480 Vegetation m ttigb Latitudes. [Book V. 



Every habitable latitude muft enjoy a heat of 60 

 at leaft for two months in the year, in order to pro- 

 duce and bring to maturity corn and the other ve- 

 getable productions. The quicknefs with which ve- 

 getation proceeds in high latitudes is chiefly owing to 

 the long duration of the fun above the horizon during 

 their fummer. Dr. Halley, indeed, has proved, that, 

 abftracting from the intervention of fogs, mifts, and 

 mountains of ice, the hotted weather might take place, 

 even under the poles, the duration of the fun's light 

 compenfating for the obliquity of its direction. 



Among the caufes of the changes of weather in thefe 

 climates, efpecially with refpect to heat or cold, we 

 muft account the circumftance of the air being charged 

 with vapour. The air, when cloudy, is capable of 

 receiving and retaining more of the fun's heat, than 

 when clear, for the obvious reafon, that a tranfparent 

 medium permits thofe rays to pafs through it, which 

 are intercepted if the medium is thicker and lefs pel- 

 lucid. Hence a cloudy air is frequently found warmer 

 than the earth, on which it is incumbent. The air is 

 alfo warmed by the condenfation of vapour, and hence 

 the origin of hail, which is rain condenfed by patting 

 through air which is colder than that which produced it. 



A continuance, however, of cloudy or mifty weather 

 will intercept the fun's rays from reaching the earth, 

 which will therefore be prevented from receiving its 

 due portion of heat v The winter of the year 1783-4 

 was unufually fevere j and it is to be remarked, that 

 during feveral of the fummer months which preceded 

 it, where the effect of the fun's rays to heat the earth 

 fhould have been the greateft, the whole continent of 

 Europe was covered with a. kind of fog, fuppofed to 

 proceed from the fmoke of fome volcanoes, near 

 Mount Hecla, in Iceland. This fog was of a dry kind, 



and 



