538 



PROTECTION AGAINST WINDS. 



becomes condensed owing to the confined space in which it 

 moves, and this increases the violence of the storm. 



It is found that storms do more damage down-hill than 

 up-hill, and for the following reasons : 



i. The root-system of individual trees is usually weaker 

 down-hill than up-hill. Not only the anchor-roots, but also 

 the prop-roots are more oblique in the soil to the wind- 

 direction on the easterly 

 aspects, i.e., away from the 

 wind. The descending storm 

 therefore finds only a feeble 

 resistance in the roots ; the 

 wind blowing up-hill has to 

 overcome not only the resis- 

 tance of the anchor-roots, but 

 a * so those of the prop-roots, 

 ii. An ascending wind has 

 to contend with the friction 

 of the tree-crowns, as well, 

 as of the soil, which reduces 

 its force. Moreover, the 

 prop-roots of the trees on 

 the western slopes, exposed 

 to the wind-direction, are 

 more vertically inclined to 

 the latter, and therefore 



Fig. 246. 



offer a stronger resistance, than on the easterly slopes. 



iii. The centre of gravity of a tree has to be raised less 

 when blown over from above than from below, as. the annexed 

 diagram shows : 



Here s is the centre of gravity of the tree, and w, wi, two 

 roots, and if the tree is to be thrown up-hill by the wind, s 

 must be raised through a b ; if down-hill, through c d, which 

 is less than a b. The greater the gradient of the slope, the 

 more endangered is the tree. 



The nature of the soil exercises an unmistakeable influence 

 on the extent of the damage done by storms. 



Shallow, loose, spongy soil affords a bad root-hold ; this 

 explains the great amount of windfall which occurs on moors 



