THE FORMATION OF DEW
The writer of the Book of Job gravely asked the important question, "Who
hath begotten the drops of dew?" We repeat the question in another form,
"Whence comes the real dew? Does it fall from the heavens above, or does
it rise from the earth beneath?"
Until about the beginning of the seventeenth century, scientific men held
the opinion of ordinary observers that dew fell from the atmosphere. But
there was then a reaction from this theory, for Nardius defined it as an
exhalation from the earth. Of course, it was well known that dew was
formed by the precipitation of the vapour of the air upon a colder body.
You can see that any day for yourself by bringing a glass of very cold
water into a warm room; the outer surface of the glass is dimmed at once
by the moisture from the air. M. Picket was puzzled when he saw that a
thermometer, suspended five feet above the ground, marked a lower
temperature on clear nights than one suspended at the height of
seventy-five feet; because it was always supposed that the cold of evening
descended from above. Again he was puzzled when he observed that a buried
thermometer read higher than one on the surface of the ground. Until
recently the greatest authority on dew was Dr. Wells, who carefully
converged all the rays of scientific light upon the subject. He came to
the conclusion that dew was condensed out of the air.
But the discovery of the true theory was left to Dr. John Aitken, F.R.S.,
a distinguished observer and a practical physicist, of whom Scotland has
reason to be proud. About twenty years ago he made the discovery, and it
is now accepted by all scientific men on the Continent as well as in Great
Britain. What first caused him to doubt Dr. Wells' theory, so universally
accepted, that dew is formed of vapour existing at the time in the air,
and to suppose that dew is mostly formed of vapour rising from the ground,
was the result of some observations made in summer on the temperature of
the soil at a small depth under the surface, and of the air over it, after
sunset and at night. He was struck with the unvarying fact that the
ground, a little below the surface, was warmer than the air over it. By
placing a thermometer among stems below the surface, he found that it
registered 18 deg. Fahr. higher than one on the surface. So long, then, as
the surface of the ground is above the dew-point (_i.e._ the temperature
when dew begins to be formed), vapour must rise from the ground; this
moist air will mingle with the air which it enters, and its moisture will
be condensed and form dew, whenever it comes in contact with a surface
cooled below the dew-point.
You can verify this by simple experiments. Take a thin, shallow, metal
tray, painted black, and place it over the ground after sunset. On dewy
nights the _inside_ of the tray is dewed, and the grass inside is wetter
than that outside. On some nights there is no dew outside the tray, and on
all nights the deposit on the inner is heavier than that on the outside.
If wool is used in the experiments, we are reminded of one of the forms of
the dewing of Gideon's fleece--the fleece was bedewed when all outside was
dry.
You therefore naturally and rightly come to the conclusion that far more
vapour rises out of the ground during the night than condenses as dew on
the grass, and that this vapour from the ground is trapped by the tray.
Much of the rising vapour is generally carried away by the passing wind,
however gentle; hence we have it condensed as dew on the roofs of houses,
and other places, where you would think that it had fallen from above. The
vapour rising under the tray is not diluted by the mixture with the drier
air which is occasioned by the passing wind; therefore, though only cooled
to the same extent as the air outside, it yields a heavier deposit of
dew.
If you place the tray on bare ground, you will find on a dewy night that
the inside of the tray is quite wet. On a dewy night you will observe that
the under part of the gravel of the road is dripping wet when the top is
dry. You will find, too, that around pieces of iron and old implements in
the field, there is a very marked increase of grass, owing to the deposit
of moisture on these articles--moisture which has been condensed by the
cold metal from the vapour-charged air, which has risen from the ground on
dewy nights.
But all doubt upon this important matter is removed by a most successful
experiment with a fine balance, which weighs to a quarter of a grain. If
vapour rises from the ground for any length of time during dewy nights,
the soil which gives off the vapour must lose weight. To test this, cut
from the lawn a piece of turf six inches square and a quarter of an inch
thick. Place this in a shallow pan, and carefully note the weight of both
turf and pan with the sensitive balance. To prevent loss by evaporation,
the weighing should be done in an open shed. Then place the pan and turf
at sunset in the open cut. Five hours afterwards remove and weigh them,
and it will be found that the turf has lost a part of its weight. The
vapour which rose from the ground during the formation of the dew accounts
for the difference of weight. This weighing-test will also succeed on bare
ground.
When dealing with hoar-frost, which is just frozen dew, we shall find
visible evidence of the rising of dew from the ground.
You know the beautiful song, "Annie Laurie," which begins with--
"Maxwelton's braes are bonnie,
Where early fa's the dew"--
well, you can no longer say that the dew "falls," for it rises from the
ground. The song, however, will be sung as sweetly as ever; for the spirit
of true poetry defies the cold letter of science.