34-3 



'On the change of CLIMATE 



I 



countries the tops of mountains are always covered with 

 fnow. Whoever will carry a thermometer on a very warm 

 day to the top of an high fteeplc, will find that the mer- 

 cury immediately falls feveral degrees, and rifes again as 

 he defcends. From this it is obvious that nothing is want- 

 ing in the midft of fummer to render the country agreea- 

 bly cool, but a proper mixture of the cold air which is 

 above, with the warm air below. This would be effeded 

 by any caufe that might increafe our fummer winds. For 

 though the fimple motion of the air does not by any means 

 produce cold, yet, moderate blafls will naturally introduce 

 a colder atmofphere, efpecially when they pafs over hills 

 or any unequal furface, by which the equilibrium of the 

 atmofphere is deftroyed, the cold air always tending to- 

 wards the furface. Hence a fummer's guft is generally 

 attended by a fudden change in the temperature of the air. 

 Tall timber greatly impedes the circulation of the air, for 

 it retards the motion of that part which is near the furface, 

 and which, from its denfity and fituation being moft heat- 

 ed, becomes the general origin of fuch agitations as take 

 place in the upper regions. Wc fhall often find it ex- 

 tremely fultry and warm in a fmall field, furrounded by 

 tall woods, when no fuch inconvenipncy is perceived on 

 an extenfive clear plain in the neighbourhood. From thefe 



particulars wc may conclude, that when this country fliall 



be diverfified, as it muft be in a feries of years, by vaft 

 trads of clear land, intcrfe£led here and there by great 

 ridges of uncultivated mountains, a much greater degree 

 of heat being refleded by the plains than from the neigh- 

 bouring mountains, and an eafy circulation of air produced 

 on the plains; our land winds in fummer, to fay nothing 

 of thofe which come from the fea, or from the lakes, muft 

 certainly be much frefhcr and more frequent than they 

 now are, and confequently our fummer heats be more 



temperate. 



A confiderable change In the temperature of our feafons 

 may doubtlcfs effcd a change in the produce of our lands. 



Temperate 



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