The Anderson intelligencer. [volume] (Anderson Court House, S.C.) 1860-1914, 1881-01-20
Dew.

the functions ofthe aqueous varor ofHthe
atmosphere. We explained its Office in
checking evaporation fror, animal and
vegetable organisms; in forming rain,
the food, and the vehicle Of the food, Of
plants; in forming snOw. pature's blank
ct for The protection Of win,eri vegeta-
ton; and we showed also how water
freezing in the soil prevents compactness
and agglutination Of the soil, and so rem
ders the land light, friable, and mellow,
when stirred by the plow 10 spring. = ~
We notice (5) the Junction Of aqueous
vapor in the formation OF dew. In dis.
cussng this subject, we must strip it Of
its poetical features. We are not to con-
slider the dew.drops as the heeting dia-
monds with which nature bedecEs even
the grass Of the fields, nor as the tears
which plying. angels shed. upon the sor,
rowing earth. The subject may be so
treated, however, that it will gain in true
dignity and importance. :. ~
Acyhady cen sEeke desy at pleasure,
and just as nature makes IT. When we
put ice and water into pitcher, the
outer surface Of the vessels j covered at
once with drops of water, or dew. There
is always aqueous vapor in the atm0s-
phere, and aqueous vapor is water with
certain amount Of latent heat in it.
When this heat is taken away, the vbpor
becomes water again. The chilled sur
face ofthe pitcher robbing the aqueous
vapor of its latent heat, converts the va
por into water. Now, the earth aud all
objects III its surface are areaiwaysrndiating.
or tho ling off, heat into free space.
During the day the heat received from
the sun is greater thah the abloilnt lost
by radiation. Hence, there is no reduc-
tion, but rather an elevation, Of tempera-
ture. But when the sun sets, the earth
continues to lose heat, and receives none.
It's temperature, therefore, is reduced,
and the aqueous yapor Of the atmosphere,
oming in contact with the chilled earth,
loses its heat, and is condensed and de-
posted rs dew.
Let us notice some of the circumstances
that modify the deposit OF dew. =,
~ The greater the amount Of aqueous
vapor in the atmosphere, the fIreateS,
other things being equal, will be the de.
post. When the atmosphere is absolute.
ly saturated with aqueous vapor, that is
holds us much as possible for its temper.
aturc. the slightest reduction of tempera-
ture is attended with the formation oi
dew, aud If the reduction Is considerable
the deposit of dew 10 correspondingly CO.
pious. Hence, not unfrequently, the
grass becomes wet even before tho sun
has set. Hence, also, We find the grass
in the morning, when the atmosphere Is
moist, perfectly drenched with dew, pro.
vided the preceding night has been
clear and still. On the other hand, in
drought, the grass isfrequently 90 dry in
the early morning thnt we may walk in
it at will without dimming tho polish OF
our boots.
~ Another modifying circumstance IL
the clearness of the heavens. Clouds
reflect back the radiant heat and prevent
the necessary reduction of Temperature
The most oppressive nights in summer
are generally cloudy nights. The radiant
heat Is not thrown of and lost in free
spacc, 5a; il :!!;;rawr hack 70 the earth by
the clouds, and hence the sultry, sufo
eating Feeling we experience on clouds
summer nights. Tnke certain weigh,
ofwool and lay it on the brass ut sunset
and stretch three or four feet aLove 1t on
uprghts thin piece of cloth. Place
near by on the grass, but unprotected
an equal weight of wool, and on each
parcel put n thermometer. On examina
tion the next morning at sunrise, the
thermometer lying on the unprotected
wool will be lower and that parcel will
a!so weigh more than the other. Why;
One explanation covers both facts. The
unprotected wool radiated, without OBJ.
structon, its heat into free space. Hence
Its temperature was mo-e reduced and
the deposit of dew, increasing its weight
was accordingly greater. Hence the
practice of gardeners in stretching loose
mating, or even extending q board
over tend vegetation when fros
is feared. Like the clouds, tho con.
trivances reflect back the ridant heat
aud less reduction Of temperature is thi
re ,ult.
(3) For the deposit ofdew to be copious
the atmosphere must be still. Why
Consider a meadow thickly set In
grass. The grass is radiating its
heat, as rapidly becoming chilled.
If the atmosphere be Rtill, II
will also become chilled by contact with
the cold grass, and its vapor will be con
densed aud deposited. But when the at.
mosphere is in motion, the same stratum
Of air does not brood over the Geld grnss
long enough to have It's temperature
sufficiently reduced to condense its vopor.
It Is for these reasone, that the, farmer,
particularly the tobacco grower, when
tiicutcuCu NY lux - -WuICE ~ Out frozen
dew-fears tho still, clear night. At the
close of an anxious day, his fears
vanish II clouds appear, or. the sysiud
rises. We summarize these points
by saying, that for ccpiuus depasits of
dew, the atmosphere must be humid,
still and serene.
(4) There is another matter connected
with the formation of dew, which Is we'll
calculated to arrest attention. All OBJ.
jccts possess different radiating powers,
and hence lose heat at night with differ.
ent rates of facility. Living vegetation
radiates haat far more freely than dead
vegetation or any form of organic matter.
Consequently when the supply of dew Ia
sparse, living vegetation, which needs it
gets all or nearly NIL dead matter. gets
none cr nearly none. Heuc6 as all is
discoverable on the gravel walks, ON the
plank fences, or ricks Of straw. Nature,
though prodigal of her gifts, IL not the
less frugal. When the supply Of dew is
moderate, she bestowsit exclusively on
objects that need itslife,giving powor, aud
gives it to the objects that need it less
only when the supply is large and she
can afford to be lavish. What wonder.
fu wisdom 10 that which has endowed
the different forms ofamattes with the
power vi IWIIusIng winter from the inviiaii.
bleatmosphere in exact proportion to the 3
several wants? Can it be other than
divine? Somebody else, besides the
undevout astronomer, Is mad.
According to the foregoing, plants
must be chilled in order to obtain the do.