230 Dr FoiTy on the Climate of the United States. 



semble that of the sea coast is very apparent ; but that the 

 region intermediate, or the one beyond, ever maintained such 

 a relation, is an assumption conti'ary to the laws of nature. 



Dense forests and all growing vegetables, doubtless, tend 

 considerably to diminish the temperature of summer, by af- 

 fording evaporation from the surface of their leaves, and pre- 

 venting the colorific rays from reaching the ground. It is 

 a fact equally well known, that snow lies longer in forests 

 than on plains, because, in the former locality, it is less ex- 

 posed to the action of the sun ; and hence, the winters, in 

 former years, may have been longer and more uniform. As 

 the clearing away of the forest causes the waters to evapo- 

 rate, and the soil to become dry, some increase in the mean 

 summer temperature, diametrically contrary to the opinion 

 of Jefferson, Lyell, and others, necessarily follows. It is re- 

 marked by Umfreville that, at Hudson's Bay, the ground in 

 open places thaws to the depth of four feet, and in the woods 

 to the depth only of two. Moi*eover, it has been determined 

 by thermometrical experiments, that the temperature of the 

 forest, at the distance of twelve inches below the surface of 

 the earth, is, compared with the adjacent open field, at least 

 10° lower during the summer months ; while no diflFerence 

 is observable during the season of winter. 



" The mere effect of cultivation," as stated by a writer on 

 Physical Geography, " can never be very considerable in 

 changing a climate ;'' but although cultivation of the soil may 

 not be productive of a sensible change in the mean annual 

 temperature, yet such a modification in the distribution of 

 heat among the seasons may be produced, as will greatly in- 

 fluence vegetation. 



The unqualified decision of this question, as exhibited by 

 Lyell, in the following quotation from his " Principles of 

 Geology," is unsustained by any well observed facts : — " In 

 the United States of North America, it is unquestionable that 

 the rapid clearing (f the country has rendered the winters 

 less severe, and the summers less hot ; in other words, the 

 extreme temperatures of January and July have been ob- 

 served from year to year, to approach nearer to each other. 

 Whether in this case, or in France, the mean temperature 



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