Fort Worth gazette. [volume] (Fort Worth, Tex.) 1891-1898, 1891-09-17
THE pEy-PoISr QUESTION.

| All that has been said on thissubject in the
| last two issues of The Sunday GAZETTE, Ia
| been carefully read, and still l 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; 1t sounds plausible; but
| do not think it is sustained by scientific
| writers, and my question was to learn
| whether ii was corrOborated by actual ex-
| pertinent BY positive, reliable testimony
| ] meant that made by careful experiment,
as distinguished from that which is only
| theoretical
! The authorities you cite are all right
| enough, but I do not draw the same infef-
ence that you do from the sentence Sou
| quote. Let us see: ga body. then, must
| be colder than the contiguous atmosphere
before the dew can be deposited upon it. -
Certainly it must; if the surrounding air
and the earth were above dew-pSnt, no
| phenomenon would be apparent; if at dew
| point there would be drizziing rain or
clammy, oppressive atmosphere. Ii, how
ever, by radiation tho requisite temperature
has been reached by some bodies before the
| mass Of the air has sunk to its dew-point, a
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 tho following from Profess
ser LOOMS:
The amount of dew thus deposited is
greatest upon those substances whose tenn
perature is the lowest, being proportional
to the amount Of their depression below the
dew point.
Lest my conception should be the wrong
one. l have reviewed the authorities in
search ana find nothing leading to a differ
ent conclusion. The only sentence which
detached from its connection might favor
-. y. xf,&.- Il,
ture is found IN Beschanel's Natural Philos-
opby, 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 4 height of a
few feet, difference viii n is found by
observation 10 amount sometimes to S or 10
dogs. c., and it is this which causes the
deposition OF dew.
Now how is this deposition brought
abouts The next sentence explains: 1.-The
surface of the earth, as it gradually cools,
lowers the temperature Of the adjacent air,
which thus becomes saturated, ana ON fur
Their cooling Yields up a por ton Of its vapor
in the liquid FORM The dew-point is
reached when the air is saturated; an in
finiteSmal cooling after that causes de-
post. The greater the humidity of the at
mosphere 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 pointf In the former paper gave,
THAT point on the Thermometer, that de-
gree of temperature, at which the moisture
r,rn N in hA typn Sraha
am more than willing to drop the first part
OF the definition, 1t being used TO give con-
creteneSs anu effectiveness. This is nearly
WeDster's definition. more nearly Professor
Cleveland Abbe's. Let US read from Pro
fesSor LOOMS. In his Treatise on Me
teorology, page 81, we find
1.-The amount of vapor in the air NAV 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-poiut. 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
Sel.h We see here that the dew point
measures the amount of vapor in the air:
therefore the greater the amount tine more
quickly the dew point is reached, the less
the decline and the higher the dew point.
again. turning to Deschanels Natural
Philosophy, translated by Professor J. D.
Everett, Belfast, page S65. we find:
when air containing aqueous vapor is
gradually cooled at constant pressure, its
density increases, and the rate OF increase
is sensibIy the same for the vapor as for the
dry air with which i. is mixed, until point
IS reached at which the density Of the vapor
becomes equal to the maximum density
corresponding to 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 a portion of the vapcr
which will take the form of dew, rain, snow
or hear frost, according to circumstances
The formation Of dew consists essen.
tisliy-in the deposition Of moisture on the