THE DISTRIBUTION OF WATER 17 



orates. On wash-day wet clothes are put out on the line 

 and presently they are dry. You can't see the water go, 

 but you know that it has gone. How are we going to ex- 

 plain this? 



A drop of water is composed of millions of particles. It 

 is impossible to see these particles, but we know that they 

 are there by the way they behave. These particles are 

 always in motion. They move in straight lines. They 

 strike each other, and rebound. At the surface they are 

 always flying off into the air. It is this flying off of the 

 particles of water that we call evaporation. There is al- 

 ways some water in the air in the form of these invisible 

 particles. When there is a good deal of water in the air 

 we say that it is moist or humid. 



Evaporation does not always go on at the same rate. 

 Sometimes it is rapid, sometimes slow. The more moisture 

 there is in the air the slower the rate of evaporation. Heat 

 and wind affect it. It is most rapid when the air is hot and 

 dry and the wind is blowing. It is slowest when the air 

 is moist and cold and there is no wind. 



Evaporation is a cooling process. As the water passes 

 away, heat passes away with it. On hot days if the air is 

 dry, we are much more comfortable than if it is moist. 

 This is because rapid evaporation of moisture from our 

 bodies keeps us cool. But if the air is moist as well as hot, 

 we are very uncomfortable. Moisture stays on our skin, 

 and heat stays with it. 



Evaporation goes on all over the world all the time. 

 All the time water is passing into the air from all the seas 

 and lakes and streams, and from all the leaves of plants. 

 Each year enough rain falls to make a sheet of water be- 

 tween thirty and forty inches deep all around the world, 



