PNEUMATICS. 317 



currents already mentioned come in contact with 

 one another. This is at the height of 18,000 feet 

 and upwards, which agrees very well with the me- 

 dium height of the clouds. 



429. The clouds thus formed, have their parti- 

 cles united into larger masses or drops by different 

 causes, such as the mutual attraction of aqueous 

 particles, the force of the wind, or the operation of 

 electricity, and so fall down in rain on the surface 

 of the earth. 



The intimate connection between rain and the exist- 

 ence of different currents of air, is evident from ma- 

 ny appearances. 



1. When the Trade Winds blow uniformly, hardly 

 any rain falls ; but when the monsoon changes, 

 heavy falls of rain seldom fail to take place. 



2. In the tropical climates, the rainy season is always 

 on the sun's approach to the zenith, at which time 

 also the winds are most variable. 



3. There are some spots of continual rain, which seem 

 to be where opposite streams of air constantly meet 

 one another. 



4. There are several tracts on the earth's surface, 

 where it hardly ever rains. These are usually far 

 inland, and are extensive plains, without any of 



those 



