890 Professor Mitchell on the Proximate Causes 



tion > £. A similar motion of air, but in an opposite direction, 

 will be produced by the condensation of the air at s. 



In every case of wind, the primary movement is upwards or 

 downwards in a vertical plain. Of this the horizontal current 

 felt at the earth's surface, is only a secondary result. It is not 

 possible that it should be generated by those causes which affect 

 the condition of our atmosphere, except according to the me- 

 thods here represented ; and we are warranted in laying down 

 the following proposition : The phenomena of winds and storms 

 are the result of a vortex or gyratory movement, generally of 

 no great extent, established in that region of the atmosphere 

 where they prevail. 



To such persons as have been much conversant with writings 

 on the subject of meteorology, no apology will be necessary for 

 the formal enunciation of this proposition, and the subjoined 

 illustrations. They must be well aware that winds ai'e generally 

 spoken of as long aerial rivers, flowing from one part of the 

 earth's surface to another, with scanty and imperfect, if there 

 are any, notices of the fact, that they oioe their existence to ano- 

 ther movement of the air at right angles to their ozcm course. 

 These obscure and erroneous views of the nature of that motion 

 of the air which constitutes winds, seem to pervade most of the 

 meteorological speculations of an individual holding a high rank 

 amongst the philosophers of the age — Mr Leslie of Edinburgh. 

 See his investigation of the cause of the oscillations of the mer- 

 cury in the barometer, and his illustrations of the Huttonian 

 theory of rain, — tJiat it is produced by the mingling of air of 

 different temperatures, charged xvith moisture, referred to by 

 riayfair (Outlines, vol. i. p. 316.) with approbation, as contain- 

 ing a correct exhibition of the rationale of falling weather. 

 " To explain the actual phenomena, we must have recourse to 

 the mutual operation of a chill and of a warm current driving 

 swiftly in opposite directions, and continually mixing and 

 changing their conterminous surfaces*." (Leslie on Heat and 



• This passage appears a second time, without any alteration of the language, 

 in the article Meteorology, drawn up by the same author, for the Supplement 

 to the Encyclopijedia Britannica, ten yeju-s after the publication of the account 

 of experiments respecting heat and moisture ; so that he seems to regard this 

 theory either as not admitting of, or not requiring, any conrection. In the 



