282 Proximate Causes of certain Winds and Storms. 



they touched again. The men saved themselves, some by getting 

 under a canoe, others by putting sundry articles on their heads; two 

 were knocked down, and seven had their legs and thighs much 

 bruised. " Capt. Lewis remarks, under April 1, " I have observed 



fi 



it all thunderclouds , in the western part of the continent, proceed 

 om the westerly quarter, as they do in the Atlantic States." 

 4. Adopting the views of the phenomena of thunderstorms, taken 

 in this paper, we shall find no difficulty in receiving as true and ac- 

 curate, the well attested, but hitherto inexplicable, accounts of the 

 effect of the surface of the ground over which they pass, upon their 

 progress and direction ; that they should turn aside from their path, 

 to follow the course of a stream, or to avoid a forest or rising ground ; 

 that these last should check their progress,* or that one storm should 

 show a disposition to advance in the track of its predecessor, where 

 of course the air is more highly charged with aqueous vapor. How 

 is it, we have been ready to inquire, that the earth can exert this kind 

 of influence upon the clouds hovering over its surface? Is it some 

 unknown attraction or repulsion ? But it is well known that straits, 

 valleys and defiles, in the mountains, change the direction of winds. 

 It is evident therefore, that they may exert an influence upon the 

 lower rim of a revolving current, where it touches the earth, and 

 change the direction of its plane, or retard its progress. 



5* Finally, it may be of advantage to recur once more to our 

 original and fundamental proposition respecting the cause of winds 



»/ 



of a movement, or rather 

 and the other doivnwards, in a 



vertical plane, and that they are accompanied by a counter-current in 

 the upper regions of the atmosphere. They can be generated in no 

 other way, nor can they continue for any length of time after the verti- 

 cal motion has ceased. If the horizontal current is rapid and violent, 

 the same must be true of the vertical. If the requisite number of 

 points in its track were ascertained, we might determine its whole 

 route in the same way, though not with the same degree of accuracy 

 that we investigate and ascertain the orbit of any other revolvin 

 body. Whereas, in the case of a thunderstorm, the space covered 

 by the tempest is small, and the path of the wind is a tangent to the 

 earth's surface through a small distance only ; no inference in Phys- 





See the last Number of this Journal. 



