Fort Worth gazette. [volume] (Fort Worth, Tex.) 1891-1898, 1891-08-30
THAT DEFY QUESTION AGAIN.

| THAT DEW QUESTION AGAIN.
Your article on dew, appearing in reply
| to questions from school teacher. preSum-
| ably, was of special interest to me, as it
| gave me clearer idea of how dew is formed
| than had ever before been able to secure.
But there are still some things that / do not
| well understand. How could a pitcher of
| ice-water have the effect that you attribute
! to it, How is it that on cloudy nights there
| is no dew, If dew is merely moisture con-
| densed from the atmosphere by colder
| earth surface? Why IS there no dew on a
windy night: Viii you please explain
| these things through THE GAzETTEH have
| a cyclOpedla which treats on dew, but fails
| TO make plain the things just referred TO-
| l think understand you as to 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 our article of last week, was typograph-
ical error-we had given it to the printers
"a pitcher Of ice-water''
| One Of the best English authorities on
dew $50 generally acknowledged b sci
| entists), was the late James f. W John
| ston, NJ. A, z. r. S. s., of the university of
| Durham. We never think of working the
|
rect science. In Professor Johnston's
| Chemistry of Common Life,' page II,
| please read:
! When the surface of the earth cools by
| radiation, the air in contact with it must
| cool also and like the warm currents on the
| mountain side. must forsake portion of
| the watery vapor it has hitherto retained
| This water, like the floating mist on the
| hills, descendS IN particles almost infinitely
| minute. These particles collect on every
| leatet, and suspend themselves from every
| blade of gr. SIS in drops of ''pearly dew"
| And mark here beautiful adaptation. Diff
| ferent substances are endowed with the
| property Of radiating their heat, and thus
becoming cool, with different degrees of
| rapidity. Those substances which in the
| air become cool first must also attract first,
| and hence most abundantly In the course of
| the night. Thus. in the cool of the sum
mer's evening the grass-plat is wet while
the gravel walk is dry; and the thirsty
| pasture and evergreen leaf are drinking in
| the descending or 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 E. NJ. Pendleton, NJ. D., late of the
| University of Georgia. In his ''Text Book
| of Scientific Agriculture page II, he
| says:
| Dew, another important source of moist
! ure to plants in certain seasons, is sponta-
| neously 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 so
| falls in temperature. But the earth loses
| its heat more rapidly than the air, there
| being sometimes as much as 10 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.
A body, then, must be colder than the
| contiguous atmosphere before the dew can
| be deposited upon II, and the greater the
| difference in temperature, other things be-
| ing equal, the greater the amount of dew.
Dew can be formed only when the atmos-
| phere is calm and clear; hence it is not de-
| posted on cloudy or windy nights. The
| clouds reflect back the radiated heat of the
earth, and keep the intermediate atmos-
| phere of an equable temperature, so that it
| cannot be formed. Dr.. Wells demonstrated
| this as follows: On a clear night a Their
mometer laid upon the grass stood at %
| deg. above zero. The sky suddenly became
overcast with clouds and the mercury rose
| IN twenty minutes to 90 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 a volume of air cannot long enough
remain in contact with the cold surface of
the earth for its moisture to 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, If an atmosphere
carries more moisture than it will carry
when far inland, Gas 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 4
colder atmosphere, as at the sea- would rsss
quire correspondingly lower earthstem-
perature to condense moisture thansskould
warmer atmosphere of the interior. This,
being undoubtedly the case, the l1ew-poiA
as reckoned on the earth fmpersfsre
necessary to produce dew, coAd no 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
ier-the question hangs on the highness or
.ownsxs OF earth temperature necessary to