[ 453 1 [BookV. 



CHAP. XI. 



WINDS. 



Different Opinions concerning the general Caufe of i'T'inds. Of General 

 or Trade Winds. Of Monfoons.Of Sea and Land Breezes. 

 Caiifes of thefe. Variable Winds. Storms. Hurricanes. Tbf 

 Harmattan. The Sirocco Wind. The Samiel. Moving Pillars of 

 Sand.' The Simoom* Wbifkuoinds, Waierfpcuts. Tornadoes. 



TH E opinions of philofophers have varied much 

 refpecling the caufe of winds, and many of their 

 theories are little more than mere conjectures ; but it 

 muft be confefied, that electricity and a chemical 

 knowledge of air have latterly in ibrne degree im- 

 proved our imperfect acquaintance with" thefe aerial 

 currents. 



It has been already obferved, that air is expanded 

 by heat, and its fpring confequently increafed ; and it 

 is well known alfo that its elafticity is weakened by- 

 cold or freezing mixtures. From experiments which 

 have been made for the illustration of thefe properties 

 of air, we are enabled to point out the cauies of many 

 phenomena that occur in the atmofphere. 



When a fire j is made in the open air, the rarefied 

 part of that fluid will afcend in a current, and the 

 cooler and denfer air will rulh in on all fides, in confe- 

 quence of which a 'wind is generated, and blows 

 constantly towards the fire. The wind produced in 

 this manner will be too inconfiderable to be perceived 

 at any great diftance; but the rarefactions which arife 

 from natural caufes may be fuch as to agitate pur at- 

 mofphere fufficiently to produce thofe torrents of air 



which 



