Miscellany [pp. 340-344]

1873.]JIIISCELLANY 341
to whom the ocean is a thing unknown.
Dwellers on extensive alluvial plains have to
take their notions of a water-fall from a lock
or a mill-weir. The great American fresh water lakes-their aspect, phenomena, fish,
and birds-are separated from us by a hemi sphere.
There are regions where the form which
water takes when we witness its balloon per formances, in the shape of a passing cloud, is
a marvel and a rarity. One of Captain Mar ryat's sailors joyously hails the black squall
he meets in the Channel because it is "no
more of your d-d blue skies." Rain is
most partially and unequally distributed.
There is a spot near Bangor, in Wales, where
it rains more or less every day in the year.
Another, in Borrowdale, competes with its
rainfall. At Perpignan, chief town of the
Oriental Pyrenees, France, it rains so seldom
that, when the phenomenon does occur, little
boys and girls call each other out to see it,
and catch the drops on their inquisitive
tongues.
In the Pampas there occur long droughts
which, Mr. Darwin was told, are almost peri odical, the interval being about fifteen years.
Note here that Mr. G. J. Symons calls atten tion to the periodicity of wet seasons in the
United Kingdom. A few years since, any one
who expressed belief in the periodicity of me teorological phenomena received more criti cism than credit. Not being much afraid of
satire, in 1865 he pointed out the fact that,
of the fifty years between 1815 and 1864, the
wettest were'36,'41,'48,'52, and'60, and
that, out of these, three were equidistant,
giving what looked like a twelve-year period.
Now that such speculations are more favora bly received, it may be permissible to state
that'72 is just twelve years after'60, and
that, while this is written, it is raining stead ily, with plenty of inundations in plenty of
quarters.
In contrast with this, during the "gran
seco" in the Pampas, between the years 1827
and'30, the vegetation, even to the thistles,
failed. The brooks were dried up, all the
small rivers became highly saline, causing the
death of vast numbers of animals. The whole
country assumed the appearance of a dusty
high-road. In fact, such quantities of dust
were blown about that, in that open country,
the landmarks became obliterated, and people
could not tell the limits of their estates. Dis putes arose in consequence. Multitudes of
birds, and wild and domestic animals, perished
for want of food and water. The deer came
into a court-yard to a well which a man had
been obliged to dig to supply his own family
with water.
More than this, there are localities, as in
the Great Desert, where it never rains at all;
also within the Arctic and Antarctic Circles,
where the deposition of water on the earth
occurs only in the shape of snow (and that
the very finest) and frozen vapor, or minute
particles of ice, floating in the air. Neither
does it hail there, hail being frozen rain.
Water has even an invisible state, in
which it increases the clearness of the atmosphere. Among the traditional signs of rain
are:
"Along the stream the swallows fly
The distant hills are looking nigh."
There is no better example of invisible water
than that given by Dr. Tyndall. At every
puff of a railway-locomotive, a cloud is projected into the air. Watch it sharply: you
notice that it first forms at a little distance
from the top of the funnel. Give close attention, and you will sometimes see a perfectly
clear space between the funnel and the cloud.
Through that clear space the thing which
makes the cloud must pass. What, then, is
this thing which at one moment is transparent
and invisible, and at the next moment visible
as a dense opaque cloud? It is the steam,
or vapor of water, from the boiler. Every
bit of steam shrinks, when chilled, to a much
more minute particle of water. The liquid
particles thus produced form a kind of water dust of exceeding fineness, which floats in the
air, and is called a cloud.
And'not only is it called, but it is a cloud.
On a chain of mountains you often see a bit
of cloud fastened, like a flag, to the summit
of every peak, while the intervals between
them remain perfectly clear. The fact is so
familiar as to have given rise to popular
names. The Wrekin has his nightcap, the
Table Mountain his table-cloth. Dr. Tyndall
figures and describes the cloud-banner of the
Aiguille de Dru. I have seen a cloud-flag
hang to Mont Ventoux for hours and days to gether, apparently unaltered and unmoved.
But its fixity is only apparent. When the
streamer of cloud drawn out from an Alpine
peak is many hundred yards in length, we
wonder at its obstinate persistence in spite
of a high wind which may be blowing all the
while. But, in reality, its substance is ever
changing. The invisible vapor, forced up the
mountain-side, is chilled and condensed into
fog at the top. The banner, which is inces santly dissolved at the farther end, is inces santly renewed at its points of contact with
the peak. In consequence of this equalization
of consumption and supply, the cloud appears
as changeless as the mountain to which it
clings. "When the red evening sun," writes
Dr. Tyndall, "shines upon these cloud-stream ers, they resemble vast torches with their
flames blown through the air."
Air, at a certain temperature, can hold
only a certain quantity of invisible watery
vapor. That is, the quantity of moisture
contained by air, when saturated with it, is
constant and fixed for every degree of tem perature. The drier the air, and the hotter
the air, the greater is the amount of cloud
which can thus be dissolved in it. Conse quently, the invisible water - vapor in air
becomes visible when a lowering of tempera ture, or an increase of moisture, brings it to
the point of saturation. What we call a
cloud, therefore, is water-vapor which the air
cannot absorb when it is saturated, and which
differs from the vapor already absorbed by
passing into the state which Dr. Tyndall calls
water-dust, consisting, according to recent in vestigations, of minute vesicles, or bladders.
By watching a small cloud which hangs low
in the air, we may often make a good guess
at the weather. If it grows smaller, melts
away, and is dissolved in the air, we may ex pect a dry day, or at least a few dry hours.
If it grows heavier, and amplifies its propor tions, we may take our umbrella under our
arm, with the likelihood of having to hold it
overhead.
This change of water from the gaseous to
the molecular state can take place at any alti tude. When it occurs at the ground-level, we
call it fog; but there is no essential difference
between a cloud and a fog. While traversing
clouds in a balloon, no resistance is felt; the
air is simply more or less opaque, chilly, and
moist, exactly as happens on the ground, according to the nature of the fog or mist. The
same with clouds encountered on mountains.
But, although there is no essential difference
between clouds and fogs, there really is one
of fact or circumstance. A fog is the produce
of a place or locality in which water-vapor
passes from the invisible to the visible state;
a cloud is a free, individual object, an unattached grouping of vapors into forms so determinate that clouds are classified according
to their shapes. The one is fixed, local, and
uniform; the other is movable, and of variable
aspect.
Examined with a magnifying-glass, fog is
composed of tiny bodies, which are found to
consist of water, obeying the laws of universal
gravitation. The water-molecules are little
balls, like shot or melted lead fallen from a
height, or mercury spilled on a mahogany
table. Whether those spherules are hollow
or not, is a question on which meteorologists
are not agreed. Halley, with apparent reason,
maintained that they are. The deadening of
sound by fog confirms the idea. Gas-bubbles
in water have the same effect. Probably, in
mists, the vesicles are mingled with a consid erable quantity of minute droplets of water.
Take a cupful of any dark-colored liquid,
as, for example, coffee. Heat it, and set in
the sunshine. If the air is calm, a vapor rises,
and soon disappears. With a magnifying-lens,
globules are seen to rise. The smallest rapid ly cross the field of the lens; the others fall
back on the surface of the liquid. De Saus sure declares that the little vesicles which
mount are so completely different from those
which fall back again that it is impossible to
doubt that the former are hollow.
Their behavior with light confirms the
opinion. Everybody has remarked the irides cent hues that gleam on the surface of soap bubbles. In order that those colors should
appear, it is optically necessary that the film
containing the bubble of air should be exces sively thin. Kratzenstein examined with a
magnifying- glass, in sunshine, the vesicles
that steamed up from the surface of hot wa ter, and saw on their surface colored rings,
exactly like those on soap-bubbles; and not
only was he convinced respecting their struct ure, but he was able to calculate the thick ness of their envelop.
De Saussure and Kratzenstein tried to
measure with the microscope the diameter of
the vesicles composing visible water-vapor;
but hot-water steam can hardly be expect ed to give the same results as natural fog.
Kaemtz made numerous measurements on
mists in Central Germany and Switzerland.
He found that in winter, when the air is very
moist, the diameter of the vesicles is twice as
great as in summer, when the air is dry. But
in the course of the same month the diameter
varies. The average diameter of mist-vesicles
may be taken at one-fiftieth part of a milli metre. The length of the millimetre is three
hundredth parts of an English inch. Their
minimum diameter occurs in very fine weather;
when rain threatens, it increases; and imme diately before a downfall it is very unequal in
the same cloud, probably in consequence of
the mixture of hollow vesicles with full drop lets.
When we behold a cloud resolve itself into
rain and pour out thousands of gallons of
water, we marvel that such an enormous weight
of fluid should be capable of suspension in the
atmosphere. The cause consists simply in its
extreme divisibility. The fiftieth part of three
hundredth parts of an English inch is smallness
beyond our clear conception; and this is not
the minimum, but the average size of the particles of water-dust. Currents of warm air
ascending from the earth's surface are quite
sufficient to keep such tiny atoms afloat. They
hang together in groups and masses in consequence of their mutual attraction; for-attractive influences are the only obvious explanation of the very distinct forms and clearlydefined outlines which clouds exhibit.
Thick fogs are sometimes odorous, by impregnation with diverse exhalations pervading
the lower strata of the atmosphere. In Belgium and the North of Europe, they not unfrequently smell of turf. In Paris, during the
chilly fogs of October, 1871, especially in the
evening of the 14th, a most disagreeable taint
of petroleum was painfully perceptible.
The forms of clouds are infinitely diversified, from the flat, thick mist which carpets
the meadow to the bright-white flakes which
hover in the heights of the firmament. The
convenience of some sort of classification, for
literary and scientific purposes, led the meteorologist Howard to give names to the prinei
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