The daily dispatch. [volume] (Richmond [Va.]) 1850-1884, 1882-10-13

xznxs eARuyHs MAY BE FORSwARNED AGAINST
INV TEurERATuREs--A VALUABLE CoNlss-
atrtoN FROM THE UNITED STATES siSSaL
SERvIC1.
From time TO time General IIasen, the
ehief cf1icer Of the Signal Service, l'fs had
several special papers prepared on sub,
iects most interesting to the farwT ana
NAVIGATOR. The nxsuner in which cse Gun
anu south Atlantic coasts were warned Oh
the approach OF tic cyclone lass a,onth III
dt ated the valucci the sySem OF coserving
st,tions that wore established " ms oruc'
throughout the West Indies. Indeed, un-
III s,ch Gordon Of meteorological videttes
was established the inhaitants OF the
southern Stsses were completely at the
mercy of the treacherous equatorial storm
The nayies.ors whose vessel plough the
waters of the Gulf OF Mexico or the ATTAIN
tic off the Florida coast car now he assured
that they will not he caught by a cyclone
II they only take heed Of Jlic warnings Is
sued in the port Of departure.
AID FOR TBE FARMER.
The latest paper cmanating from the Sig
nal Office is one that calls for the strict AZ
tention OF farmers and all others dealing IN
goods affected by changes in temperature.
"To Foretell Frost is the title. Al all
Temperatures" SAYS the'writer, Lieutenant
lanes Allen. "eyen the lowest, moisture
exists i'm the atmosphere In an invisible
state; the air is never absolutely dry. Inc
Intervals between the particles OF air are
narfly filed withfhc vapor that is constant
IL rising from the earth. This capacity OF
the air for moisture has limit ana when
this is reachedth air Is said TO he saturated
A rise OF temperature increases the capa-
city of the air 5or moisture. On the otneg
hand. a fall al temperature dlminlshes the
capacitv. But the capacity OF the air Tor
moisture increases more rapidly IMAM the
femperature Thus, air can contain at ~
degrees the I60th part OF its own weight, at
so degrees the'S0th part. and at 81 degrees
the 4Oth pat. the law being that for every
increase OF 81 degrees the capacity Is dou-
bhd." After.showiug how the amount OF
yapor in the air may be measured DE
noting the temperature at which the moi~-
IRE begins 10 be condensed on cola YES
sci. the writer gives full description OF
the dry and wet bulb hygrometcr. and
illustrates the manner in which the dew
point can be ascertained by its Use. Re
searding it, usefulness TO agricuturistS, he
saws. @.The ascertainmcnt OF the dew
point is of great practical importance. par
ticularly TO hOrticulturists, since it shows
~ the MONT near which the descent OF the
temperature of the air during the night
viii he arrested. For when the air
has heen cooled down by radiation
10 this point dew Is deposited and latent
heat is given out. The amount OF heat
thus set free being great, the tenn
peratrre OF the surrounding air is imme-
diaielv raised. The same process con-
tinues 10 be repeated, and thus the tenn
perature OF the air in contact with plants
or other radiatng surfaces may be cOnSid-
cIed as gently oscillatin@ about the dew
point; Tor If it raises higher the loss of
heat by radiation speedily lowers it. Thus
the dew-point determines the minimum
temperature of the surface OF leaves on the.
ground during the night. The minimum
Temperature in the instrument-shelter
would be higher than this.
HOW CHANGES ARE INDICATED.
"This suggests an important practical
use Of the hygrometer. II the dcw-pont
he ascertained by it, the approach Of low
temperatures or of frost may be foreSeen
and provided against. Thus, suppose on
fine. clear Gay toward evening that the dry
bulb is 50 degrees and the wet bulb 40 de-
grees. the dew-point at the time is there
fore 20 degrees Fahrenheit. Frost on the
ground may then be predicted with GER
tanty, and no time ought TO be lost in pro-
tecting such tender plants as may be eX-
posed in the open air. If, on the other
hand, with q sky qste as clear, the
dry bulb is so degrees and the wet
bulb A. degrees, the dew-point being
thus 43 degrees, no frost need be
feared. Assuming the sky to remain
perfectly clear Of haze or clouds, the raising
or depressing Of the dew-point during the
night usually with a change OF wind are
the only circumstances that can happen to
interfere with the predictions founded on
the hygrometer. Frequently the presence
of haze at high attitudes during the night
prevents the radiation OF heat from the
earth, and thereby the frost which other
wise would have occurred. Hoar-frOst j.
formed under the same circumstances as
dew. with the exception of lower
temperature. When the temperature of
the surface OF plants falls below 81
degrees the moisture of the air is
condensed upon them in the solid state
and forms layer Of Snow crystals like
spongy ice. Hoar-frost therefore is not
frozen dew, but the moisture Of the air is
deposited in the solid form without having
passed through the liquid condition. IIoar-
frost, like dew, is deposited chiefly upon
those bodies which radiate best, suclsas
pants anu inc leaves OF vegetables, ana
the deposit is made principally on those
parts which are turned towards the sky.
Since plants sometimes become cooled by
radiation from twelve to fifteen degrees
below the temperature OF the surrounding
air, q frost may occur although a Their
mometer few feet above the ground, in
an Observation-Disclosures, may not sink to 81
degrees. During u clear and still night,
when thermometer six feet above the
ground sinks to SO degrees, heavy frost
may be expected. ASlight frost may occur
when the same thermometer sinks only to
Al degrees. Whatever prevents the radia-
ton OF heat serves also to check the forma-
ton Of hoar-frost. During the cold
nights OF spring plants which are
sheltered by trees are less liable TO be
injured by frost than those which are
fully exposed, and thin covering OF
cloth or straw will generally afford entire
protection. \ garden may frequently be
saved from injury by kndling a small
smudge fire, which Will envelope the plants
in 4 cloud OF smoke. Fogs and clouds also
protect vegetation from the effects OF frost ,,
All persons desiring to use the hvgrome.
ter will be supplied with them by the CHIEF
officer OF the Signal Service at cost price
-. Full instructions as to the manner o,
reading them will also be furnished free Of
charge.
The Comets Terrible (rists