256 



FROST IN VALLEYS. 



On hills, where the wind blows freely, it tends to 

 restore to plants the heat lost by radiation, which is 

 the reason that hills are not so liable to sharp frosts as 

 still valleys. When the air is cooled it becomes heav- 

 ier, and, rolling down the sides of valleys, forms a lake 

 of cold air at the bottom ; this adds to the Uability of 

 frosts in low places. The coldness is frequently still 

 further increased by the dark and porous nature of the 

 soil in low places radiating heat faster to the clear sky 

 than the more compact upland soil. 



A knowledge of these properties teaches us the im- 

 portance of selecting elevated places for fruit-trees, and 

 all crops liable to be cut off by frost ; and it also explains 

 the reason that the muck or peat of drained swamps is 

 more subject to frosts than other land on the same lev- 

 el. Therefore, corn and other tender crops upon such 

 porous soils must be of the earliest ripening kinds, so 

 as to escape the frosts of spring by late planting, and 

 those of autumn by early maturity. 



REMARKABLE EFFECTS OF HEAT ON WATER. 



The effects of heat and cold on water are of a very 

 interesting character. Without its expansion in freez- 

 ing, the soil would not be pulverized by the frost of 

 winter, but would be found hard, compact, and diffi- 

 cult to cultivate in spring ; without its expansion into 

 steam, the cities which are now springing up, and the 

 continents that are becoming peopled, through the in- 

 fluence of rail- ways, steam-ships, and steam manufac- 

 tures, would mostly remain unbroken forests ; without 



