PHYSICAL GEOGRAPHY. METEOROLOGY (_continued_).
ATMOSPHERIC PHENOMENA—THUNDER AND LIGHTNING—AURORA BOREALIS—THE
RAINBOW—MOCK SUNS AND MOCK-MOONS—HALOS—FATA MORGANA—REFLECTION AND
REFRACTION—MIRAGE—SPECTRE OF THE BROCKEN.
There are a great number of interesting, and to inhabitants of these
islands uncommon,—perhaps we might say fortunately uncommon,—phenomena,
which overtake the traveller in other countries. We have referred to
whirlwinds and tornados, and will now mention two phenomena connected
with these storms. There is the water-spout, for instance, and
sand-pillars in the desert, which are whirled up by these winds in
spiral columns of water and sand respectively. The tiny whirlwind at
cross-roads, which picks up straws and leaves, is the common appearance
of whirling or crossing currents of air.
[Illustration: Fig. 724.—The Waterspout.]
Waterspouts, when they are permitted to come near a ship at sea, or
when they break upon land, which is seldom, are very destructive. The
waterspout is begun generally by the agitation of the sea, and the
cloud above drops to meet the water, which at last unites with it, and
then the column of whirling liquid, tremendously disturbed at the base,
advances with the prevailing wind. Its course is frequently changed,
and ships within its influence would be speedily wrecked. The only way
to save the vessel is to fire a cannon ball through the column and
break it.
[Illustration: Fig. 725.—Thunderstorm and shower of ashes from
Vesuvius.]
A waterspout once devastated a district in the Hartz mountains of
Saxony. “A long tube of vapour descended to the earth, and several
times was drawn upward again; but at last it reached the ground, and
travelled along at the rate of four-and-a-half miles in eight minutes,
destroying everything in its way.”
On another occasion at Carcassonne in 1826, “a reddish column was seen
descending to the ground, and a young man was caught up by it and
dashed against a rock.” His death was instantaneous.
The cause of these whirling winds is supposed to be in the action of
vertical currents of air which ascend heated, and return rapidly as
cold air. The “waterspouts,” etc., are quickly formed. The tornado
is a monster whirlwind like a waterspout in form, and advances at a
tremendous rate—eastward as a rule. It moves in leaps and bounds,
passing over some portions of the ground and descending again. The
current of air is directed to the centre; the cyclone, as mentioned,
has a spiral or rotatory movement.
Thunder and lightning have been, to some extent, described under the
head of ELECTRICITY, but some observations may also be introduced here,
as storms of that nature appertain to meteorology distinctly.
Electricity is always present in the atmosphere, and arises from
evaporation and condensation as well as from plants. As the air becomes
moist, the intensity of the so-called “fluid” increases, and more in
winter than in summer. Clear skies are positively electric, and when
large, heavy clouds are perceived in process of formation in a sky
up to that time clear, a storm is almost certain to follow. These
“thunder clouds,” in which a quantity of electricity exists, attract
or repel each other respectively. The cloud attracts the opposite kind
of electricity to that within it; and when at last a tremendous amount
has been stored up in the cloud and in the air, or in another cloud,
the different kinds seek each other, and lightning is the result,
accompanied by a reverberation and commotion of the air strata, called
thunder.
Lightning most frequently darts from cloud to cloud, but often strikes
the ground, whereon and in which are good conductors, such as wet
trees, metals, running water, etc. The “electric fluid” assumes
different forms—“forked,” “sheet,” and “globular.” The second is
perhaps the most familiar to us, and the third kind is the least known
of all. There are many well-authenticated instances on record in which
lightning with the form and appearance of fireballs has entered or
struck houses and ships.
“Fulgarites” are vitreous tubes formed in sandy soils by the lightning
in search of subterranean water-courses, for running water is a great
conductor of electricity.[34] The fire-ball form of lightning has been
known to enter a school-house where a number of children were, and
to singe the garments of some, killing others. The ball passed out
through a pane of glass, in which it bored a hole, breaking every other
pane, however, in its transit. Another instance occurred in which the
lightning ran about the floor of a room, and descending the stairs,
exploded without doing any injury.
Lightning, like the electric current of the laboratory, will not
always set fire even to inflammable objects. An electric spark can be
passed through gunpowder without setting fire to it, and lightning
will often shatter the object without firing it. Death by lightning
is instantaneous, and in all probability quite painless; for we may
argue from analogy, that as those who have been rendered insensible by
lightning have had no remembrance of seeing the flash which strikes so
instantaneously, nor of hearing thunder after it, it is instantaneous
in its effects. Besides, the natural attitude is preserved, and the
face is usually peaceful and limbs uncontorted after death by lightning.
There are some curious electrical phenomena, such as St. Elmo’s Fire,
already noticed under ELECTRICITY; and in some parts of America, in
very hot weather, such a light is perceived to issue from trees as
the fire glides through the forest. Many instances are on record
concerning the luminosity of pointed sticks, and even of the tails and
manes of horses in certain conditions of the atmosphere, and of the
universal power of electricity and its pervading influence in nature.
The benefits conferred by thunderstorms in purifying the air, and in
the production of ozone and nitric acid, are very great, and apart from
the magnificent phenomena exhibited, are well worth our attention,
though beyond our reach.
[Illustration: Fig. 726.—Aurora Borealis.]
Terrestrial magnetism, however, is still more puzzling in its action
than is electricity, and the study of the needle, its destination,
inclination, and intensity, which are marked upon charts, just as
are the weather reports of the _Times_, is an interesting one. These
magnetic maps are termed the charts of ISOCLINIC and ISODYNAMIC lines.
The declination of the magnetic needle from the _true_ north is its
_deviation_ from that point, and the “inclination” is its dip towards
the horizon. The line of its direction being known as the magnetic
meridian, its divergence from this line constitutes its declination.
There are places where it does not deviate, and these, in direction
north and south, are called lines of “no variation.” There are also
places in the equatorial regions where the needle does not “dip.” The
line connecting such places is termed the Magnetic Equator, and north
or south of this the needle dips respectively to north or south in
degrees coinciding with the distance from the equator.
The earth, then, acts as a magnet, and attracts the needle, but the
magnetic poles are not identical with the terrestrial poles. The north
magnetic pole was reached in 1831 by Sir James Ross, when the dip was
only one minute less than 90°, and the south magnetic pole was very
nearly reached also by him in 1840. The magnetic equator passes between
these two points.
[Illustration: Fig. 727.—Paraselenæ, or mock moons.]
It is to magnetic atmospherical disturbance that the aurora is due.
These northern (or southern) phenomena are extremely brilliant
and diversified. In temperate regions the aurora does not present
such grand forms as in the extreme north. There the spectacle is
astonishingly beautiful. The sky at first clouds over, and mist is
developed. Humboldt has eloquently described the aurora borealis, and
the beautiful changes of light, the constant movement, flashes, etc.,
denoting a “magnetic” storm, as electrical discharges indicate an
electric storm, although the area affected by the former is far more
extensive than that of the latter, and there is no thunder accompanying
the magnetic storm, with the production of which the electricity of
the earth is unassociated. To the continuous flow of this electricity
the aurora is due, and the flashes are only the electric current
descending towards the earth. But the true reason of the phenomena may
have to be yet discovered, for nothing absolutely certain is known as
to the origin of the aurora.
Amongst the numerous effects of refraction and reflection of light the
RAINBOW is most common and the most beautiful. If we hold a chandelier
“drop” in the sunlight, we shall see a brilliant representation of the
rainbow on the wall or on the carpet. The three colours—red, yellow,
and blue—mingle or shade away into seven—red, orange, green, blue,
yellow, indigo, and violet. These colours are all found in the rainbow.
[Illustration: Fig. 728.—Parhelia, or mock suns.]
The colour of the atmosphere—the usual blue tint of the sky—arises from
the blue rays of the spectrum being reflected more than the rest by the
aerial particles, and the less vapour the bluer the sky, because the
vapour gives it a whitish or misty tint. At sunset and sunrise the sky
is red or yellow, like gold, or of crimson hue. This is because the
sun’s rays have so much farther to come to us at sunrise or sunset, as
you will readily perceive if you draw a line from the sun to the sides
and then to the top of the arc of the heavens. The blue rays are thus
lost in space, while red and yellow, which travel so much faster than
blue, are transmitted to the eye, not giving the air time to absorb
them.
If you go under water and look at the sun it will appear very fiery
indeed, and we may likewise imagine that fiery crimson rays, which
betoken atmospherical disturbance, very often are due to the moisture
through which they are transmitted. Wet and storm frequently succeed
a crimson sunset, which betokens much moisture in the air. The sun
is similarly seen through the steam issuing from an engine, and the
colours vary according to the density of the steam in its stages of
condensation.
[Illustration: Fig. 729.—Mirage at sea.]
Vapour, we know, is invisible and transparent, but when it has been
condensed into rain-drops, and the sun is shining, if we stand with
our backs to the sun we see what we call the rainbow, because a ray of
light entering the drop is reflected, and as all rays are not of equal
refrangibility, the light, which is composed of three simple rays, is
divided and reflected into those and the complementary colours. When
the sun is at the horizon, the rainbow, to an observer on the earth
(but not on a mountain), will appear to be a semi-circle. The higher
the sun rises the lower is the centre of the rainbow. So we can never
see rainbows at noon in summer because the sun is too high. A second
rainbow is not uncommon, the second reflection producing the colours in
a different order. The colours in the “original” range from violet to
red; in the “copy” they extend from red to violet. “Rainbows” are often
visible in the spray of waterfalls and fountains.
HALOS are frequently observed surrounding the moon, and then we are apt
to prognosticate rain.
“The nearer the wane
The farther the rain,”
is an old couplet referring to the appearance of the moon, and is
supposed to foreshadow the weather by the size of the halo, which
is caused, as we know, by the existence of vesicular vapour in the
atmosphere.
MOCK SUNS, or _parhelia_, and mock-moons, or _paraselenæ_, are
continually observed in cold climates, where the tiny ice particles
are so abundant in the air. These phenomena were recognized by the
ancients, and halos round the sun can be observed by means of darkened
glasses. We annex an illustration of a mock sun and moon seen on the
continent of Europe. Readers of Mr. Whymper’s “Scrambles in the Alps”
will remember the gorgeous, and to the guides mysterious, fog-bow or
sun-bow seen as the survivors of the first and most fatal ascent of the
Matterhorn in 1865 were tremblingly pursuing their descent over the
upper rocks of that mountain.
The MIRAGE, or _Fata Morgana_, is a very curious but sufficiently
common phenomena, and in the Asiatic and African plains it is
frequently observed. When the weather is calm and the ground hot,
the Egyptian landscape appears like a lake, and the houses look like
islands in the midst of a widely-spreading expanse of water. This
causes the _mirage_, which is the result of evaporation, while the
different temperatures of the air strata cause an unequal reflection
and refraction of light, which give rise to the mirage. Travellers are
frequently deceived, but the camels will not quicken their usual pace
until they scent water.
The Fata Morgana and the inverted images of ships seen at sea are not
uncommon on European coasts. Between Sicily and Italy this effect is
seen in the Sea of Reggio with fine effect. Palaces, towers, fertile
plains, with cattle grazing on them, are seen, with many other
terrestrial objects, upon the sea—the palaces of the Fairy Morgana.
The inverted images of ships are frequently perceived as shown in the
illustration (fig. 729), and many most extraordinary but perfectly
authentic tales have been related concerning the reflection and
refraction of persons and objects in the sky and on land, when no human
beings nor any of the actual objects were within the range of vision.
It will be well to explain this phenomenon, and the diagrams will
materially assist us in so doing, for the appearances are certainly
startling when realized for the first time. The Spectre of the
Brocken we see mimics our movements, and we can understand it. But
when apparently solid buildings appear where no buildings have been
erected,—when we see—as has been perceived—soldiers riding across a
mountain by a path, or ledge, perfectly inaccessible to human beings
even on foot, we hesitate, and think there is something uncanny in the
sight. Let us now endeavour to explain the mirage.
Suppose that in the annexed diagram the space enclosed between the
letters A, B, C, D, be a glass vessel full of water. The ship is below
the horizon, the eye being situated at E—the glass vessel of course
representing the atmosphere charged with moisture. The eye at E will
perceive the top of the mast of the ship, S, and we may imagine a
line drawn from E to S. Then put a (short focus) convex lens at _a_
just above this (imaginary) line, and a concave one, _b_, just over
it. Through the former an inverted ship will be seen, and an erect
one through the latter at S′ and S″ respectively. We now have the
effect in the air just as reproduced in nature by the difference in
temperature in air strata, which cause it to act like a concave lens
when the density of the water diminishes towards the centre, and like a
convex lens when it is increased.
[Illustration: Fig. 730.—Explanation of Mirage.]
This can be proved by heating the air (by hot irons) above the glass
vessel filled with oil, and the effects will be just the same as
through the lenses. Dr. Wollaston obtained the mirage by using a clear
syrup,—about one-third of the vessel full,—and filling it with water.
The gradual mingling of these fluids will produce the phenomenon. The
illustration in the margin (fig. 731) shows us the rays proceeding from
the ship’s hull, and refracted into the line reaching the eye, above
the line proceeding from the mast, so the ship appears hull uppermost;
the rays cross at _x_. But if they did not cross before they reach the
eye, the image would appear as at _s´_ _p´_ in an erect position.
[Illustration: Fig. 731.—The Mirage.]
The Spectre of the Brocken arises from a different cause. Such
appearances are only shadows,—projected on thin clouds or dense vapours
at sunrise, or when the sun’s rays are directed horizontally,—for of
course vertical rays will throw the shadow on the ground on to the
zenith. Balloons are also reflected thus, and much interest has been
caused by the appearance of a twin balloon, until the aerial voyagers
have discovered the cheat by seeing the shadowy aeronaut imitating
their actions, and the second balloon has been discovered to be an airy
nothing.
FOOTNOTES:
[34] “Fulgarites” are composed of melted quartz, and on Mount Ararat
many have been found which give a character to the formation of the
lesser Ararat.