Proximate Causes of certain Winds and Storms. 279 



breeze that succeeds them, and this President Dvvight has shown to 

 descend from the higher regions of the atmosphere. 



(c.) It has been shown that in all parts of the world, thunderstorms 

 generally commence in the after part of the day. They have there- 

 fore some close and intimate connexion with the transit of the sun, 

 either over the meridian under which they occur, or over one at 

 some distance west of it. Their occurrence in a particular part of 

 the day, proves the very powerful action of the cause to which the 

 trade winds are attributed by Halley, and that the sun in his daily 

 passage excites a great commotion in the atmosphere, and creates a 

 current in the air moving from west to east. 



{d.) Thunderstorms are strictly local phenomena. One of the 

 greatest errors of those who have investigated the motion of the 

 winds, has been that of attributing those prevailing in the particular 

 country where they made their observations, or to which their re- 

 marks applied, to general currents, traversing extensive tracts of the 

 earth's surface. Thus Kirwan represents the south winds, which 

 prevail in England in the winter season, as part and parcel of a great 

 current, which, sweeping by the north pole, passes at length down 

 the eastern side of Asia, between the coast of Malabar and the Mo- 

 luccas to the equator. If winds in general, and especially those 

 bringing rain, were not local, there would not be the wide difference 

 actually observed, between different parts of the earth's surface in 

 regard to dryness and humidity. We should have no burning des- 

 arts of Sahara. But a glance at a collection of tables where the 

 direction of the winds in places not very remote from each other, is 

 exhibited, is sufficient to satisfy any one that they are the result of 

 local causes. 



The water recently drawn from a well in New Haven on a hot 

 summer day, is poured into a tumbler ; dew is seen to be deposited 

 upon the outer surface of the tumbler ; a coming storm is foretold, 

 and the prediction is commonly verified. Its existence in that par- 

 ticular place, and at that time, is determined by what happens to be 

 the condition of the air in regard to warmth and moisture there. 

 When it comes on, the limits and route of the cloud are perfectly 

 defined. The cause therefore of the storm, its progress and effects, 

 are local, and have nothing to do with distant regions. 



(e.) The wind which precedes and accompanies a thunderstorm, 

 has a vertiginous motion, and brings the masses of hot and cold air 

 into a state of intimate mixture, by a process that has already been 



