208 



IMPOETAE'CE OF S]!^OW. 



elevated ridges, wMch. supply the natural irrigation of the soil 

 and feed the perennial fountains and streams, the ground remains 

 covered with snow during the winter ; for the trees protect the 

 snow from blowing from the general surface into the depressions, 

 and new accessions are received before the covering deposited by 

 the first fall is melted. Snow is of a color unfavorable for radia- 

 tion, but, even when it is of considerable thickness, it is not 

 wholly impervious to the rays of the sun, and for this reason, 

 as well as from the warmth of lower strata, the frozen crust 

 of the soil, if one has been formed, is soon thawed, and does not 

 again fall below the freezing-point during the winter.* 



thorougMy moisten the soil, and it is a common saying in the North that " the 

 ground will not freeze till the swamps are full." 



* Dr. Williams, of Vermont, made some observations on the comparative 

 temperature of the soil in open and in wooded ground in the years 1789 and 

 1791, but they generally belonged to the warmer months, and I do not know 

 that any extensive series of comparisons between the temperature of the 

 ground in the woods and in the fields has been attempted in America. Dr. 

 Williams's thermometer was sunk to the depth of ten inches, and gave the fol- 

 lowing results : 



Time. 



Temperature 



of ground in 



pasture. 



Temperature 



of ground in 



woods. 



Difference. 



May 23 



" 28 



June 15 



" 27 



July 16 



" 30 



Aug. 15 



" 31 



Sept, 15 



Oct. 1 



" 15 



Nov. 1 



" 16 



52 



57 



64 



62 



62 



651 



68 



59i 



59i 



m 



49 

 43 



m 



46 



48 



51 



51 



51 



55i 



58 



55 



55 



55 



49 



43 



43i 



6 



9 

 13 

 11 



11 

 10 

 10 



4i 



4i 



4i 















On the 14th of January, 1791, in a winter remarkable for its extreme 

 severity, he found the ground, on a plain open field where the snow had been 

 blown away, frozen to the depth of three feet and five inches ; in the woods 

 where the snow was three feet deep, and where the soil had frozen to the 

 depth of six inches before the snow fell, the thermometer, at six inches below 

 the surface of the ground, stood at 39°. In consequence of the covering of 



