44 INDIRECT UTILITY OF FORESTS. 



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 humus and 

 the retention of moisture. In this way, the Landes of France 

 have from a dreary waste been converted into extensive 

 forests intersected by cultivated fields and vineyards. 



c. Protection against Avakmches. 



Although most avalanches in the higher hills originate 

 above the upper limit of tree growth, there are many cases 

 where the presence of a well-preserved forest protects towns 

 and villages lying below them by preventing the formation of 

 avalanches, or by stopping their forward movement and 

 increase whilst they are still small. Hence many forests 

 in the Alps are maintained as a protection against avalanches. 



d. Protection cuiainst 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. Whether such effect is beneficial or other- 

 wise, depends on the geographical position, the local climate 

 and the season of the year. Woodlands afford also shelter to 

 cattle, game and useful birds. Their importance in this 

 respect should not be overlooked : forest grazing is frequently 

 of greater value than grazing on open ground ; the presence 

 of birds, which are the great enemies of injurious insects, 

 depends often on that of woodlands. 



4. Hygienic Pjfect 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. 



