FROST IN VALLEYS. 279 



a sheet of thin muslin, stretched on pegs over garden 

 vegetables, has afforded sufficient protection, when those 

 around were destroyed. 



FEOST IN VALLEYS. 



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

 store 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 heavier, and, 

 rolling down the sides of- valleys, forms a lake of cold air 

 at the bottom ; this adds to the liability of frosts in low 

 places. The coldness is frequently still fuither increased 

 by the dark and porous nature of the soil in low places 

 radiating heat faster to the clear sky than the more com- 

 pact upland soil. 



A knowledge of these properties teaches us the import- 

 ance 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 level. 

 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. 



REMAKKABLE EFFECTS OF HEAT ON WATER. 



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

 teresting character. Without its expansion in freezing, 

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

 but would be found hard, compact, and difficult to culti- 

 vate in spring ; without its expansion into steam, the 

 cities which are now springing up, and the continents that 

 are becoming peopled, through the influence of rail-ways, 

 steam-ships, and steam manufactures, would mostly re- 



