LETTER XIII. 



THE WINDS. 



The winds are currents of tlie air exactly the 

 same in kind as those of the waters in rivers and in 

 the seas. Every disturbance of the balance between 

 neighbouring masses of air, an increase of density, 

 and consequently of pressure on the one side, or 

 a diminution of density on the other, immedi- 

 ately sets up a movement from the heavier towards 

 the lighter air ; just as water is set in motion, if 

 it suffers a greater pressure on one side than on 

 the other. The most usual and general cause of 

 such disturbances of the balance of the air is 

 unequal heating. The causes upon which the 

 draught of our lamps, the burning of the fuel 

 in our fire-places, and the airing of our rooms 

 depend, are the same as those which we find at 

 work as the moving power of the slightest breezes 

 and of the mightiest gales, which stir the restless 

 air and mingle it in every climate. 



