168 THE PRINCIPLES OF SOIL MANAGEMENT 



Changes of temperature affect the flow of water 

 through soils in several ways. It affects the gravita- 

 tional water directly by changing its viscosity. Warm 

 water is more limpid and flows more freely than cold 

 water, just as oil is thinned by heating. Consequently 

 soils drain more readily in summer than in winter. (See 

 also page 183.) 



Changes in temperature also affect percolation 

 indirectly through their effect on the free air in the soil, 

 and the air in the water in the soil. Air, in common 

 with all gases, expands very greatly with a small in- 

 crease in temperature, and it thus exerts a pressure 

 which may force water out of the soil into the larger 

 drainage channels. Conversely, a lowering of the temper- 

 ature contracts the air, and causes water to be sucked 

 into the soil. 



In the same way, barometric changes affect the drain- 

 age of soils. Alternate periods of low and high pressure 

 sweep over the country at intervals of a few days apart, 

 and the changes in volume of the outer air are trans- 

 mitted to the air in the soil, which expands or contracts 

 and tends to draw water into the soil, or forces it out 

 as the pressure is decreased or increased. The suctional 

 effect of winds may have a similar effect. Strong winds 

 considerably modify the air pressure, and where this 

 is brought to bear on the soil through a tile drain or 

 other underground channel it increases the flow of 

 water. 



Water does not necessarily percolate vertically into 

 the soil. It may flow off nearly horizontally, depending 

 on the character of the soil and its conditions. A hard 



