The aegis & intelligencer. [volume] (Bel Air, Md.) 1864-1923, 1876-02-25

below the horizon, the darkened portion
of the earth's surface begins to lose its heat,
to radiate it into this cold space every
object gives out its heat and grows cold.
The air, surcharged with moisture, rests
against cooling spear of grass, the mois-
ture is condensed upon the grass and APP
pears upon it, first in intinitesimal drops,
which, as the whole surface becomes wet,
run together and form large drops This
water in no way differs from that which
gathers on the surface of cool vessels, and
its method of deposition is perfectly the
same. 1t IS distilled" water raised into
| the air by evaporation, and under very IA
yorable circumstances is nearly pure, but no
| doubt often contains organisms that were
lloatlng IN the air, miasmatic particles, and
| other impurities, but may be regarded as
purer than rain water which falling through
a large field of air, washes away from it
great deal OF dust, smoke particles, organ
lsms and carbonic acid. The dew which
forms on the windows of crowded room
would certainly be very doubtful test for
purity, as it is the moisture breathed out
from the lungs, and contains the waste
particles so thrown off, besides being sur
charged, no doubt, with carbonic acid. A
plank, rock, or any substance, which,
through its non-conducting powers will re-
tain its heat through the night, receives no
dew upon its surface, while piece ofm
al, as an iron tool, or any material which
soon cools, condenses the moisture which
in the form of invisible vapor comes in
contact with it, and is soon covered with
dew. A hard bare surface is almost as good
q non-conducting substance as q brick,
hence field left in this state receives lit
tie or no dew, and the scattered plants
growing on such field also suffer, while
on the contrary loosened, pulverized sur
face radiates heat very fast, soon cools
down after sunset, and receives an abun-
dance of dew. In this we see hint to the
farmers to keep their corn ground well
worked up during the May weather. The
dew is not wasted ON the field of the slug
gard. "From him who has nothing shall
be taken away even what he hath" All
forms OF vegetation are excellent radiators,
soon cool down, and so condense an album
dance of moisture those that evaporate
most moisture during the day, cool most
quickly and so receive most dew during
the night.
When the sky is quite obscured with
clouds through the whole night, no dew is
observed the reason is obvious. These
clouds wrap the earth around like cloak,
prevent the radiation of the heat into the
cold space beyond the earth and the sur
face not cooling down, no dew is condens-
ed. A tree of thick foliage will prevent
the radiation from objects beneath it, and
hence there is little or no dew beneath a
tree, or in the woods.
A very slight wind will prevent de-
position of dew, by continuing evapora-
ton through the night.
In the fall the cold becomes so severe be-
fore morning, that the deposited dew con-
geals and we have frost. This congela-
ton must usually take place before the
dew gathers into drops, as we see the frost
particles evenly covering the whole sur
face of the plant in very small crystals
Sometimes with the mercury very low |
have observed the atmosphere to be filled
with frost crystals, glttering like dia-
monds, but never in this latitude.
The vapor of water is as perfectly inyisi- |
bic as air itself until it is condensed. In
warm weather, the breath of animals is in
visible; in cold weather the PARTICLES of
dew are formed in the cold air, the instant
the breath meets it, and become visible. -.
Were our engine boilers ofglass, the space
above the wator would appear as empty as
air itself G.. G. G.