12 



trees ainl tin- layers <>!' Immus keep together, and protect the soil against the 

 action of water, l>e>ides the crown intercepts and retains, at any rate for a time 

 a cotiMderaMe portion , ,f the water. On the whole a series of obstacles are, 



posed to tin- movement of the water, which reduce its velocity and force, or at 



anv rate divide it into numerous small channels. The beneficial effect of tree 



tioti in this reaped ean be observed in most mountain ranges, and especially 



in the Alps iYom France to Austria. Wherever, in these parts, extensive defor- 



ations have taken place, the consequence has been the gradual formation of a 

 series ,,f torrents, in all places where the surface did not consist of hard rock ; the 

 </' bria brought down has covered more and more fertile land at the base of the 

 torrents, and this evil lias grown to such an extent, that not only in France but 

 also in the other Alpine countries great efforts are now made to re-afforest the 

 denuded areas at a great outlay. When once the evil has been created immediate 

 ali'ore.vtation is not possible; it must be preceded by the construction of dams, 

 i likes, walls, etc., to steady the soil until the young forest growth has had time 

 to establish itself, and once more to lay hold of the surface soil. Forests protect 

 the soil not only in the hills, but also in low lands, wherever it consists of so- 

 called moving or shifting sand along the sea coast, as well as in the interior of 

 countries. The action in this case is due partly to their moderating the force of 

 the air currents and partly by keeping the soil together through their roots, by 

 the formation of huinus and the retention of moisture. In this way the Landes of 

 France have from a dreary waste been converted into extensive forests inter- 

 sected by cultivated fields. 



) Protection against air currents. 



Forests break or moderate the force of air currents, and in this way afford 

 protection to lands lying beyond them against cold or dry winds. Woodlands 

 afford also shelter to game and useful birds. Their importance in this respect 

 should not be overlooked ; the presence of birds, which are the great enemies of 

 injurious insects, depends often on that of woodlands. 



HYGIENIC EFFECTS OF FORESTS. 



Forests, in forming a substantial part of the vegetation of the earth, are an 

 important agency for the production of oxygen obtained by the decomposition of 

 carbon dioxide.^ Direct observations have also shown that forest air (like sea air) 

 is much richer in ozone than the air of open countries, and especially of towns. 



forests, then, produce oxygen and ozone and protect human habitations 



injurious air currents, they may exercise a beneficial effect upon the 



s of adjoining lands. Instances are not wanting where forests are said 



have given protection against the germs of malaria, but there are others where 



ire said to have had the opposite effect. As far as India is concerned, in 



the medical authorities of military cantonments ordered forests to be 



au.l in others to be cut down. Whether certain species, such as 



alyptus, really possesses the quality of frying up soil, and thus remove 



swampiness, has yet to be proved. 



No general rule can be laid clown showing whether forests are required in a 

 mtry or what percentage of the area should be so used. The forest question 



