454 WATER SUPPLY AND SEWAGE DISPOSAL 



Throughout the thousands, even millions of years, which have 

 passed since the beginning of this earth, the water which has 

 been falling in the form of rain has been soaking down through 

 these layers of soil till the earth's crust in most places is quite 

 saturated. This water is called the GROUND-WATER. 



Over much of the earth's surface, then, the rainfall has been, 

 and is, sufficient nearly to saturate the soil with water. This 

 cloes not mean that the ground is completely filled with water 

 from bed rock quite to the surface all the year around. But it 

 does mean that the GROUND-WATER has sunk deeper into the 

 earth than man has as yet been able to penetrate, and that 

 over much of the earth's area it comes nearly to the surface of 

 the soil. When rains are frequent and heavy, the ground may 

 be completely saturated even to the surface. During most 

 of the months of the year, however, the spaces between the soil 

 particles are not filled with water for some distance from the 

 surface downward. If we penetrate the ground deeply enough, 

 however, we come to a place where there is so much water in 

 the soil that it fills the spaces between soil particles com- 

 pletely. This leads us to the point where we must state defi- 

 nitely the meaning of a new term, WATER PLANE, or WATER 

 TABLE. 



532. The Water Table, or Water Plane. While all of the 

 soil is more or less moist, the moisture in the upper portions of 

 the soil usually is not free, unabsorbed water. It is water 

 which adheres closely to the soil particles and cannot be re- 

 moved by ordinary means. Of this moisture, FILM WATER, 

 we shall learn more, later. At present we are interested in the 

 portion of GROUND-WATER which is unabsorbed. This water 

 does not adhere as moisture to the soil particles, but lies as free, 

 unabsorbed water between the soil particles. It is the surface 

 of this free, unabsorbed ground-water which is catted the WATER 



TABLE, Or WATER PLANE. 



533. Relation of the Water Table to the General Surface 

 of the Land. It has been determined by experiment that the 

 water table follows, in the main, the general surface of the land. 



