TRADE-WINDS--STORMS--THEIR EFFECTS--MONSOONS--THEIR VALUE--LAND AND SEA
BREEZES--EXPERIMENTS--HURRICANES--THOSE OF 1801--ROTATORY STORMS--THEIR
TERRIBLE EFFECTS--CHINA SEAS--HURRICANE IN 1837--WHIRLWINDS--WEIGHT OF
ATMOSPHERE--VALUE OF ATMOSPHERIC CIRCULATION--HEIGHT OF ATMOSPHERE.
Before proceeding to speak of the power and the dreadful effects of
wind, it is necessary to say a word or two about the trade-winds.
It is supposed that the "trades" derived their name from the fact of
their being favourable to navigation, and, therefore, to trade. They
consist of two belts of wind, one on each side of the equator, which
blow always in the same direction.
In the last chapter it was explained that the heated atmosphere at the
equator rises, and that the cooler atmosphere from the poles rushes in
to supply its place. That which rushes from the south pole is, of
course, a south wind, that from the north pole a north wind; but, owing
to the Earth's motion on its axis from west to east, the one becomes a
north-east, the other a south-east wind. These are the north-east and
the South-east "trades." They blow regularly--sometimes gently,
sometimes fiercely--all the year round. Between the two is a belt of
calms and changeable breezes, varying from 150 to 500 miles broad--
according to the time of the year--where there are frequent and violent
squalls, of very short duration, accompanied with heavy rains. This
region is called by seamen the "doldrums," and considerable trouble and
difficulty do ships experience in crossing it.
It has already been explained that about latitude 30 degrees, the upper
current of wind from the south descends. At the same point the upper
current from the north also descends. They cut through each other, and
the point where these two cut each other is the northern limit of the
north-east trade-winds. The same explanation holds in regard to the
southern limit of the south-east trades.
In the accompanying diagram the arrows within the circle point out the
direction of the north-east and the south-east "trades" between the
tropics of cancer and capricorn, and also the counter currents to the
north and south of these, while the arrows around the circle show how
counter currents meet and rise, or descend, and produce the calm belts.
We have hitherto enlarged chiefly on the grand currents of the
atmosphere, and on those modifying causes and effects which are
perpetual. Let us now turn to the consideration of those winds which
are produced by local causes, and the effects of which are partial.
And here we are induced to revert to the Gulf Stream, which has been
already referred to as a _local_ disturber of the regular flow of the
atmosphere. This immense body of heated water, passing through cold
regions of the sea, has the effect of causing the most violent storms.
The hurricanes of the West Indies are among the most violent in the
world. We have read of one so violent that it "forced the Gulf Stream
back to its sources, and piled up the water in the Gulf to the height of
thirty feet. A vessel named the _Ledbnry Snow_ attempted to ride it
out. When it abated, she found herself high up on the dry land, having
let go her anchor among the tree-tops of Elliott's quay! The Florida
quays were inundated many feet; and it is said the scene presented in
the Gulf Stream was never surpassed in awful sublimity on the ocean.
The water thus dammed up rushed out with frightful velocity against the
fury of the gale, producing a sea that beggared description."
The monsoons of the Indian Ocean are among the most striking and regular
of the locally-caused winds. Before touching on their causes, let us
glance at their effects. They blow for nearly six months in one
direction, and for the other six in the opposite direction. At the
period of their changing, terrific gales are frequent--gales such as we,
in our temperate regions, never dream of.
What is termed the rainy season in India is the result of the south-west
monsoon, which for four months in the year deluges the regions within
its influence with rain.
The commencement of the south-west monsoon is described as being sublime
and awful beyond description. Before it comes, the whole country is
pining under the influence of long-continued drought and heat; the
ground is parched and rent; scarcely a blade of verdure is to be seen
except in the beds of rivers, where the last pools of water seem about
to evaporate, and leave the land under the dominion of perpetual
sterility. Man and beast pant for fresh air and cool water; but no cool
breeze comes. A blast, as if from the mouth of a furnace, greets the
burning cheek; no blessed drops descend; the sky is clear as a mirror,
without a single cloud to mitigate the intensity of the sun's withering
rays. At last, on some happy morning, small clouds are seen on the
horizon. They may be no bigger than a man's hand, but they are blessed
harbingers of rain. To those who know not what is coming, there seems
at first no improvement on the previous sultry calms. There is a sense
of suffocating heat in the atmosphere; a thin haze creeps over the sky,
but it scarcely affects the broad glare of the sun.
At length the sky begins to change. The horizon becomes black. Great
masses of dark clouds rise out of the sea. Fitful gusts of wind begin
to blow, and as suddenly to cease; and these signs of coming tempest
keep dallying with each other, as if to tantalise the expectant
creation. The lower part of the sky becomes deep red, the gathering
clouds spread over the heavens, and a deep gloom is cast upon the earth
and sea.
And now the storm breaks forth. The violent gusts swell into a
continuous, furious gale. Rain falls, not in drops, but in broad
sheets. The black sea is crested with white foam, which is quickly
swept up and mingled with the waters above; while those below heave up
their billows, and rage and roar in unison with the tempest. On the
land everything seems about to be uprooted and hurled to destruction.
The tall straight cocoa-nut trees are bent over till they almost lie
along the ground; the sand and dry earth are whirled up in eddying
clouds, and everything movable is torn up and swept away.
To add to the dire uproar, thunder now peals from the skies in loud,
continuous roars, and in sharp angry crashes, while lightning plays
about in broad sheets all over the sky, the one following so close on
the other as to give the impression of perpetual flashes and an
unintermitting roar; the whole scene presenting an aspect so awful, that
sinful man might well suppose the season of the Earth's probation had
passed away, and that the Almighty were about to hurl complete
destruction upon his offending creatures.
But far other intentions are in the breast of Him who rides upon the
storm. His object is to restore, not to destroy--to gladden, not to
terrify. This tempestuous weather lasts for some days, but at the end
of that time the change that comes over the face of nature seems little
short of miraculous. In the words of Mr Elphinstone, who describes
from personal observation--"The whole earth is covered with a sudden but
luxuriant verdure, the rivers are full and tranquil, the air is pure and
delicious, and the sky is varied and embellished with clouds.
"The effect of this change is visible on all the animal creation, and
can only be imagined in Europe by supposing the depth of a dreary winter
to start at once into all the freshness and brilliancy of spring. From
that time the rain falls at intervals for about a month, when it comes
on again with great violence; and in July the rains are at their height.
During the third month they rather diminish, but are still heavy. In
September they gradually abate, and are often suspended till near the
end of the month, when they depart amid thunders and tempests, as they
came."
Such are the effects of the monsoons upon land and sea. Of course the
terrific gales that usher them in and out could not be expected to pass
without doing a good deal of damage, especially to shipping. But this
is more than compensated by the facilities which they afford to
navigation.
In many parts of the world, especially in the Indian Ocean, merchants
calculate with certainty on these periodical winds. They despatch their
ships with, say, the north-east monsoon, transact business in distant
lands, and receive them back, laden with foreign produce, by the
south-west monsoon. If there were no monsoons, the voyage from Canton
to England could not be accomplished in nearly so short a time as it is
at present.
And now as to the cause of monsoons. They are, for the most part,
_deflected trade-winds_. And they owe their deflection to the presence
of large continents. If there were no land near the equator, the
trade-winds would always blow in the same manner right round the world;
but the great continents, with their intensely-heated surfaces, cause
local disturbance of the trade-winds. When a trade-wind is turned out
of its course, it is regarded as a monsoon. For instance, the summer
sun, beating on the interior plains of Asia, creates such intense heat
in the atmosphere that it is more than sufficient to neutralise the
forces which cause the trade-winds to blow. They are, accordingly,
arrested and turned back. The great general law of the trades is in
this region temporarily suspended, and the monsoons are created.
It is thus that the heated plains of Africa and Central America produce
the monsoons of the Atlantic, the Pacific, and the Gulf of Mexico.
We think it unnecessary to explain minutely the causes that produce
variation in the monsoons. Every intelligent reader will readily
conceive how the change of seasons and varied configuration as well as
unequal arrangement of land and water, will reverse, alter, and modify
the direction and strength of the monsoons.
Land and sea breezes are the next species of wind to which we would
direct attention. They occur in tropical countries, and owe their
existence to the fact that the land is much more easily affected by
sudden changes of temperature than the sea. Thus, the land in warm
regions is much heated by the sun's rays during the day; the atmosphere
over it becomes also heated, in virtue of which it rises: the cool
atmosphere over the sea rushes in to supply its place, and forms the
_sea breeze_: which occurs only during the day.
At night the converse of this takes place. Land heats and cools
rapidly; water heats and cools slowly. After the sun sets, the cooling
of the land goes on faster than that of the sea. In a short time the
atmosphere over the land becomes cooler than that over the sea; it
descends and flows off out to sea; thus forming the _land breeze_. It
occurs only at night, and when the change from one to the other is
taking place there is always a short period of calm. Land and sea
breezes are of the greatest use in refreshing those regions which,
without them, would be almost, if not altogether, uninhabitable.
In "The Tempest," an interesting work on the origin and phenomena of
wind, published by the Society for Promoting Christian Knowledge, a
curious and simple experiment is described, whereby the existence of
upper and under currents of air and the action of land and sea breezes
may be clearly seen and understood. We quote the passage:--
"The existence of the upper and under currents of air which mark the
phenomena of the trade-winds, and of land and sea breezes, may be
beautifully illustrated in two adjoining rooms, in one of which a good
fire is burning, while in the other there is none. If the door between
the two rooms be thrown open, the cold air will enter the heated room in
a strong current, or, in other words, as a violent wind. At the same
time the heated air of the warm room ascends and passes the contrary way
into the cold room, at the upper part of the same doorway; while in the
middle of this opening, exactly between the two currents, the air
appears to have little or no motion. The best way to show this
experiment is to introduce the flame of a candle into the doorway
between a hot and a cold room. If the flame be held near the bottom of
the doorway, where the air is most dense, it will be strongly drawn
towards the heated room; and if held near the top of the door it will be
drawn towards the cold room with somewhat less force; while midway
between the top and bottom the flame will be scarcely disturbed.
"There is also another pretty experiment which illustrates well the
theory of land and sea breezes. Take a large dish, fill it with cold
water, and in the middle of this put a water-plate or a saucer filled
with warm water. The first will represent the ocean, and the latter an
island made hot by the rays of the sun, and rarefying the air above it.
Take a lighted wax candle and blow it out; and, if the air of the room
be still, on applying it successively to every side of the saucer, the
smoke will be seen moving towards the saucer and rising over it, thus
indicating the course of the air from sea to land. On reversing the
experiment, by filling the saucer with cold water (to represent the
island at night) and the dish with warm water, the land breeze will be
shown by holding the smoking wick over the edge of the saucer; the smoke
will then be wafted to the warmer air over the dish."
We have just tried the first of these experiments, with complete
success. We would, however, recommend a piece of twisted brown paper,
lighted and blown out, instead of a wax candle, because it gives out
more smoke and is probably more obtainable on short notice. The
experiment of the doorway, moreover, does not require that there should
lie two rooms with a door between. We have found that the door of our
study, which opens into a cold passage, serves the purpose admirably.
Were we treating chiefly of the atmosphere in this work, it would be
necessary that we should enlarge on all the varieties of winds, with
their causes, effects, and numerous modifications. But our main subject
is the Ocean. The atmosphere, although it could not with justice have
been altogether passed over, must hold a secondary place here; therefore
we will conclude our remarks on it with a brief reference to hurricanes.
It has been ascertained that most of the great storms that sweep with
devastating fury over the land and sea are not, as was supposed,
rectilinear in their motion, but circular. They are, in fact, enormous
whirlwinds, sometimes upwards of one hundred and fifty miles in
diameter; and they not only whirl round their own centres, but advance
steadily forward through space.
In the year 1831, a memorable and dreadful series of storms passed over
some of the India Islands, and caused terrible havoc, especially in the
island of Barbadoes. The peculiarity of these hurricanes was that they
ravaged the different islands at different dates, and were therefore
supposed to be different storms. Such, however, was not the case. It
was one mighty cyclone, or circular storm,--a gigantic whirlwind,--which
traversed that region at the rate of about sixteen miles an hour. It
was not its progressive, but its rotatory motion, that constituted its
terrible power. On the 10th of August it reached Barbadoes; on the
11th, the islands of Saint Vincent and Saint Lucia; on the 12th it
touched the southern coast of Porto Rico; on the 13th it swept over part
of Cuba; on the 14th it encountered Havanna; on the 17th it reached the
northern shores of the Gulf of Mexico and travelled on to New Orleans,
where it raged till the 18th. It thus, in six days, passed, as a
whirlwind of destruction, over two thousand three hundred miles of land
and sea. It was finally dissipated amid heavy rains.
The effect of a hurricane is well described by Washington Irving.
"About mid-day," he says, "a furious gale sprang up from the east,
driving before it dense volumes of cloud and vapour. Encountering
another tempest from the west, it appeared as if a violent conflict
ensued. The clouds were rent by incessant flashes, or rather streams,
of lightning. At one time they were piled up high in the sky, at
another they descended to the earth, filling the air with a baleful
darkness, more impenetrable than the obscurity of midnight. Wherever
the hurricane passed, whole tracts of forest were shivered and stripped
of their leaves and branches; and trees of gigantic size, which resisted
the blast, were torn up by the roots and hurled to a great distance.
Groves were torn from the mountain-precipices, and vast masses of earth
and rock precipitated into the valleys with terrific noise, choking the
course of the rivers.
"The fearful sounds in the air and on the earth, the pealing thunder,
the vivid lightning, the howling of the wind, the crash of falling trees
and rocks, filled every one with affright, and many thought that the end
of the world was at hand. Some fled to caverns for safety, for their
frail houses were blown down, and the air was filled with the trunks and
branches of trees, and even with fragments of rocks, carried along by
the fury of the tempest. When the hurricane reached the harbour, it
whirled the ships round as they lay at anchor, snapped their cables, and
sunk three of them to the bottom with all who were on board. Others
were driven about, dashed against each other, and tossed mere wrecks
upon the shore by the swelling surges of the sea, which in some places
rolled for three or four miles upon the land. This tempest lasted for
three hours."
The China seas are the most frequently visited by severe tempests, or
typhoons; yet of all vessels, the Chinese junks, as they are called,
seem to be least adapted by their build for encountering such storms.
A terrible hurricane burst upon the China seas in the month of January
1837, as we learn from the "United Service Journal" of that year. An
English vessel was exposed to it. The sea, rising in mountains around
and over the ship's sides, hurled her rapidly on her passage homeward,
when suddenly a wreck was discovered to the westward. The order to
shorten sail was given, and promptly obeyed; and when they neared the
wreck they found her to be a Chinese junk without mast or rudder--a
helpless log on the breast of that boiling sea.
There were many Chinamen on deck vehemently imploring assistance. The
exhibition of their joy on beholding the approach of the stranger was of
the wildest and most extravagant nature; but it was doomed to be
suddenly turned to despair, as the violence of the storm drove the ship
past the wreck. It became necessary to put her on the other tack, a
manoeuvre which the poor creatures construed into abandonment, and the
air rang with the most agonising shrieks of misery. But hope was again
raised, when a boat was lowered and a rope thrown on board for the
purpose of towing the junk to the ship. This intention was frustrated
by the windlass breaking. At sight of this one man, in a paroxysm of
despair, jumped overboard after the rope; but he missed it. Being a
good swimmer, he tried to reach the boat; but his feeble power could
avail him nothing in the midst of such raging elements: he speedily sank
to rise no more.
Another rope, however, was secured to the junk, and by means of it the
rest of the crew (eighteen in number) were saved. Their gratitude was
boundless. They almost worshipped the officers, the crew, and the
vessel, prostrating themselves and kissing the feet of the former, and
the very planks of the latter.
Well-built ships, however, are not always able to withstand the violence
of rotatory storms. Instances occur in which the tightest built and
best manned ships are destroyed as suddenly as the clumsiest of
ill-managed junks. Not many years ago, a vessel was proceeding
prosperously on her voyage, when signs of a coming tempest induced the
wary captain to reduce, and, finally, to take in all sail. But his
precautions were in vain. The storm burst on the devoted ship, and in a
few minutes the masts went over the side, and the hull lay a total wreck
upon the sea.
These hurricanes or cyclones, although in reality whirlwinds, are so
large that man's eye cannot measure them, and it is only by scientific
investigation that we have arrived at the knowledge of the fact. The
whirlwind, properly so called, is a much smaller body of atmosphere.
Sometimes we see miniature whirlwinds, even in our own temperate land,
passing along a road in autumn, lifting the leaves and dust into the air
and carrying them along in the form of a rotatory pillar. In other
regions they exert a power quite equal to the tempest, though in a more
limited space, overturning houses, uprooting trees, cutting a track
twenty or thirty yards wide through the dense forest as thoroughly as if
a thousand woodmen had been at work there for many years.
When whirlwinds pass from the land to the sea they create waterspouts;
of which we shall have something to say in another chapter. Meanwhile,
we think it may be interesting to give the following miscellaneous
information regarding the atmosphere, gathered from the work of Dr
Buist, who devoted much earnest study to the subject of atmospheric
phenomena.
"The weight of the atmosphere is equal to that of a solid globe of lead
sixty miles in diameter. Its principal elements are oxygen and nitrogen
gases, with a vast quantity of water suspended in them in the shape of
vapour; and, commingled with these, a quantity of carbon in the shape of
fixed air, sufficient to restore from its mass many-fold the coal that
now exists in the world. Water is not compressible or elastic; it may
be solidified into ice or vaporised into steam: but the air is elastic
and compressible. It may be condensed to any extent by pressure, or
expanded to an infinite degree of tenuity by pressure being removed from
it. It is not liable to undergo any changes in constitution beyond
these, by any of the ordinary influences by which it is affected."
If the heating and cooling process--which we have described as being
carried on between the equator and the poles--were to cease, we should
have a furious hurricane rushing perpetually round the globe at the rate
of one thousand miles an hour,--ten times the speed of the most violent
tornado that has ever carried devastation over the surface of the earth.
The air, heated and dried as it sweeps over the arid surface of the
soil, drinks up by day myriads of tons of moisture from the sea,--so
much, indeed, that, were none restored to it, the surface of the ocean
would be depressed eight or ten feet annually.
We do not certainly know the height of the atmosphere. It is said that
its upper surface cannot lie nearer to us than fifty, and can scarcely
be further off than five hundred, miles. "It surrounds us on all sides,
yet we cannot see it; it presses on us with a weight of fifteen pounds
on every square inch of the surface of our bodies--in other words, we
are at all times sustaining a load of between seventy and one hundred
tons of it on our persons--yet we do not feel it! Softer than the
finest down, more impalpable than the lightest gossamer, it leaves the
cobweb undisturbed, and, at times, scarcely stirs the most delicate
flower that feeds on the dew it supplies; yet it bears the fleets of
nations on its wings round the world, and crushes the most refractory
substances with its weight. It bends the rays of the sun from their
path to give us the aurora of the morning and the twilight of evening.
It disperses and refracts their various tints to beautify the approach
and the retreat of the orb of day. But for the atmosphere, sunshine
would burst on us in a moment and fail us in the twinkling of an eye,
removing us in an instant from midnight darkness to the blaze of noon."
We have written a good deal on this subject, yet the thousandth part has
not been told of even the grand and more obvious operations of the
atmosphere, much less the actions and results of its minor and invisible
processes. Were we to descend with philosophers into the minuter
laboratories of the world, and consider the permeating, ramifying,
subtle part the atmosphere plays in the innumerable transformations that
are perpetually going on around and within us, we should be constrained
to feel more deeply than we have ever yet felt, that the works of the
Creator are indeed wonderful beyond all expression or conception.