OBSERVATIONS Otf THE WEATHER. 277 



Violent winds generally abate towards fun-fet. Strong winds 



If we admit that wind is only a current of* air put in motion a 

 by the rarefaction of the atmofphere in fome particular place, c jj ange f te m- 

 and that this current of air is moving towards the point of peratu re charges 

 rarefaction to reftore the equilibrium, we mull fuppofe, that as 

 the fun declines the rarefaction will diminifli, and confequently 

 the velocity of the wind decrcafe. But this obfervation, in 

 my opinion, rather applies to the temperate than to the torrid 

 zone; for in whirlwinds and hurricanes the contrary may very 

 often occur. 



When the wind follows the courfe of the fun, it is gene- Weather fteady 

 rally atteuded with fair weather. This frequent and regular J^JJ^^J, 

 change of wind, which is never more than a moderate breeze, 

 proves that there is no point of confiderable rarefa&ion near, 

 and therefore, the current of air follows immediately the fun's 

 courfe : it always happens in fummer, but very feldom when 

 the fun's meridian altitude is lefs than 40 degrees. 



The changes which take place in the atmofphere are prin- A ftorm follows 

 ci pally marked by the riling and falling of the barometer, a ca m ' 

 which apparently is caufed by heat and cold, the hands with 

 which nature performs her meteorological operations : by the 

 former the atmofphere is rarefied, and confequently becomes 

 light ; by the latter it is condenfed, and confequently becomes 

 heavy. Hence probably the old remark, that a ftorm gene- 

 rally follows a calm ; for during a calm the air is rarefied and becaufe the air 

 expanded, and the cold air will rufh forward in a firong cur- ^^hited™ ,S 

 rent to reftore the equilibrium, and neceflarily produce what when qufcfcealf, 

 is generally called a gale of wind, the violence of which alfo \ Q . 



P ^ J \ O ' ^ < rounding air 



will of courfe be in proportion to the degree of its preceding ru/hes in. 

 rarefaction. 



For thei'e reafons, the barometer falls fuddenly whilft the The barometer 

 air is expanded before a gale of wind, and rifes again gradu- £°" firms the 

 ally as the condenfed air returns, and the gale in like manner 

 by degrees fubfides. 



It muft how r ever be obferved, that art extraordinary fall of Summer mowers 

 the mercury will fometimes take place in fummer, previous to J n £ te ? Jj a fali 

 heavy fhowers of rain, particularly if attended with thunder 

 and lightning ; but in fpring, autumn, and winter, the fudden 

 extraordinary defcent ef the barometer indicates principally 

 violent wind. 



Upon 



