392 



Woods and Water Supplies. 



when the dead leaves, moss, &c. containing the cocoons may be 

 raked together and destroyed. Ornamental trees in parks and 

 gardens may be speedily cleared by spraying with hellebore 

 wash or arsenate of lead. 



Relationship of Woods to Domestic Water Supplies. 



This subject has, for more than twenty years, occupied much 

 cf the attention of Forest Experimental Stations, especially in 

 Germany, France, Austria, and Switzerland, and in view ot 

 its importance the conclusions arrived at may be usefully 

 summarised. 



It has been asserted, and theoretically the contention is 

 doubtless correct, that masses of woodland increase the rainfall. 

 The causes of this result are sought for in the reduction 

 of temperature associated with forests, and in the . greater 

 absolute and relative humidity of the air in woods. But 

 although it may be possible to obtain experimental proof by 

 means of elaborate and long-continued observations in a region 

 where extensive afforestation or deforestation is taking place, it 

 may at once be said that such tree-planting as is practically 

 possible in Britain can have no appreciable influence on the 

 rainfall. Trees do, however, under certain conditions of the 

 atmosphere, condense dew on their leaves and branches, and 

 this effect may often be seen in the wet state of the ground 

 underneath trees on a foggy morning when the surface else- 

 where is comparatively dry. 



But the case is materially different where the fate of the rain 

 and snow that falls on a tract of woodland is considered. The 

 foliage, branches and stems of the trees intercept much of the 

 rain and snow, so that it never reaches the ground at all, the 

 amount so intercepted usually ranging from 30 to 45 per cent, 

 of the total, but much depends on the character of the rainfall 

 and on the species of tree. In a district of heavy annual rainfall 

 a smaller proportion of the precipitation is caught by and 

 evaporated from the trees than where the rainfall is light. 

 Similarly, in the case of heavy and long-continued rain, as con- 

 trasted with gentle showers ; in the latter case, in fact, but little 



