iS 9 9- 



AMERICAN FORESTRY ASSOCIATION, 



! 49 



through underbrush on a mistv mornino- 

 will carry conviction to the most sceptical. 

 A notable instance of the economic value 

 of this feature of vegetation is to be seen 

 at Ascension a place that I visited in 



l8 79-. 



It is a volcanic island lving a few de- 



grees south of the equator and about mid- 

 way between Africa and South America. 

 It has an area of 30 to 40 square miles and 

 is used as a naval station by the British 

 Government. The water supply is ob- 

 tained from near the summit of Green 

 Mountain, so named from the fact of its be- 

 ing about the only green spot visible on the 

 island. The summit elevation is about 

 2,Soo feet above the sea, and its green cap 

 of vegetation is maintained by the almost 

 constant drip from trees and rocks of the 

 moisture mechanically collected from 

 clouds and fogs, eked out by light passing 

 showers. 



That the drip from trees should play so 

 prominent a part in a domestic water 

 supply is very satisfactory testimony to the 

 efficiency of woods in mechanically in- 

 creasing precipitation. Interesting experi- 

 ments have been made as to the amount 

 of condensation of aqueous vapor by 

 leaves, but it does not appear that the 

 velocity, if any, of the surrounding air 

 was taken into account. 



Cloud or fog is a manifestation of water 

 in suspension, and it is obvious that the 

 more rapidly the cloud is moved against 

 any surface the more water will be brought 

 into contact with that surface in a given 

 time and the more it will collect. Unless 

 condensation tests are conducted with 

 reference to air velocity they will not fur- 

 nish complete values. As before said, it 

 is questionable whether a numerical value 

 can ever be satisfactorily established for 

 the action of forests in a direct increase of 

 rainfall, but it is without question that 

 their effect is in that direction the point 

 of uncertainty being one of quantity only. 



In the matter of conserving of the water 

 that has fallen, forests are important fac- 

 tors. They intercept the sun and rain, 

 saving the earth from packing hard under 

 the baking of the one and the persistent 

 beating of the other. They appreciably 



decrease the quantity that would other- 

 wise pass rapidly off into the runs and 

 waterways and be lost in floods. Not 

 only do they lessen the wasteful and de- 

 structive expenditure of water in floods, 

 but they afford greater time for the earth 

 to absorb to its full capacity the water held 

 back by the mechanical obstructions of the 

 forest floor. They also reduce the quan- 

 tity lost by evaporation. 



These things we enter on the credit side 

 of the forest account with water supply, 

 and on the debit side make the sole entry 

 of the water used in supporting plant life. 

 It remains to ascribe values to these various 

 items and strike a balance. 



All permanent water-supplies are drawn 

 directly or indirectly from the rainfall ab- 

 sorbed and stored within the earth. Di- 

 rectly by the means of wells, tunnels, in- 

 filtration galleries and similar structures ; 

 indirectly through the medium of running 

 surface streams, which in turn draw their 

 supply from visible springs and the unseen 

 accretions that enter along their beds from 

 groundwater at high elevation. 



The surface water which flows into the 

 streams after rains gives but a temporary 

 and passing supply. The permanent flow 

 comes from ground storage. It must not 

 be thought from this that all ground-water 

 reappears at some time or other in the sur- 

 face streams. Much passes on unseen to 

 the sea. Its place of discharge into the 

 ocean is at times well marked. 



Off the east coast of England there is a 

 sub-marine valley, called the Silver Tit, 

 20 miles long by from 50 to 250 feet deep 

 below the general surface of the adjacent 

 ocean bed. The extraordinary depth pre- 

 cludes it being due to currents, and from the 

 circumstance of the depression occupying, 

 as it were, the focus of the concave chalk 

 formation of eastern England it is held to 

 he the place at which the inland ground- 

 waters are discharged through the chalk. 



Coming right home we have a discharge 

 of oil into a sub-marine valley of great 

 depth off Redondo, indicating a probable 

 discharge from the inland oil tie Ids. The 

 direction of these under-ground flows is at 

 times difficult to trace. Latham, the noted 

 sanitary engineer, in some of his sewage 



