FOREST LANDS FOE THE PROTECTION OF WATERSHEDS. 77 



dew is practically unknown. The quantity deposited in the open country in 

 a single night is quite large under favorable conditions, leaving the effect on 

 shrubbery and on the ground of a considerable shower. As it gathers in greater 

 or smaller quantities on every clear, still night in the eastern sections of the 

 country, except in the colder season of the year, the total quantity must be 

 quite large. 



One authority holds that dew does not come entirely from the air, but in part 

 from the ground. It is said that water which in the daytime passes from the 

 ground and plants into the air is prevented from doing this at night, because 

 the air can not receive it, and therefore it gathers in visible form on the ground 

 and vegetation ; but if this were true, it really makes no difference in the benefit 

 which comes from the dew. Whether the low temperature due to radiation 

 causes a deposit of moisture from the air or prevents the air from absorbing 

 moisture which it otherwise would, the result, so far as the ground and vegeta- 

 tion are concerned, is practically the same. 



This may be as good a place as any to note one important characteristic of 

 precipitation, and that is its tendency to move in cycles. It is well known that 

 dry years often follow each other for long periods with great regularity, and 

 that these are succeeded by wet periods. Take the region of the upper Missis- 

 sippi reservoirs where the normal precipitation, based upon twenty-one years' ob- 

 servation, is 27.1 inches; in the ten years 18SG-1895 this normal was exceeded 

 only once; in the succeeding ten years the record fell appreciably below it only 

 once. Omitting these two years, the mean for the two periods of nine years 

 was 24.7 and 30 inches, respectively, an average yearly difference of nearly one- 

 tifth of the normal. Following the well-known law that the percentage of run- 

 off increases and diminishes with the precipitation, the disparity between the 

 run-offs for the two periods was greater still. 



This phenomenon is also admirably illustrated in the rise and fall of the levels 

 of the Great Lakes, for these immense storage reservoirs not only absorb and 

 distribute annual variations of run-off, but equalize to a large degree the varia- 

 tions from year to year. During the period of the eighties there wns a general 

 rise in the lake levels, except Superior, and many people ascribed this fact to de- 

 forestation, which allowed the water to find its way more quickly into the Lakes. 

 During the nineties there was a period of general subsidence, occasioning con- 

 siderable anxiety, and it was frequently asserted at that time that this was due 

 to deforestation, which was drying up the sreams. For some years now the 

 Lakes have been rising, Ontario being the highest in forty years; and with 

 another wet year the levels will almost reach record heights. 



The long record of the Danube floods already referred to is another example. 

 Almost invariably high floods would follow each other for several years in 

 close succession, and then would come long intervals of ordinary high waters. 



These periodic changes are not, of course, due at all to the presence or absence 

 of forests, for they occur just the same whether forest conditions remain un- 

 changed or not. It is an order of .nature not at all understood, but nevertheless 

 fully established as a fact. Just now we are in an era of high precipitation 

 and consequently of high waters. There is a disposition to " view with alarm " 

 these exaggerated conditions. Rarely does one stop to think how far better it 

 is to the country to have these wet periods, even with all their floods, than the 

 dry periods that will surely follow. A single dry year may cause more loss to 

 the country through the shrinkage of crops than the floods of an entire cycle of 

 wet years. 



Related to the subject of precipitation is that of evaporation as affecting the 

 quantity of water that remains upon the ground. Generally speaking, the sur- 

 face evaporation in summer should be greater in the open than in the forest 

 because of the more direct action of the sun and wind : but in the height of sum- 

 mer the forests arrest precipitation to such an extent in the leaves and humus 

 that more of it escapes through evaporation than in the open. The effect of 



a The author has never seen any data as to the actual quantities of dew de- 

 posited in different localities and conditions, and hopes that the discussion of 

 this paper may bring some to light. He has, however, vivid recollections on 

 the subject when, as a lad on a dairy farm, it was his unlucky lot to go bare- 

 footed after the cows every morning without waiting to see whether the sun 

 was going to shine or not. He knows from experience how near zero the dew 

 point can get. and how wet dew is; and also that the warmest place in the 

 world at such times is where a cow has lain all night, and next to that the 

 dry precincts of the tall woods. 



