130 FOREST INFLUENCES. 



able author does not fail to observe. It is the fact that these obser- 

 vations refer to a nioiintainous region throughout. That means tliat the 

 influence which the forest areas might exercise in the distribution of 

 hailstorms is vitiated or wiped out by the more potent influence of 

 slope and elevation. It is, therefore, impossible to study the isolated 

 forest influence in such regions where other known but quantitatively 

 undetermined disturbing influences must be discounted. For such 

 studies the plain otters a more satisfactory held. The forest on hilly 

 or mountainous terrain is never effective by itself; it could, therefore, 

 here only be the question whether it plays a determinative or only a 

 subordinate part. The forest besides is not like hill and mountain, a 

 constant and evenly effective factor, since its condition and in most 

 cases its area is constantly changing. The author also recognizes the 

 fact that his material does not permit the discussion of the practically im- 

 portant question, whether and why, as is often observed, hailclouds will 

 pass by single fields or confined areas within the general route of the 

 storm. The careful observations of the route and extent of a hailstorm 

 on June G, 1891, in Thurgau, Switzerland, recorded in Meteorologische 

 Zeitschrift, 189 L, p. 403, would also show that at least in this case, in 

 mountainous regions and with extraordinarily violent hailstorms, no 

 effect of forest distribution was noticeable. The writer would recall 

 here a case of his own observation on the North German plain, where 

 the farm of one of his relatives of 5,000 acres is on three sides sur- 

 rounded by forest areas of considerable extent. The owner for a long 

 series of years has had no damage from hail, while his neighbor, only 

 one mile outside of this forest-inclosed district, reaps the benefit of his 

 hail-insurance money every other year. 



INFLUENCE OF FORESTS UPON THE DISPOSITION OF THE WATER 



SUPPLY. 



(1) ELEMENTS OF DISSIPATION. 



In analyzing the relation of forests to the conservation of the water 

 supplies, we shall examine first the factors of dissipation, or those 

 Avhich diminish the available supply. They are represented in the 

 quantity of water which is prevented by interception from reaching 

 the ground, in the quantity dissipated by evaporation, in the quan- 

 tity used by jilants in their growth, and in transpiration during the 

 process of growing. 



IntercepHon. 



The amount of rainfall and snow which is prevented by a forest 

 growth from reaching the soil varies considerably according to the 

 nature of the precipitation and the kind of trees which form the forest 

 as well as the density and age of the growth. 



A light drizzling rain of short duration may be almost entirely inter- 



