STORMS. 531 



kept moist by the drip from overhanging trees. * This is 

 greatly preferable to lopping the border trees along a road- 

 side, which, besides its unsightly appearance, admits the 

 wind into the forest, and exposes the bark to sun-scorching. 



(/) All undergrowth which springs up along easterly and 

 north-easterly forest boundaries affords a natural protection 

 belt and should be carefully preserved. 



(0) Coppice should be cut from the west, south-west or 

 north-west towards the opposite bearings. In high forests 

 this is only permissible on plains which are exposed to the 

 east and for storm-firm trees such as the oak. 



SECTION II. STORMS. 

 1. Origin of Storms. 



All winds are caused by differences in atmospheric pressure 

 resulting from unequal temperatures of the air in different 

 localities. Whenever the equilibrium of the atmosphere is 

 thus disturbed, a current of wind sets in to restore it. 



A storm is a wind with a velocity of at least 20 to 25 meters 

 in a second. Its approach is accompanied by a barometric 

 minimum. A hurricane has at least a velocity of 35 meters 

 per second. 



A line joining all places having the same atmospheric pres- 

 sure, as indicated by the height of the barometric column of 

 mercury, after compensation for the elevation of the place 

 above sea-level and for temperature, is termed an isobar. 



If maps are drawn, as in the Times' weather reports, show- 

 ing the different isobars for every tenth of an inch, it will be 

 noticed that they surround tracts from which either the pres- 

 sure decreases in all directions, termed barometric maxima or 

 anticyclones ; or, from which the pressure similarly increases, 

 which are termed centres of depression, barometric minima or 

 cyclones. 



The wind always blows from the regions of high pressure 

 towards the depressions, i.e., from an anticyclonic region 

 towards a centre of depression ; it does not, however, blow in 

 a direction normal or perpendicular to the isobars, but greatly 

 inclined to this, owing to the effects of the earth's rotation, 



M M 2 



