ii6 Deserts 



tons to the acre, or under three inches— three hundred 

 tons during the whole month, or less than ten tons each 

 day. But an acre of pasture-grass actually gives up 

 more than ten times this quantity in the course of 

 twenty -four hours — io6 tons— that is to say, in a single 

 day and a night. So that an acre of pasture which 

 has received three hundred tons of rain in a month, 

 gives up more than three thousand tons in the same 

 time. 



The question as to where this immense quantity 

 comes from will have to be considered later. At 

 present we are concerned only with the fact that so 

 much water is returned 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 vapour rise from it, which we com- 

 monly 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 visible, 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 



