268 FORESTS, RESERVOIRS, AND STREAM FLOW 
favorable conditions, leaving the effect on shrubbery and on the ground 
of a considerable shower. As it gathers in greater or smaller quanti- 
ties 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 day- 
time passes from the ground and plants into the air, is prevented from 
doing this at night, because the air cannot 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 vegetation 
are concerned, is practically the same. 
This may be as good a place as any to note one important charac- 
teristic 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 Mississippi reservoirs where the 
normal precipitation, based upon 21 years’ observation, is 27.1 in.; in 
the ten years (1886-1895), 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 9 years 
was 24.7 and 30.0 in., respectively, an average yearly difference of 
nearly one-fifth 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 variations from year to year. During the period of 
the Eighties, there was a general rise in the lake levels except Superior, 
*The writer 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 barefooted 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. 
