iO{. 



The theory of the auto-irrigator has been de- 

 scribed in the papers of Livingston and of Hawkins cited 

 above. In brief, it is as follows. It will be reraam- 

 bered that water is held in moist but unsaturated soils 

 in the from of a system of water-films in the capillary 

 spaces between the soil grains. 1 Because of surface 

 tension on the curved surfaces of these films the water 

 in this film system is always under a hydrostatic ten- 

 sion. The individual films tend to thicken and the 

 film-system as a whole tends to draw more water into 

 the soil. It is this capillary tension which draws 

 water upward into a soil against gravity. The actual 

 amount of the tension depends on the nature of the 

 soil, being greater in soils composed of finer parti- 

 cles; and also on the wetness of the soil, being greater 

 when the soil is relatively dry which means that the 

 films are thinner and more curved. Accordingly water 

 is drawn by capillarity from wetter portions of 

 a soil to dryer portions, or, between two soils, from 

 the soil of coarser particles to that of finer; always 

 provided, of course, that the capillary film-system is 

 in contact throughout, ./hen the porous clay cup of the 

 auto-irrigator is placed in a soil the water in the cup 

 penetrates the porous clay of the cup and establishes 



1. oee pages (?2,t> r US LJ.m il ') , above. 



