GENERAL EFFECT ON EVArORATION. 139 



saturated and an additional pressure is exerted by water standing or 

 falling- on it, a further supply of water may be given up to the soil; if, 

 however, the moss is only just saturated and no further access of water 

 takes place from above, then there is no physical law by which a sur- 

 render of this saturation water to the soil could take place as long as 

 the underlying soil is of a gravelly or nonabsorbing nature. If its 

 nature is like clay, marl, flue sand, capable of attracting water, then the 

 further process of water absorption depends upon the difference be- 

 tween the water capacity of the cover and that of the soil. 



In a sand soil in which the upper strata lose their water rapidly to 

 the lower, the moss-cover, which holds water more tenaciously, can be 

 made to give up water to the soil as long as the capacity for absorption 

 by the sand is greater than the capacity for retention by the moss. 



A loam or clay soil takes up water very slowly, but takes up a 

 great deal before it is saturated, and the process of filtration goes on 

 very slowly; if, therefore, a plentiful rain falls, there is formed 

 under the moss-cover a shallow, nearly saturated layer of soil, which 

 acts as an impermeable stratum. This layer is protected by the cover 

 against rapid surface drying, and since it gives up its water only slowly 

 to the lower strata, it remains moist so long as the moss-cover is not 

 dry. As soon as by evaporation the cover has lost its water, the soil 

 must give up some of its moisture by capillary attraction to supply the 

 deficiency in the cover. It must be noted here, however, that accord- 

 ing to Oltman's experiments, moss does not take up water from an only 

 moderately moist soil. A deficiency of moisture occuring in soil earlier 

 than in the cover can be presumed only when the water is utilized by 

 the roots and transpired, which is not likely to occur. 



These are the extreme cases between which in nature many interme- 

 diary conditions occur. The litter cover does not act analogously to 

 the moss-cover or to a sponge. A difference must here be noted between 

 the newly fallen loose litter of the previous year and the closely packed 

 and felted litter accumulations of former years. The former allows a 

 rapid filtration ; the latter, according to Eiegler's experiments, is nearly 

 impermeable, and the water practically can enter the soil only where 

 the litter is intermitted. The compacted litter serves admirably to re- 

 tard evaporation. In reality there rarely exists an uninterrupted cover 

 of such litter or a cover of one uniform nature ; open spaces, moss-covers, 

 varying thicknesses of litter-cover interchange, and accordingly the 

 water penetrates readily, while the cover performs its duty as a conserv- 

 ing agent against evaporation. 



There is an additional conservative action of the forest floor to be 

 noted, which will be more fully dis(?ussed farther on, as an influence 

 upon the distribution of the run-off. It is the mechanical protection 

 which the cover affords against the compacting of the soil by the fall- 

 ing raindrops; by this protection the soil is kept porous, permits ready 

 percolation, and therefore less water remains at the surface to fall a 

 prey to evaporation. 



