58 



THE PRINCIPLES OF AGRICULTURE 



63o. Irrigation is of primary value, of course, in all arid coun- 

 tries; but as complete systems of land culture develop, it must be 

 employed also in countries of free rainfall in order to tide over 

 periods of drought and to enable the husbandman to control his con- 

 ditions. Irrigation will come more and more to be a truly national 

 problem. 



66rt. Capillary action, or capillarity, is due to the attraction 

 of matter for mattei . Capillary attraction is that force which 



Fig. 10. How to show that plants give 

 off moisture. 



Fig. 11. To determine how much 

 water a soil can hold. 



causes a liquid to ascend or descend or move laterally through 

 very small openings or tubes, or the interstices between fine par- 

 ticles of solid matter, or by which it is held to the surface of the 

 particles themselves. The teacher should illustrate capillarity by 

 the familiar experiment of standing tubes of glass in water. The 

 smaller the bore of the tube, the higher the water rises. The oil 

 rises in the wick by means of capillarity. The principle may be 



