92 The Great World's Farm 



to the air. Whether it falls again on the same spot is 

 another matter, and we have no proof that it does so. It 

 may do so under certain circumstances, or it may be 

 carried away by the wind and fall elsewhere, perhaps 

 close by, or perhaps a long way off. 



But if the air immediately over a certain district is 

 being constantly cooled by the evaporation day after day 

 of large quantities of water, does this produce no effect 

 upon the air above .'* 



What happens when water is boiled over a fire ? 

 Clouds of visible vapor rise from it, which we commonly 

 call "steam." They are not properly steam, however, 

 for steam is invisible. These are clouds, true clouds, 

 consisting of minute globules of water, steam made vis- 

 ible, converted into water again by coming into contact 

 with the air of the room, which is cooler than that within 

 the kettle. 



As heat converts water into gas or steam, so cold turns 

 it back into water again. So when the earth is chilled at 

 night the moisture of the air is also chilled on a large 

 scale, and dew is formed — first on grass and leaves, 

 because they are cooler than the soil. 



If this be so, then when a current of warm, moist air 

 comes in contact with the cool air over a forest, or over 

 acres of pasture, will not some of its moisture be con- 

 densed into a cloud, as the steam from a kettle is con- 

 densed into a cloud when it escapes into the air, and may 

 not this cloud discharge itself upon the grass or the trees.'* 



Of course the cloud may be carried away; but it seems 

 likely that, in some cases at all events, it will water the 

 district above which it is formed. 



There is a further question as to whether trees actually 



