278 Proximate Causes of certain Winds and Storms. 



air in the upper regions of the atmosphere, directed eastward and 

 downward. If it meet in its descent with a stratum of moist air, 

 such as is known to exist in the lower regions of the atmosphere on 

 those sidtry days when thunderstorms are to be expected, there will 

 be a condensation of vapor, and the formation of a cloud. Into the 

 partial vacuum thus created, there will be a more rapid rush from 

 the upper regions of the atmosphere, and a vortex or horizontal 

 whirlwind will be formed, which will move forward before tlie cur- 

 rent to which it owes its existence, creating a wind by its gyratory 

 motion, and a rain, by whirling the lower strata of the atmosphere 

 aloft, and bringing down the higher strata to the ground. That the 

 actual origin and progress of thunderstorms is as here stated, is ren- 

 dered probable by the following facts and arguments. 



(a.) We have already shewn by numerous quotations, that thun- 

 derstorms generally move from west to east in all parts of the world. 

 See also the last number of this Journal, page 192, where it is stated 

 that twenty-one thunderstorms, whose course has been distinctly tra- 

 ced in France, extended from N. N. W. to S. S. W., and that no de- 

 structive thunderstorm has come from any other points of the hori- 

 zon. Within the limits of the United States, it is believed that these 

 storms more frequently come from a point between west and north, 

 than from a point betv/een west and south. Both the wind that car- 

 ries the cloud forward, and that which accompanies it, and seems to 

 be more intimately connected with it, come ordinarily in the same di- 

 rection. We cannot therefore attribute the effects observed to a wind 

 coming from the north, or from the south, or from both of these points 

 together, or to a wind coming from any other quarter than the west. 

 If the fact of their coming from the west is not particularly and dis- 

 tinctly noticed in our theory, it is defective, and if the supposition of 

 their being produced by winds blowing from any other point, be in- 

 volved in it, it is erroneous. They are commonly succeeded by a 

 cool breeze from the north-west, and are therefore the first burst of a 

 wind that is to blow from that quarter for a number of hours. 



{h.) Thunderstorms are produced, in the first instance, by a wind 

 proceeding from the higher regions of the atmosphere. For, com- 

 ing from the west, it is only a wind proceeding from the higher re- 

 gions of the atmosphere, that will be cold enough to determine even 

 the commencement of the precipitation, d)at is an essential part of 

 these storms. The generating and attendant wind are apparently of 

 the same nature, and have the same origin with the north-western 



