GRAVITATIONAL WATER 219 



duction of cracks through its property of shrinkage as well as its 

 effect on granulation, both favoring the movement of water. 



4. Viscosity. Changes in temperature affect the viscosity or 

 mobility of water to such an extent that it moves more readily under 

 high than low temperatures. The effect of temperature on capil- 

 lary movement was shown on page 204. King found that the 

 amount of water flowing through soil at 9 degrees C. was 6.15 

 grams per minute, and at 32.5 degrees it was 10.54 grams. Briggs 

 explained this greater flow on the theory of lessened viscosity, and 

 showed that while the ratio between the flows is 1.71, the ratio 

 between the viscosities is 1.77. These correspond so closely thai 

 there is no doubt that his conclusion was right. Water will perco- 

 late through soils faster in summer than winter. Water at 32 

 degrees F. and at 70 degrees F. was alowed to flow from a milli- 

 meter opening under the same pressure in each case. Twice as 

 much water flowed out at 70 degrees as at 32 degrees. At 32 

 degrees the water did not come out in a stream, but dropped rap- 

 idly from the tube, while at 70 degrees it flowed in a steady stream. 



The viscosity is frequently affected by substances dissolved in 

 the soil water. Some substances increase while others decrease 

 viscosity, as shown on page 20(5. In the case of organic substances 

 in solution percolation may be aided by the lessened viscosity. 



5. Atmospheric Pressure. The changes in pressure of the 

 atmosphere, with its expansion and contraction accompanying the 

 "lows" and "highs," affect percolation to some extent. The 

 decrease of pressure allows the air in the soil to expand, thus forcing 

 out some of the water into the drainage channels. King- found 

 the discharge from a spring to be X per cent greater with a falling 

 than a rising barometer and a variation of 15 per cent in the flow of 

 water from a tile under similar conditions. 



(I. Shrinkage Cracks. The movement of water by percolation 

 is aided greatly by the cracks that arc produced in clayev soils hv 

 shrinkage during periods of drouth. These cracks 'do not fully close 

 upon subsequent wetting and may thus leave passageways for 

 water. This is very important in heavy soils. The burrows of 

 animals, especially insects and earthworms, penetrate the soil in 

 all directions and furnish a ready means for movement of water 

 both laterally and vertically. The greatest amount of work done 

 by earthworms is in heavy soils where percolation is naturallv 

 slowest. These animals are not abundant in acid soils and those 



