Deserts 117 



on a lar^e scale, and dew is formed— y^rs/ 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 

 condensed into a cloud, as the steam from a kettle is 

 condensed 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 attract the clouds or not, and this still waits 

 for a satisfactory answer ; but it is certainly the popular 

 opinion that they do, and it is a very common thing to 

 hear it said that the clouds have gone over to a neigh- 

 bouring park or wood, when the farmer would have 

 been better pleased that they should water his fields. 



As we began by saying, the subject is a difficult one ; 

 but though we may not be able to explain precisely 

 the how, there is no doubt at all as to the fact that the 

 presence or absence of all vegetation, not of trees only 

 does very greatly affect climate, and the climate in its 

 turn affects vegetation. 



For instance, Tacitus, the Latin historian, writing 

 some eighteen hundred years ago, mentions that not 

 even a cherry would ripen on the banks of the Rhine; 

 and he certainly would not have believed that in cen- 

 turies to come the same region would have become 

 warm enough to be famous for its vineyards. But 

 in his day forests abounded all about the river, and 

 it is the removal, or great diminution, of these which 



