ICE, WATER, AND STEAM 



125 



142. Amount of water evaporated by the air. When we 

 consider that the oceans, lakes, and rivers make up the larger 

 part of the surface of the earth, and that water is evaporating 

 from this surface all the time, it is plain that a very large 

 amount of water evaporates. How much it is no one knows 

 exactly, but we get some idea when we remember that all the 

 water that falls as rain or 



snow has evaporated from 

 the ocean or from the land. 

 A study has been made 

 of the rate of evaporation in 

 the United States (fig. 63), 

 and it is believed that about 

 30 inches of water evapo- 

 rates each year from the 

 surface of Lake Michigan 

 and about 70 inches from 

 the Great Salt Lake; in 

 some of the desert regions 

 of the Southwest perhaps 

 as much as 100 inches 

 would evaporate annually 

 if there were any body of 

 water there. 



143. Evaporation from 

 plants transpiration. The 



evaporation of water from land is greatly accelerated by 

 plants. Trees, grasses, weeds, cultivated crops in fact, all 

 common land plants absorb water from the soil, and most 

 of this water may afterwards evaporate from the leaves. 



Usually we are not aware that water is evaporating from 

 the leaves of plants, since it passes off as a gas and is therefore 

 not visible. A demonstration of its presence may be made 

 by inclosing several leaves in a bottle or a tumbler (fig. 64) 

 in such a way that there is little chance for water to enter or 



FIG. 64. Transpiration by leaves 



Some of the water which evaporates 



from the leaves condenses upon the inside 



of the bottle 



