EXrERIMEXT STATION BULLETINS 135 



The influence of forests in diminishing evaporation by restraining tlie velocity 

 of tlie v>-incl at the surface of the ground is worthy of consideration. I do not 

 refer to the well-lvnowu influence of tall forests in restraining the violence of the 

 wind as a body, but to the influence of forests, shrubs and vegetable growth of 

 every kind in keeping the air quiet at the ground level, where alone soil evaporation 

 takes place. 



To obtain some measure of the relative rapidity of evaporation in a draft and in 

 comjiarativi'ly still air. the following experiment was tried: The air in one part of 

 the chemical lecture room was so nearly still that a feather would not move 

 perceptibly; by opening windows in an other part of the room a strong draft could 

 be secured at one window, the wind blowing 12 miles an hour. Two square pieces 

 of Turkish toweling of same size were thorouglily wet, just short of dripping, then 

 weighed separately, one suspended in the still air of the room for an hour, while 

 the other was similarly hung in the draft by the window, or in the mouth of the 

 ventilating shaft. Both were left for an hour, then weighed again, and the loss in 

 weight showed the amount of water evaporated in each case. The trial was made 

 seven times, with the result that the evaporation was four times greater In the draft 

 than in the still air. The actual amount evaporated was not the same for each 

 liour, but the ratio of evaporation was almost identical in the whole series, viz: 

 four times as much in the draft as took place in the still air. 



The reason for this is not hard to find. A volume of perfectly still air surround- 

 ing a wet body will take up moisture with progressive slowness till the air is 

 saturated; but if this damp air is blown away and replaced by relatively dry air, 

 evaporation will go on with increased rapidity, and if the air is constantly renewed 

 as in a draft of wind, the evaporation will be more rapid. Every washer-woman 

 knows that the clothes will soon blow dry when hung out in a stiff breeze, but will 

 take hours if lined up indoors. 



The principle of my laboratory experiment will hold in the broad open of Nature. 

 The air in contact with the moist ground will take up water till it is saturated and 

 then evaporation will be suspended, provided the air remains still and undisturbed. 

 But if this bottom air is swept away by wind, evaporation will be renewed and the 

 drying of the soil will again go on. The influence of trees, shrubs and even the 

 grasses in preserving in some degree this shallow pool of quiet air at the ground 

 level and thus diminishing evaporation from the soil may seem a trifling matter at 

 first thought, but become of great moment ou the large scale of nature. 



RELATION OF SOIL TO MOISTURE. 



The capacity of a soil to imbibe and retain water is intimately connected with 

 its composition. The open and porous soils of a sandy character take up water 

 most readily, retain it in least quantity and part Avith it most rapidly by evapora- 

 tion. One hundred parts of pure sand will take up 25 parts of water, and will lose 

 22 parts by evaporation in four hours when spread out in a thin sheet in dry air. 

 Such soil is well called leachy and very subject to drought. 



If to such sand, clay, oxide of iron and marl, carbonate of lime, are added, the 

 power of such mixed soil to retain water is much improved, but the material that 

 has the greatest influence, and the one most within our control is humus or the 

 organic matter of soil. An experiment showed this clearly. A glass funnel was 

 used holding a very small filter paper in its throat (and thoroughly wet) to pre- 

 vent the sand flowing through. Into this funnel 100 grains of dry sandy soil from 

 Grayling were placed, and water poured over the sand till completely wet, and the 

 excess of water drained off till dripping had ceased. This soil contained 95 per cent 

 of sand and 2% per cent humus. It took up and retained by capillary attraction 

 29 per cent of water. Left undisturbed for three days in the shade it lost 27.7 parts 

 of water, retaining only 1.3 parts out of the 29. 



A soil was prepared by mixing two parts of Grayling sand with one part of 

 Kalamazoo muck (containing Go.7G per cent of humus). The materials were 

 thoroughly mixed and sifted and 100 grains of the mixed soils were placed in a 

 glass funnel, the same as in former experiment, and water poured over it till 

 saturated, when it was found that 100 parts of this soil took up and retained 65 

 ))arts of water. On leaving it exposed for three days in the same way, the soil lost 

 45 parts of water, but still i-etained 20 parts of the original 05. In other words, the 

 addition of 21 per cent of humus to a sand containing 2^4 per cent of organic 

 matter, more than doubled its capacity to take up water by capillarity, and in- 

 creased by more than 15 fold its power to resist evaporation. 



