EFFECT OF WOODLANDS UPON EVAPORATION. 297 



tbis list may be added a portion of Soutbem Russia, wbere 150 to 200 years ago tbere 

 existed large forests, uow cbanged iuto naked plains, -wbere tbe bills are witbout water, 

 and tbe population is forced to settle in tbe valleys. We may also mention tbe Volga 

 and tbe Dnieper in Southern Russia, wbere tbe forests around their sources bave been 

 cleared to such an extent tbat in tbeir middle and lower portions, where these two 

 rivers, so important to the commerce of Russia, pass through a wholly cleared country, 

 the high water reaches points never before attained when the upper forests were stand- 

 ing. Every one knows of the changes made yearly in the beds of these rivers by these 

 'floods, and tbe consequent inconvenience and even danger which these occasion to 

 navigators. Tbe fact is also generally known tbat tbe deep gulfs, which in summer 

 and winter are without water, become wild torrents after heavy rains and tbe melting 

 of snows in spring, carrying with them acres of tbe finest soil. We believe tbat these 

 evils would have appeared in less degree if the country adjoining tbese rivers bad not 

 been cleared of its woods.' 



EVAPORATION— ABSORPTION OF SOLLS.^ 



Experiments (other than those already given) made upon the evap- 

 oration of plants, and from soil entirely naked and that covered with 

 turf, have led to the following conclusions : 



1. That tbe evaporation from naked soil (most abundant after a rain), diminishes 

 rapidly as the surface becomes dry. The turning and breaking up of the soil increases 

 and prolongs the evaporation ; but still its effect is limited to a comparatively moder- 

 ate depth. 



2. That while plants arrest tbe direct evaporation of the soil, they give rise to a 

 greater amount of loss to the air than naked soil, and especially to a more prolonged 

 evaporation, because their roots reach the water in strata from whence tbe evaporation 

 proceeds very slowly. This effect is particularly noticed in the trees that retain their 

 fresh appearance during tbe estremest droughts of our climate, because they draw their 

 supplies from as low as tbeir roots reach. 



3. It appears well established that in summer the naked soil — still more, cultivated 

 soil, especially if sandy— lose by evaporation a notably greater amount of moist- 

 ure than they receive "by the rains. For this reason tbe summer rains, and even 

 tbe first rains of autumn, do not raise the water of springs and wells, although if ex- 

 cessive they may find a surface-discbarge iuto the channel-ways quickly and in de- 

 structive amount. Such rains must first sux^ply the losses of the soil before their 

 effects are felt in tbe deeper strata from which wells and springs are fed. 



4. In autumn and winter, tbe evaporation being feeble, and vegetation dormant, or 

 at least languid, almost all of the rains are absorbed by the soil, or delivered in the 

 water-courses. This explains the influence of tbese rains upon springs and wells, 

 which are at least annually replenished, if not wholly furnished with a perennial sup- 

 ply flora tbe rains of winter. Tbe differences in amount of rains one year with 

 another appear to occasion those oscillations of level observed in great rivers and 

 lakes, wbicb extend through series of years, and are sometimes serious in their effects 

 upon navigation, and upon property along their shores. 



5. The cultivation of the soil notably increases its permeability, depending nponthe 

 character of the soil, however, for the permanence of this eft'ect. When covered with 

 crops the evaporation is much greater, but still derived from a comparatively small 

 depth, as their roots are feeble as contrasted with those of trees. So long as the grain 

 is green, the evaporation has a cooling effect, as well upon the air as the soil. This 

 effect becomes sensible in meadows covered with grass, and especially when they bave 

 been sprinkled by the rain, the evaporation and absorption of beat becomes enor- 

 mous. 



The foliage of trees evaporates less than tbat of cultivated plants, but the great ex- 

 pansion of surfaces more than compensates for tbe difference. Their action is more- 

 over more steady and prolonged. 



Under woodland shade the air and the soil are generally cool and damp. The air is 

 cool from tbe evaporation, and tbe soil damp because the superficial evaporation is fee- 

 ble, and because tbe evaporation of the leaves is drawn from the deeper supplies. An 

 evaporator placed on tbe ground, in a piece of wood, would therefore give no true idea 

 of the action going on in the forest. 



Woods, and perhaps more still, well-sprinkled lawns, have tbe tendency to render 

 our summers more cool and humid. But if they give more to the air they take more 

 from the soil, and although this coolness and humidity may tend to produce rains, it 

 is rather a restitution than a gain. Nor could tbe restitution be complete excei)t in an 

 atmosphere perfectly calm, and in the entire absence of winds. 



> MontagsUatt der St. Peteraiiirger Zeitung, No. 102, 1876. 



"Notes by H. Mari6 Davy, in the i?er«e dea Eaux ct Forets, viii (1869), p. 267. 



