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EVAPORATION. 267 



contains more or less water, and as the clouds are higher 

 or lower, the drops will vary in size. Perhaps we may 

 hence discover a reason why large drops are considered 

 a sign, that the rain will not last long. Large drops, it 

 is supposed, come most generally from elevated clouds, 

 and the reason why they are large is, the number of par- 

 ticles accumulated during their passage through so large 

 a portion of the atmosphere. But at a height sufficient 

 to form these large drops, the air is not heavy enough to 

 support large quantities of vapor. Hence the rain con- 

 tinues but a short time. But when the clouds are low, 

 the particles of vapor, passing through but a small space, 

 do not coalesce in sufficient numbers to form a large drop. 

 The air near the earth being dense, can support more 

 vapor. Hence the storms from these clouds are long. 

 But thunder-clouds are low ; yet the rain from them often 

 comes in large drops, and does not generally continue 

 long. These clouds are formed of very dense collections 

 of vapor, so that in falling a short distance, a number of 

 particles sufficient to form a large drop come in contact. 

 In addition to this, the concussion given to the clouds by 

 the motion of electricity, probably serves to condense 

 the vapor still more. There is another marked difference 

 between thunder-clouds, and those which produce long 

 storms. The former extend over but a small space : the 

 latter sometimes cover a large part of a continent. This 

 will account for the difference in their duration. In ad- 

 dition to this, during a storm clouds probably often re- 

 ceive fresh accessions of vapor from places beyond the 

 limits of the storm. Daily experience shows that the 

 duration of storms depends very much on the direction 

 of the wind. Winds which blow from the ocean, come 

 loaded with vapor, which contributes to swell the amount 

 already collected, and consequently to prolong the storm. 

 IIe,nce clear weather succeeding a storm, while the wind 

 still continues in the northeast, east, or southeast, is sel- 

 dom of long continuance. The wind soon load? the air 

 again with vapor, and another storm is the consequence. 

 On the contrary, north-northwest and west winds usually 

 bring fair weather. They not only drive the clouds to- 

 wards the ocean, but coming from a colder region into a 



