132 Hp:Ff'i{r S. A.A. Advaxckmknt of Scii:n{m:. 



distance at least ten times the height of the trees.* Water is lost nor 

 only when it juns off into a stream, but also when the winds entice it 

 awav from growing plants l)y evaporation. This increases greatlv 

 with the increase of the wind velocity, and a plantation on the wind- 

 ward side of a farm will greatly lessen the loss. Nearly all soil would 

 furnish abundant water for the growth of plant life were it not for the 

 less by evaporation. The conservation of this moisture may prove 

 more important than an increase of rainfall, and more feasible than 

 the application of water by irrigation. The value of wind-breaks tor 

 minimising the evapoi'ation fiom the soil may not appear of practical 

 interest to those who live in a humid climate : but in the arid and 

 semi-arid legions the desiccating effect of dry or hot winds is tremen- 

 dous. The author has seen tlourishing crops wilted in a few hours bv 

 hot winds, and giving every appearance of suff'ering from a prolongc*! 

 drought. It is stated that" in lyf>8, 21,000,000 bushels of corn were 

 lost in the JState of Kansas through the influence of hot, dry winds. t 

 Experiments show that, given a certain condition of soil anrl a brisk, 

 dry wind, the surface ground may lose in exceptionally favourable days 

 2, 3, and even 4 lbs. of w^ater per square f*>ot per day, eijuivalent to more 

 than 40, 60 and 80 tons per acre. These high rates of exaporation. 

 however, are not maintained for long peiiods : but losses during seven 

 days have amounted to 9-13 lbs. per' square foot, or a dail}' i-ate of 

 1-3 lbs., and in four days as high as 1'77 lbs. per square foot.:;: Sucli 

 rates of evaporation, if applied to the surface of a reservoir, would re- 

 sult in a verv serious loss of storage (see p. 117). The enormous losses 

 from e\aporation on the Karroo — estimated at about 90 ins. per annum 

 — are undoubtedly due to exposure to hot, (hying winds. Many 

 foresters are perfectly satisfied to devote their attention to tree grow- 

 ing pure ;ind simple, without consideiing flie effect 07i the district or 

 the influence on public works. The civil engineer, again, often accepts 

 existing conditions without realising how far they could be modified 

 by the work of the forester. An instructive example of the efi'ect of 

 forestry on engineering occurs at Port Elizabeth, where the whole 

 prol>lem of harbour works has been revolutionised by the afforestation 

 of the neighbouring sand dunes.§ The Department of Pouts et 

 Chaussees in France has had its expenses for maintenance and con- 

 struction of roads and bridges very materially reduced by the reboise- 

 ment of the Alps and other mountains. The extension of forests 

 woukl undoubtedly have a similar effect in lessening the losses from 

 wash-c>uts on the South African railwavs. ft is to the investigations 



* (;. I.. Clotliiei ill ^'ear Uook of Agricultnial [department (T.S.A.), If«t4, 



i». -im. 



^ See Amuial Kei.ort of UIkmU' Islaiitl \'j,r\. K\jt. Stii., I!)0(» 1. p. 2Ki. 

 Wiiidf Iiijiij-ioiis to \'r(irtfit'u»i and Crn/t.s. h\ ('. \'.. ('uv{\>. linllrliii II. 

 \\cather i'.ureaii (I'.S.A.), part ii, ISH.l. 



:J: See valualdc information in I nii/tifinii (mil Ih-aniinii , hy I'rof. !•'. II. 

 Kin-(, p. 14S. 



!? Ecclaniatioii n/ Ihift Sinnh in ('(t/n I'liloini. .\liniitf> of rroceediii.us of 

 the Institute of Civil Kngineers, \(>1. d. 



