On the Proximate Causes of' certain Winds and Storms. 289 



which these movements in the atmosphere are generated. This 

 theory is indeed abundantly simple and familiar to philoso- 

 phers, but too much neglected by them when treating of meteo- 

 rological phenomena. Let AC, BD be two adja- 

 cent columns of air, of which AC rests upon a 

 sandy plain, 'and BD upon a forest or some other sub- 

 stance at D, less susceptible of being heated by the 

 sun's rays. Let s y, S >i, /3 ^ be corresponding strata 

 of the two columns, of equal thickness and eleva- 

 tion. The pressure on the opposite sides of the 

 plain separating the two columns at i and >, will 

 in the first instance be equal ; but the portion e of 

 the column A C being heated by its contact with 

 the hot sand at C, will be expanded so as to till 

 both t and a part, greater or less, of 5. The strata 

 of air lying immediately above, will be lifted up 

 out of their natural positions J into /3, and /3 into «. 

 The elevation will not be extended to the whole 

 column, but limited to its lower strata, it being in ^ D 

 all cases the effect of the expansion of a given portion of air, 

 to produce a condensation and displacement of the air in its 

 neighbourhood, to which the immediate effect is confined : S will 

 therefore be condensed, and at the same time lifted into the po- 

 sition /3, where, exerting in the direction of 6 the same pressure 

 as when in its original situation, this pressure will not be fully 

 counteracted by the elasticity of 6, but a part of S will flow into 6. 

 Up to this time, there could be no motion in the lower strata « 

 and y, the original pressure upon each remaining unchanged ; 

 but as soon as a part of S flows into 6, the pressure upon i being 

 diminished, and the pressure upon y increased, «, the lighter, will 

 give way, and y move in to supply its place. At the same time 

 ?, now in the position i, will descend into x. By a continuance 

 of the motion it will sink to y, pass into s, and being heated 

 there, will ascend into its original position 5. The air thus set 

 in motion, retaining the momentum it has gained, and receiving 

 a new impulse from time to time, a horizontal whirlwind, mov- 

 ing with greater or less rapidity, will be formed. A person 

 living at the foot of the columns at C and D, and having no no- 

 tice of what is going on over the earth's surface, in the direc- 



