vessels wrapped in wet cloths. 

 Rapid evaporation of the water from the wet cloths keeps the 

 contents of the jars cool, and that without expense other than 

 the muscular energy needed for wetting the cloths frequently. 



Clouds, rain, snow, fog, frost, dew. The heat of the sun 

 causes constant evaporation of the waters of oceans, rivers, 

 streams, and marshes. The water vapor set free by evapora- 

 tion passes into the air, which becomes charged with vapor or 

 is said to be humid. Constant, unceasing evaporation of our 

 lakes, streams, and pools would mean a steady decrease in 

 the supply of water available for daily use, if the escaped water 

 were all retained by the atmosphere and lost to the earth. 

 But although the escaped vapor mingles with the atmosphere, 

 hovering near the earth's surface, or rising far above the level 

 of the mountains, it does not remain there permanently. When 

 this vapor, which has risen far above the earth, meets a cold 

 wind or is chilled in any way, condensation takes place, and a 

 mass of tiny drops of water or of small particles of snow is 

 formed. These droplets of water and tiny particles of snow 

 when massed together form clouds. When the drops or par- 

 ticles become large enough, they fall as rain or snow, and 

 compensate the earth for the great loss of moisture due to 

 evaporation. When the vapor in the air near the earth meets 

 a coid wind or is chilled in any way it also condenses into 

 droplets. These when massed together form a fog ; in fact a 

 fog is a cloud near the surface of the earth. Can you tell what 

 becomes of a fog ? 



