78 G. H. KNIBBS. 



water is exhausted either by evaporation alone, or by evaporation 

 and percolation together. We consider the loss by percolation 

 first. This will occur whenever the quantity of moisture is 

 greater than what would be retained in position by surface-tension. 

 Consider a horizontal stratum of soil cut away on one side, so 

 that its water may drain off. Then for any defined conditions it 

 is possible to compute the circumstances of flow, once the constant 

 k for the soil is ascertained, provided the supply to the stratum is 

 maintained, and if in the supply the question is not complicated 

 by what I have called 'partial interstitial flow.' In actual cases 

 where a porous soil is partially exhausted by draining — for it can 

 be only 'partially exhausted in such a manner — the soil above that 

 surface which marks the boundary between the portion where the 

 interstices are filled with water and where they are not so filled, 

 probably contributes afterwards only negligible quantities to the 

 flow under the action of gravity : that is to say the greater part 

 of the water left in the soil is sensibly held in suspension by 

 surface-tension, and can be exhausted only by evaporation. 



13. Exhaustion of moisture by evaporation. — Again neglecting 

 vegetation, the external factors affecting the rate of evaporation 

 from soil are : — solar radiation, temperature, the humidity of the 

 air, and its motion. The quantitative efficiency of these are 

 modified by conditions in the soil itself, e.g., it fineness, its general 

 nature as affecting the surface tension element, the presence of 

 hygroscopic materials, of vegetation of course, and the amount of 

 water it already contains. The first four elements may be fairly 

 well measured ; but their influence in promoting drying of the 

 soil is not so readily evaluated. 



14. Solar radiation. — Solar radiation raises the temperature of 

 the surface of the soil, and a wave of heat is propogated down- 

 ward at a rate depending partly on its conductivity, and partly 

 on the possibility of energy being expended in other ways, as in 

 the evaporation of contained moisture ; the promotion of growth 

 in vegetation, and so on. The intensity of this heat-wave rapidly 

 diminishes, and finally becomes insensible within a few feet — 4 or 



