Fort Worth gazette. [volume] (Fort Worth, Tex.) 1891-1898, 1891-09-13
THE DEw-PoINT QUESTION:.

All that has been said on thiSsubiect in the
| laSttwo issues of The Sunday GAZETTE. ha
! been carefully read. and still | am dissatis-
fed. 1t is TO the theory that the dew-point
depends upon the relative temperature of the
air and earth that / object. Understand
| your position; II sounds plausible: but l
| do not think it is sustained by scientific
writers, and my question was TO learn
| whether it was corroborated by actual ey-
| pertinent. BY positive, reliable testimony
l meant that made by careful experiment.
| as distinguished from that which is only
| theoretical.
| The authorities you cite are all right
| enough, but | do not draw the same infer
| ence that you do from the sentence you
| quote. Let US see: A body. then, must
| be colder than the contiguous atmosphere
1 before the dew can be deposited upon il. =
| Certainly il must: If the surrounding air
| and the earth were above dew point, no
| phenomenon would be apparent; if al dew
! point there would be a drilling rain or a
| clammy, oppressive atmosphere. II, how
| ever by radiation the requisite temperature
| has been reached by some bodies before the
mass of the air has sunk to its dew-point, d
| deposit called dew is made upon those
| bodies. The second member of the SEN
| tence, ''other things being equal, the
| greater the difference of temperature the
greater the amount of dew may be made
! still clearer by the following from Profess
sor LOOMS:
The amount of dew thus deposited is
greatest upon those substances whose tenn
perature is the lowest, being proportional
10 the amount of their depression below the
dew point.
Lest my conception should be the wrong
one. I have reviewed the authorities in
search and find nothing leading to differ
emit conclusion. The only sentence winch
detached from its connection might favor
the idea of relative earth and air tempera-
ture is found in Deschanel'S Natural PhiloS-
ophy. page 4l2:
The lowering temperature which thus
occurs Is much more marked for grass,
stones or bare earth than for the air,
whose radiating power is considerably less.
The consequence is a considerable differ
ence of temperature between the surface
of the ground and the air at a height of d
few feet, a difference which is found by
observation to amount sometimes TO or 10
dogs. c., and it is this which causes the
deposition of dew.
Now how is this deposition brought
about? The next sentence explains: 1.-The
surface of the earth, as il gradually cools,
lowers the temperature of the adjacent air,
winch thus becomes saturated, and on fur
Their cooling Yields up portion of its vapor
in the liquid FORM The dew-point is
reached when the air is saturated an in
tinitesmal cooling after that causes de-
post. The greater the humidity of the at
lnosphere the less the decline of tempera-
ture to produce saturation, hence the higher
the dew-point.
The root of the difference between the
theories we respectively advocate lies in the
differing conception Of the term dew-point.
Let US begin again at definition: What is
the dew point9 In the former paper gave,
That point on the thermometer, that de-
gree of temperature, at which the moisture
present in the atmosphere condenses."
am more than willing 10 drop the first part
OF the definition, it being used to give con-
creteness and effectiveness. This is nearly
Webster's definition, more nearly Professor
Cleveland AbDe'S. Let US read from Pro
fessor Loomis. In his Treatise on Me
teorology. page 81, we find:
The amount of vapor in the air may be
measured with great accuracy by noting
the temperature at which moisture begins
TO be condensed on a cold vessel The
moisture thus deposited is called dew, and
the temperature at which the deposition
begins is called the dew-point The dew
point, then, may always be determined by
cooling metallic vessel until dew begins
TO appear upon its surface, and noting with
thermometer the temperature of the YES
sell We see here that the dew point
measures the amount Of vapor in the air;
therefore the greater the amount the more
quickly The dew point Is reached, the less
the decline and the higher the dew point.
AGAIN. turning 10 DISCLAIMERS Natural
Philosophy, translated by Professor J. d.
Everett, Belfast, page 365. we find:
When air containing aqueous vapor is
gradually cooled at constant pressure its
density increases, and the rate Of increase
is sensibly the same for the vapor as for the
dry air with which it is mixed, until point
is reached at which the density OF the vapor
becomes equal 10 the maximum density
corresponding 10 the temperature. This
temperature IS called the dew-point of the
given mass. ana any further reduction
OF temperature will be accompanied by the
condensation of portion of the vapor,
which will take the form of dew, rain. snow
or boar frost, according to circumstances.
The formation OF dew consists essen
tally in the deposition Of moisture on the