214 LAND DRAINAGE 



of ponds in the depressions and streams in the ravines, and 

 the more severe the frost, the higher the shore line, all 

 of which shows that 32-degree air gravitates and displaces 

 air of higher temperature. 



286. Wooded areas and frosts. Obstructions to the 

 ready gravitational movements of air increase the tend- 

 ency to frosts, whether these are ridges of land or 

 stretches of wood. In cultivated areas, surrounded by 

 woods, frosts often occur that probably would not if the 

 timber on the lower side were removed. 



287. Drainage and surface temperature. While 

 drainage might not be expected appreciably to affect 

 the mean temperature of a region, it undoubtedly does 

 very materially affect the temperature of the surface 

 soil, by greatly reducing the loss of heat by evaporation, 

 and by lowering its specific heat ; and it is not unreason- 

 able to conclude that this all might result in lengthening 

 the period between late spring and early fall frosts. (See 

 paragraphs 56 and 57.) 



