55 



upon the school grounds, how much falls upon an acre of land, and 

 it will be a matter of surprise that tlie amount is so great. 



Not all the water which falls during a summer shower is carried 

 off by surface drainage, but a considerable part sinks into the soil. 

 As it passes down, each soil grain takes up a portion and surrounds 

 itself with a little lilm of water much as does a marble when dipped 

 into the water. If the rain continues long enough the soil will 

 become saturated and the water which cannot be retained will, under 

 influence of gravity, sink down to the lower layers of soil until it 

 finally reaches the level of the free water. From this free water, 

 at varying depths in the soil, wells and springs are supplied. If the 

 soil were to remain long saturated seeds would not germinate, and 

 most cultivated plants would not grow because all the air passages 



29 — a. Soil too dry. 



b. Soil in ideal condition. 



c. Soil too wet. 



of the soil are filled with water (Fig. 29). The water which sinks 

 down deep into the soil and helps supply our wells is called free 

 water. That portion which is held as a film by the soil particles 

 (as on a marble) is called capillary water. After the rain is over 

 and the sun shines, a part of the moisture which is held by the par- 

 ticles near the surface is lost by evaporation. The moisture which 

 is below tends to rise to restore the equilibrium, and thus there is 

 created a current toward the surface, and finally into the air ; and the 

 moisture which thus escapes aids in forming the next thunder storm. 

 Experiinent No. 6. — Humus enables the soil to take up and hold 

 large quantities of water. To illustrate this : Two samples of soil 

 should be obtained, one a humus or alluvial soil, rich in organic 

 matter, and the other a sandy soil. Put the two samples where they 

 will become thoroughly air dry. Procure, say, five pounds each of 



the dry soils, and put into glass tubes over one end of which there 



447 



