538 



PROTECTION AGAINST WINDS. 



becomes condensed owing to the confined space in wliicli it 



moves, and this increases the violence of the storm. 



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



up-hill, and for the following reasons : — 



i. The root-system of individual trees is usuall}^ 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 

 also those of the prop-roots, 

 ii. An ascending ^ind 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 



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 aljove 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 ,- ii down-hill, through c d, which 



is less than a h. Tiie 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 afTords a bad root-hold ; tliis 



explains the great amount of windfall which occurs on moors 



Fi-. 246. 



