66 PHILOSOPHY OF STORMS. 



have read of considerable elevations of the water, at one 

 end of the lake of Geneva, which were evidently not pro- 

 duced by the wind blowing over the surface of the lake in 

 a direction favorable to such an elevation ; if there was a 

 spout passing, near the time of the elevation, it would ac- 

 count for the phenomenon. Indeed, if the spout should 

 even pass over the middle of the lake, and the barometer 

 should fall there three inches, it would cause such a swell 

 that its reciprocations would reach its extremities after the 

 spout had passed away, and thus these swells would appear 

 to take place in the midst of a calm, and so be apparently 

 unconnected with the wind. Mr. Dalton informs us that 

 " the surface of Lake Derwent is sometimes agitated, when 

 no wind can be perceived, in so violent a manner, that it 

 exhibits large waves with white breakers. The phenome- 

 non is called a bottom wind ; but the cause of it is utterly 

 unknown." Lake Wetter, in Sweden, is affected in a simi- 

 lar manner. 1 The theory of upward vortices shows how 

 such an effect might be produced. 



91. Even as to the barometer itself, I have not seen any 

 theory which is able satisfactorily to account for its great 

 and sudden falls. It cannot be the diminished pressure 

 which takes place from the deposition of rain, for if ten 

 inches of rain were to fall so suddenly that the air would 

 not have time to rush in and restore the equilibrium, it 

 would not cause the barometer to fall one inch. 



Indeed, so great has been the difficulty on this point, that 

 the author of the art. Physical Geography, in the Edin- 

 burgh Encylopsedia, thinks these depressions are caused by 

 the destruction of large portions of the air in the higher 

 regions of the atmosphere by electricity acting on the com- 

 bustibles which ascend there from the earth. I need hardly 

 add that this phenomenon is a corollary from the theory 

 here advanced. 



1 Ed. Ency., art. Physical Geography. 



