Proximate Causes of certain Winds and Storms^ '259 



(n.) It is stated by Veicht in the Philosophical Transactions for 

 1764, that in " Bencoolen road, on the S. W. side of the island of 

 Sumatra, as well as in the strait of Malacca, you have periodical 

 winds, which blow for six months of the year from the same quarter 

 of the horizon, and the other six months from the opposite quarter ; 

 and it is observable that these thunder showers and squalls of wind 

 usually come contrary to these stated winds, which are calmed dur- 

 ing the thunder, but return to their constant quarter as soon as the 

 thunder and rain are past." Also by Shotte in the Transactions for 

 1 780, that at the mouth of the Senegal River, during the rainy or 

 sickly season, which begins about the middle of July, and ends about 

 the middle of October, " the wind is generally between the points of 

 east and south, the quarter from ivhich the tornados come.^'' It ap- 

 pears also from Major Denham's account of the rainy season at Kou- 

 ka, in Bornou, that in that country the thunderstorms are generally 

 from the north-east and south-east. These are exceptions to our 

 general doctrines, produced by local causes, such as are perpetually 

 occurring in every part of the science of meteorology. 



Of the prevalent movement of the air in winds and storms. 



It may be of use, before proceeding to account for the general 

 facts stated in the commencement of this paper, to turn our attention 

 to the general theory of winds, and the causes by which these move- 

 ments 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 meteorological phenomena. Let AC, BD be two 

 adjacent columns of air, of which AC rests upon a 

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

 face at D, less susceptible of being heated by the 

 sun's rays. Let sv, oX, pL be corresponding strata 

 of the two columns of equal thickness and elevation. 

 The pressures on the opposite sides of the plane sep- 

 arating the two columns at s and v, will in the first in- 

 stance be equal, but the portion s of the column AC 

 being heated by its contact with the hot sand at C, 

 will be expanded so as to fill both s and a part greater 

 or less of o. The strata of air lying immediately 

 above, will be lifted up out of their natural positions o 



D 



