THE WORK OF ROOTS 113 



encouraged by preventing fires, keeping out sheep, etc.^ 

 Such a cover not only prevents floods, but it prevents 

 the washing away of the soil on hillsides and slopes : 

 the damage to good farm land from this source may 

 assume enormous proportions and . it is exceedingly^ 

 difficult ever to remedy it or restore the land so 

 ruined. 



The water which runs off without penetrating the 

 soil is called the runoff : that which soaks into the soil 

 is known as the percolating water. 



For convenience we sometimes divide tlie soil -water into three 

 kinds : (1) free water, which flows under the influence of gravity and 

 percolates down through the soil ; (2) capillary water, which is held in 

 the capillary spaces or pores of the soil and is not influenced by gravity 

 but moves upwai'd or in any direction where the soil is becoming drier ; 

 (3) hygroscopic water, which is so firmly held as a film around each par- 

 ticle that it does not move about like the capillary water but can be re- 

 moved only by heating to the boiling point of water, when it passes off as 

 steam : the driest of "air- dry" soils contains considerable hygroscopic 

 water. 



In what kind of soil is percolation most rapid ; in 

 what kind of soil is water retained longest after a rain ? 

 Fill two chimneys as described above — one with clay, 

 the other with sand; put them in separate pans. Pour 

 an equal quantity of water into each chimney, taking 

 care that none runs over; through which does the 

 water run more rapidly I Why ? Which kind of soil 



1 Consult Roth, "First Book of Forestry," pp. 202-209: also Tunmey, 

 « Relation of Forests to Stream Flow," Year-Book of the U. S. Dept. of Agricul- 

 ture for 1903. 



