THE SOIL WATER 91 



lower level, are the simplest drains, but they are troublesome. 

 It is better to fill the bottoms with something to conduct 

 the water away, and then to fill in the ditches. Large pipes 

 of hard-baked clay, specially made, called drain tiles, and 

 laid with a slight slope, are the best things for this purpose. 



All this has to do with the movement of water downward 

 in the soil. But curiously, water moves upward as well. 

 To show this, let us fill four pots with our four different kinds 

 of dry soil, varying from sandy to clayey. Set the pots in a 

 pan, and in it pour an inch or more of water. Naturally 

 some of the water makes its way at once inside the pots, and 

 stands at the level of the water outside. But look at the 

 pots after half an hour. The water in the pan has lowered, 

 while at the same time it has risen in the pots, till before very 

 long we see it glistening at the surface of all of them. 



This upward movement of water is called "capillary 

 action," because it was first discovered in the case of fine 

 tubes, such as the hair-like (or capillary) tube of a ther- 

 mometer. Set a broken thermometer tube, containing 

 no mercury, and open at both ends, in water, and the water 

 in the tube will climb above the surface of the water outside. 

 Or lay two plates of glass, one above the other but kept from 

 touching by a needle laid between them, slanting into water. 

 The water will climb out between them, and will climb 

 highest where the plates lie closest. 



Capillary action is useful in lamps. We can see how a 

 wick works, if we take a tumbler half full of water, and from 

 it hang a wet wick over into an empty one beside it. The 

 water will continually climb the wick and drip into the empty 

 tumbler. In the same way the wick of a lamp continually 

 feeds oil upward to the flame. And in the soil, the particles 

 that touch act as wicks or tubes to lead the water upward 

 by capillarity. 



