Fort Worth gazette. [volume] (Fort Worth, Tex.) 1891-1898, 1891-09-03
THAT DEW QUESTION AGAIN.

I q
| 70 questions from Il school teacher. preSunl-
| ably. was Of special interest 10 ne, is IL
| gave NE Il clearer idea Of how dew IS formed
| than had ever before been able TO secure.
| But there are stl!l some things that do not
| wei understand. How could pitcher Of
| ice-water have the effect that you attribute
| 10 it, How is 1t that on cloudy nights there
| is no dew, if dew is merely moisture con
| uenseu from the atmosphere by colder
! earth surface Why IS there no dew Oh i
windy night VIII YOU please explain
| these things through The GAIErTE7 have,
| CyclOpedIa which treats on dew, but fails
| 10 make plain the things Just referred To.
think I understand you Is 10 the high
| and low dew points but cannot be entirely
sure Of it until have your answers TO the
above. C.
Fort Worth, Tex.
That picture OF ice water as appearing
in cur article of last week. was 4 typograph-
zeal error-we had given il to the printers
IA pitcher of ice-water
One of the best English authorities on
dew $50 generally acknowledged by sci
| entistsl, was the late James + W John
ston. VI. A. Z. Il. s. :, of the university Of
Durham. We never think OF working lie
cyclopPdias or anything of that sort for cor
rect science. In Professor Johnston :
chemistry of Common Life" page II,
please read:
When the surface of the earth coos by
radiation. the air in contact with il must
cool also and. like the warm currents on the
mountain side, must fOrsake q portion of
the watery vapor il has hitherto retained
This water, like the floating mist on the
hills, deScends in particles almost infinitely
minute. These particles collect on every
l..pFt c." ~ Il al,
blade of grass in drops Of "pearly dew -
And mark here beautiful adaptation. DIFF
ferent SubSt auceS are endowed with the
property Of radating their heat. and thus
becoming cool. with different degrees of
rapdity. Those substances which in the
air become cool first must also attract first.
aud hence most aDunGantlV III the course of
the night Thus. in the cool of the sum
mer's evening the grusS-plat is wet while
the gravel walk is dry: and the thirsty
pasture and evergreen leaf are drinking in
the descending OF condensing moisture,
while the naked land and the barren high
way are still unconscious of dew.
This clearly explains the causes in opera-
ton which produce dew. a leading Amer
lean authority on the same subject is Pro
fessor +. NJ. Pendleton, NJ. D., late of the
University of Georgia. In his Text Book
Of Scientific Agriculture" page Il. he
says:
Dew. another important source of moist
Ure to plants III certain seasons. j. SpOnta-
neOusIy deposited during clear, calm nights
on the surface of all bodies exposed to the
atmosphere. During the day the earth, as
well as the lower strata of the air, becomes
heated by the direct rays of the sun: as
night approaches the surface of the earth
loses its heat rapidly BY radiation. and 81
falls in temperature. But the earth loses
its heat more rapidly than the air there
being sometimes as much as - deg differ
ence. Thus the stratum of air immediately
in contact with the earth is cooled down
rapidly its vapor condensed and deposited
as dew upon the earth, and substances ~
ing upon its surface.
4 body, then. must be colder than the
contiguous atmosphere before the dew can
De deposited upon II. and the greater he
difference in temperature. other things be-
ing equal, the greater the amount of dew
Dew can De formed only when the atmos-
phere is calm and clear; hence il is not de-
posted on cloudy or windy nights. The
clouds reflect back the radiated neat of the
earth, and keep the intermediate atmos-
phere of an equable temperature, SO that il
cannot be formed. DR.. Wells demonstrated
this as follows: On Il clear night q Their
mometer laid upon the grass stood at 81
deg. above zero. The sky suddenly became
overcast with clouds and the mercury rose
In twenty minutes to 80 deg. The clouds
rolled by. and in thirty minutes after they
had passed the mercury in the thermometer
was again marking - deg.
When there IS wind no dew IS formed.
because volume OF air cannot long enough
remain in contact with the cold surface OF
the earth for its moisture 10 be condensed.
And now you have all your questions
plainly answered, not by Us, but by stan
dard scientific authorities. The dew-point
question is clearly settled by the second
paragraph as quoted from Professor Pen
dleton A body, then, must be colder than
the contiguous atmosphere before the dew
can be deposited upon it. . II an atmosphere
Immediately from the surface of the sea
carries more moisture than it viii carry
when far inland, (as all reasoning persons
must admit, then it must undoubtedly be
of lower temperature at the sea than at its
inland point, provided Of course, that the
inland point is not greatly elevated. A
colder atmosphere. as AZ the sea, would re-
quire a correspOndingly lower earth tenn
perature TO condense moisture than would a
warmer atmosphere of the interior. This
being undoubtedly the case, the dew-point,
as reckoned on the earth temperature
necessary to produce dew, could not be
otherwise than lower at our coast cities
than at Fort Worth. The quantity of dew
exhibited cuts no kind of figure in the mat
ter-the question hangs on the highness or
lowness OF earth temperature necessary to