ClouD^fformation ^Evaporation. 65 



leased in the process of evaporation coalesce 

 into one small drop or particle of water, and 

 they will continue to float in the air as mist 

 or cloud until a sufficient number have com- 

 bined into one solid mass to render that mass 

 heavier than the air, when it falls in the form 

 of rain. 



If we live in a region and there are such 

 on the face of the earth where there is very 

 little evaporation and consequently very little 

 moisture in the air, there is rarely ever a cloud 

 seen nor is there any rainfall, for the reason 

 that there is no material existing out of which 

 to form clouds, and the clouds precede the 

 rain. Hence, all the artificial attempts to pro- 

 duce rain in these arid regions have been 

 futile. If a body of warm air, when saturated 

 with invisible moisture, is suddenly chilled by 

 coming in contact with a cold wave, it is 

 squeezed like a sponge, so to speak, and the 

 invisible particles become visible because a 

 number of them have coalesced as one par- 

 ticle ; the particles gather in a large mass, and 

 we have the phenomenon of cloud formation. 



Clouds more generally form in the upper 

 regions of the atmosphere because it is nor- 

 mally colder in the higher regions. In some 

 cases clouds float very high in the air and in 

 others very low. This is due to two causes: 



If we should send up a balloon containing 

 air rarefied to a certain extent it would con- 



