102 Common Science 



cold, though, since there is no evaporation to use up 

 your heat. 



But the water problem outside is not one of mere 

 inconvenience. It never rains. How can it when the 

 water from the oceans cannot evaporate to form clouds ? 

 Little by little the rivers begin to run dry there is no 

 rain to feed them. No fog blows in from the sea ; no 

 clouds cool the sun's glare ; no dew moistens the grass 

 at night; no frost shows the coming of cold weather; 

 no snow comes to cover the mountains. In time there 

 is no water left in the rivers ; every lake with an outlet 

 runs dry. There are no springs, and, after a while, no 

 wells. People have to live on juicy plants. The crops 

 fortunately require very little moisture, since none 

 evaporates from them or from the ground in which they 

 grow. And the people do not need nearly as much 

 water to drink. 



Little by little, however, the water all soaks too deep 

 into the ground for the plants to get it. Gradually 

 the continents become great deserts, and all life perishes 

 from the land. 



All these things would really happen, and many more 

 changes besides, if water did not evaporate. Yet the 

 evaporation of water is a very simple occurrence. As 

 the molecules of any liquid bounce around, some get 

 hit harder than others. These are shot off from the 

 rest up into the air, and get too far away to be drawn 

 back by the pull of the molecules behind. This shoot- 

 ing away of some of the molecules is evaporation. 

 And since it takes heat to send these molecules flying 

 off, the liquid that is left behind is colder because of the 



