130 RELATIONS OF SOIL TO WATER 



heat through the soil. A rising temperature and a falling 

 barometer act in the same way ; the air in the soil expands, 

 and the water filling the interstices above the water level is 

 expelled, and causes a rise in the water level of the soil. On 

 the air again contracting, the water is reabsorbed by the soil, 

 and the water level again falls. 



King describes a blowing well in Wisconsin, in which the 

 expansion and contraction of the underground air is manifested 

 iri a surprising manner. The water level is in this case at a 

 considerable depth, and the soil between the surface and the 

 water is chiefly gravel, which from its free drainage is of course 

 largely filled with air. A falling barometer in this case pro- 

 duces so violent a draught of air out of the well as to blow 

 a man's hat off; while with a rising barometer in winter 

 time, the cold downward current is so severe as to freeze the 

 pipes at a depth of 70 feet. 



Wet and Dry Soils. The characters of dry and wet soils 

 have been already fully described. The dry soils are those 

 composed to a large extent of coarse particles, possessing a 

 free percolation, and little power of retaining water. The 

 wet soils are those composed of very fine particles, having 

 an enormous extent of internal surface, and therefore retaining 

 much water, and offering great resistance to the passage of 

 water through them : the colloid constituents of clay or peat 

 help greatly to intensify these properties. 



The relation of soils to water is often much modified by the 

 character of the subsoil. A sandy surface soil is agriculturally 

 a very different thing when it has a subsoil of loam or clay, 

 as the presence of this greatly increases the store of water at 

 the disposal of the crop. On the other hand, a clay soil is 

 no longer called wet when it has a subsoil of chalk to remove 



