The daily union., 1846-09-24

PAkIIkG TINE INDIAN SUMMER
The calm and pleasant weather then enjoyed is
account for the obscurty Of the air. This is mere
consequences Of that condition Of the atmosphere
which has been called comparative quiescenee.
IL is a matter of observation, and consequently
Of the aei Of the case that whenever the atmos-
phere is comparatively quiet for several days toge-
,her. IL becomes turbd, or filed with what is com
monty, from It's appearance, called smoke Some
times indeed this state Of things is ushered in by
halliday or more of considerable wind but this dies
away, as the smoke increases. II the atmosphere
be very dry the smoke rs less Dense and pies versa
| It's density goes on increasing from day TO day,
| (unless when the calm s frequently interrupted,
until il is entirely broken up by some great move
ments in the air. Nor is the observation of the sky
from this cause Of rare occurrence. Throughout
the whole year, at intervals Of every few days we
are ,enveoPed IN sky more or less densely
Tha, this murLiness is due to mol lure, will ap-
pear from the consideration Of few faets, ..
First. The morning fogs, especially those OF
August, when not followed by immediate rain, in
stead Of being dissolved in pure clear sky by the
increasing solar heat during the day, frequently
found to obscure the sky like dense smoke.
Secondly. ! have often seen both cirrus and
dense nimbus clouds, which were waited over me,
| by upper currents from an adjoining district Of
moister air, rapidly dissipsted, and leaving the sky
| obscured by dense smoke, instead Of remaining
| nearly transparent, as il was before
Thirdly. Such states Of atmosphere are frequently
broken up by copious rains which clear the sky Of
| its obscurity. SO constantly during the summer
months is this the result, that smoky sky is gen
orally considered an indication of approaching
/ rain.
.4nd tourthly. The dsto-pont indicates the exist
ence Of more than average quantity of moisture
| in the air.
These are the facts, which in general prove that
the obscurty Of the air, in the under consd-
- hua IN rTal L. hnI.. in
other words, that II has relation TO the existence
Of extensive fires, as il generally believed The ac-
cdentsl existence Of fires, at such tines is due to
the drought want Of rain, and the greater infam
inability Of combustibles The recently fallen fo-
liage OF autumn affords peculiar facilities for extern
sive fires during the Indian summer.
Weare then 10 regard what 10 ordinarily called smoke
and haze very thin cloud IL is sometimes seen at
some elevation above the earth, in great measure
dsrobng the sun, moon, and stars Of their spendor,
and producing that mild, subdued light which is
witnessed in atmosphere denominatsd hazy. At
other times il is confined more nearly to the esrth's
surface, causing terrestrial obecs through il to
have that peculiarity Of tinge denomnated smoky
tits generally found TO grow more dense during the
night, or early in the morning. in consequence Of
diminished temperature; and then, during the day,
/ again TO become more rare in consequence of the
greater solvent power Of the increased tempera-
ture.
Now cloud, which is only invisible vapor, rem
i
IN the size OF Its particles, Is formed in two ways,
viz: - either by depression of temperature or (2) by
elevation Of the dew-point, that is increase Of most
ure in the given space.
| (l.) By the former, cloud is formed either when
| the temperature of the space in which the vapor
exists Is depressed, as when fogs formed in low
and moist places, such valleys and along streams,
more extensively upon the earths surface by the
cold Of the night or when moisture, or air contain
ing moisture, is forced upwards to considerable
elevation by atmospheric pressure. In this case
the cold of the space into which il is thrown, but
mainly the cold Of eIpansion produces the conden-
salion, or forms the cloud In this manner the Gu
| mulus and nimbus clouds are formed.
(2) By the latter, viz, the elevation Of the dew
point, cirrus cloud is most generally formed if the
| increase Of moisture be rapid, beautifully exem-
phed in the formation of the band Of cirrus at the
apex of the rapidly forming nimbus cloud during
summer but If IL be pery slo,o the thin cloud often
called haze smoke, is formed. To the production
Of this, comparatively quiet state of the atmos-
phere is essential; for any considerable movements
especially if they be in a vertical direction will re-
suit in the formation Of ordinary cloud. Regarding
these things well ascertained, and therefore as
data, we may apply them to the explanation Of the
phenomena In question.
i Let now suppose that, for several months to-
gether, the atmospheric temperature and pressure
remained the same what would then take place in
reference to the vapors and transparency of the skyi
There would then be no horizontal, and, much less
vertical currents to which the formation of ordinary
cloud, and the withdrawal Of moisture from the at
mosphere, in the form ofran, 10 due. But evap-
oration would, as usual, continue to increase
the quantity of vapor from day to day, and
| ordinary cloud and rain could not be formed, until
the air would become saturated, an uninterrupted
night Of cloud would envelope the earth. Long,
however, before the point of saturation would have
been reached, the particles of increasing moisture
would begin to attach themselves to each other and
having acquired a magnitude, not indeed sufficiently
great to be seen by direct examination, but yet
enough so to intercept the light they would destroy
the transparency of the atmosphere. These part-
cles, being far less crowded than ordinary cloud,
would not in general hide from our view surround
ing objects, except those which are very distant.
Now though no state of atmospheric quiescence so
absolute that which has just been supposed, ever
actually exists; yet, as has been shown, short
sons of partial repose are found during different por-
tions of the year; among which, four are more re-
markabe than the rest, and among these one--vz
| that of the Indian summer occupies the pre-em-
At this season. the atmospheric currents being
feeble, and the average pressure, as shown by the
barometer whose oscillations are slow, being about
the mean or little above; the causes, which pro-
duce ordinary cloud, operate in slight degree as
not to be followed by its formations; or if, through
the rapid increase of MOISTURE and the greater actv
y of disturbing causes, it is formed, it is only in
light and detached masses, which pass off without
materially interfering with the repose of nature. II
must be remarked that when the equilibrium is very
unstable, and the circulation verging somewhat to-
wards activity, cloud begins to be formed, and rain
sometime descends; circumstances which are more
likely to occur at other seasons than during the In
dian summer. The difference of temperature be-
tween day and night, is not, at this season. suff-
GENTLY great to cause the formation of dense cloud
either by general cooling, or by starting vertical
rents. But the particles Of moisture, in the lower
strata Of the atmosphere, gradually attach them
selves to each other, and form the thin and more
| than semi-transparent cloud commonly called
smoke, so characteristic Of the Indian summer. A
similar process of condensation produces the smoke
Of the other times of equilibrium. The density Of
the smoke is, in general proportional to the contin
uance Of the repose.
In illustration Of our subject, numerous most in
terestng and striking examples might be given, but
for the sake of brevity they omitted. This is
hardly doing justice either to the theory the sub
ect for which this explanation has been offered but
it has been done in compliance with the earnest de-
sire of the committee Of the Institute.
These views are respectfully submitted with the
hope that they will be found to be correct account