"The atmosphere is an envelope or covering for the distribution of light
and heat over the Earth; it is a sewer into which, with every breath we
draw, we cast vast quantities of dead animal matter; it is a laboratory
for purification, in which that matter is recompounded, and wrought
again into wholesome and healthful shapes; it is a machine for pumping
up all the rivers from the sea, and for conveying the water from the
ocean to their sources in the mountains. It is an inexhaustible
magazine, marvellously stored; and upon the proper working of this
machine depends the well-being of every plant and animal that inhabits
the Earth."
An element whose operations are so manifold and so important could not
fail to engage the study of philosophic men in all ages; but so
difficult has been that study that little progress was made until very
recently, when men, acting in unison in all parts of the world, have, by
collating their observations, become acquainted with some of those laws
which govern the atmosphere, and direct its courses and velocities.
In early ages very little indeed was known about the wind beyond the
palpable facts of its existence, its varied condition, and its
tremendous power; and men's observations in regard to it did not extend
much beyond the noting of those peculiar and obvious aspects of the sky
which experience taught them to regard as evidences of approaching
storm. But, although such aspects of the heavens were, and always will
be, pretty safe and correct indicators of the Weather, they are by no
means infallible; and in some regions and under certain conditions they
are wanting altogether.
When the sea captain observes a lowering aspect of the sky, with, it may
be, a dark line above the distant edge of the sea, he knows--however
calm and unruffled may be the ocean around him--that wind may be
expected; and, calling the crew, he orders sail to be taken in, and
preparation made for the approaching breeze. But there are times when
no such warning is given, when the atmospheric is perfectly still, the
sea calm as glass, and the vessel floats motionless with her sails
hanging idly from the yards, as if she were:
A painted ship upon a painted Ocean.
Suddenly, and before preparation can be made to withstand it, the
hurricane bursts in appalling fury over the sea: the sails are blown to
ribbons; the masts, perhaps, broken down; and frequently the vessel
itself overwhelmed and sent to the bottom. Many a gallant ship, which
has left the harbour ably commanded and well manned, and never more been
heard of, has doubtless gone down in sudden storms such as those we have
referred to.
But the inventions of science have now very much lessened the danger of
these storms. The barometer, by the sudden fall of its column of
mercury, tells, as plainly and certainly as if it spoke with an audible
voice, that a storm is approaching, even though all nature should appear
to contradict the fact by its calm and serene aspect; so that the crew
thus warned have time to furl the sails, fasten down the hatches, and
otherwise prepare to face the impending danger.
The atmosphere flows in a grand harmonious system of currents and
counter-currents, with their corresponding eddies, just like the ocean;
and the grand final results of its varied action are to equalise in some
degree the temperatures of the world, to carry off and distribute
moisture where it is required, to sweep away noxious vapours, and
generally to ventilate the Earth and gladden the heart of man.
The primary cause of all wind is the combined action of heat and cold.
If the world were heated with perfect equality all round, there would
be, as far at least as heat is concerned, a perfect and permanent
stagnation of the atmosphere; and this would speedily result in the
destruction of every living thing. But by the varied and beautiful
arrangements which the Almighty has made in nature He has secured a
regular flow of atmospheric currents, which will continue unalterably to
move as long as the present economy of things exists. The intense and
constant action of the sun's rays in the torrid zone produces great
heat, while the less powerful and frequently interrupted influence of
his rays in the frigid zones induces extreme cold. Hence we have in one
region heated air, in another cool air. Now, the effect of heat upon
air is to expand it, make it light, and cause it to rise. The moment it
does so, the cold air rushes in to supply its place; and this rushing in
of the cold air is what we call wind.
It may surprise many people to be told that there are only two great and
never-ceasing courses of the winds of this world--namely, north and
south. They flow perpetually from the equator to the poles, and from
the poles to the equator. All the irregularities and interruptions that
we observe are mere temporary and partial deflections from this grand
course. The heated air at the equator rises continually and flows in an
upper current towards the pole, getting gradually cooled on its way
north. That from the pole flows in an under current towards the
equator, getting gradually heated on its way south. We speak only of
the Northern Hemisphere, for the sake of simplifying explanation,--the
action of the great wind-current in the Southern Hemisphere is precisely
similar.
But our broad simple statement about the upper current from the equator,
and the under current from the pole, requires a slight modification,
which we thought it best not to mingle with the statement itself. The
heated air from the equator does indeed _commence_ to flow in an upper
current, and the cooled air from the pole in an under current; but, as
the upper currents of air are speedily cooled by exposure to space, and
the under currents are heated by contact with the earth's surface, they
constantly change places--the lower current becoming the upper, and
_vice versa_. But they do not change _direction_. The Equatorial
Current ascends, rushes north to a point about latitude 30 degrees,
where, being sufficiently cooled, it swoops down, and continues its
Northward rush along the earth. At another point the Polar Current
quits the earth, and soaring up, in consequence of its recently acquired
heat, becomes the upper current. This change in the two currents takes
place twice in their course.
Of course, the effect of these changes is to produce north winds in one
latitude and south winds in another, according to the particular wind
(equatorial or polar) that happens to be in contact with the earth. At
the points where these two currents cross, in changing places, we
necessarily have calms, or conflicting and variable winds.
Here, then, we have the first of the constant disturbing causes, and of
apparent irregularities, in the winds. The Earth, as every one knows,
whirls rapidly on its axis from west to east. At the equator the whirl
is so rapid that the atmosphere does not at once follow the Earth's
motion. It lags behind, and thus induces an easterly tendency to the
winds, so that a north wind becomes a north-east, and a south wind a
south-east. Here we have another constant cause of variation from the
northerly and southerly flow. We thus account for an easterly tendency
to the winds, but whence their westerly flow? It is simply explained
thus:
The motion of the Earth is greatest at the equator. It diminishes
gradually towards the poles, where there is no motion at all. The
atmosphere partakes of the Earth's motion when in contact with it; and
when thrown upwards by heat, as at the equator, it keeps up the motion
for some time, as it meets with no resistance there. Bearing this in
mind, let us now follow a gush of warm atmosphere from the equator. It
rushes up, and, turning north and south, seeks the poles. We follow the
northern division. When it left the Earth it had acquired a very strong
motion _towards_ the east,--not so great as that of the Earth itself,
but great enough to be equivalent to a furious gale from west to east.
If we suppose this air to redescend whence it rose, it would, on
reaching the equator, find the Earth going too fast for it. It would
lag a little, and become a gentle easterly breeze. But now, throw aside
this supposition;--our breeze rushes north; at latitude 30 degrees it
has got cooled, and swoops down upon the Earth; but the Earth at this
latitude is moving much slower than at the equator; the wind, however,
has lost little or none of its easterly velocity. On reaching the Earth
it rushes east much faster than the Earth itself, and thus becomes a
westerly gale.
There are, however, many other agents at work, which modify and disturb
what we may call the legitimate flow of the wind; and these agents are
diverse in different places, so that the atmosphere is turned out of a
straight course, and is caused to deflect, to halt, and to turn round:
sometimes sweeping low as if in haste; at other times pausing, as if in
uncertainty; and often whirling round, as if in mad confusion. To the
observer, who sees only the partial effects around his own person, all
this commotion seems but the disorderly action of blind chance; but to
the eye of Him who sees the end from the beginning, we may certainly
conclude that naught is seen but order and perfect harmony. And to the
eye of Science there now begins to appear, in what was formerly an
atmospheric chaos, an evidence of design and system, which is not,
indeed, absolutely clear, but which is nevertheless abundantly
perceptible to minds that cannot hope in this life to see otherwise than
"through a glass, darkly."
The causes which modify the action of the winds are, as we have said,
various. Local causes produce local currents. A clear sky in one
region allows the sun's rays to pour upon, let us say, the ocean,
producing great heat; the result of which is evaporation. Aqueous
vapour is very light, therefore it rises; and in doing so the aqueous
particles carry the air up with them, and the wind necessarily rushes in
below to supply its place. The falling of heavy rain, in certain
conditions of the atmosphere, has the effect of raising wind.
Electricity has also, in all probability, something to do with the
creation of motion in the atmosphere. Now, as these are all local
causes, they produce local--or what, in regard to the whole atmosphere,
may be termed irregular--effects. And as these causes or agents are in
ceaseless operation at all times, so their disturbing influence is
endless; and hence the apparent irregularity in the winds.
But these causes are themselves, not less than their results, dependent
on other causes or laws, the workings of which are steady and unvarying;
and the little irregularities that appear to us in the form of
fluctuating and changing winds and calms may be compared to the varying
ripples and shifting eddies of a river, whose surface is affected by the
comparatively trifling influences of wind, rain, and drought, but whose
grand onward course is never for a single moment interrupted.
Among these disturbing influences, the Gulf Stream is a very important
one. It is constantly sending up large volumes of steam, which, rising
into the air, induce a flow of wind from both sides towards its centre.
And many of the storms that arise in other parts of the Atlantic make
for this stream, and follow its course.
So much has been ascertained by scientific investigation of the winds,
that we can now distinctly map out the great belts or currents which
pass right round the world. We can tell in which parallels winds with
easting, and in which those with westing, in them, will be most
frequently found; and by directing our course to such places, we can to
a certain extent count upon profiting by the winds that will be most
suitable. Before the facts of atmospheric circulation were known,
mariners sailed by chance. If they happened to get into the belt of
wind that suited them, their voyages were favourable; if they got into
the wrong region, their voyages were unfavourable,--that was all. But
they had no idea that there was any possibility of turning the tables,
and, by a careful investigation of the works of the Creator, coming at
last to such knowledge as would enable them to reduce winds and waves,
in a great degree, to a state of slavery, instead of themselves being at
their mercy.
The world may be said to be encircled by a succession of belts of wind,
which blow not always in the same direction, but almost invariably with
the same routine of variations. A vessel sailing from north to south
encounters these belts in succession. To mariners of old, these varying
winds seemed to blow in utter confusion. To men of the present time,
their varied action is counted on with some degree of certainty. The
reason why men were so long in discovering the nature of atmospheric
circulation was, that they were not sufficiently alive to the immense
value of united effort. They learned wisdom chiefly from personal
experience--each man for himself; and in the great majority of cases,
stores of knowledge, that would have been of the utmost importance to
mankind, were buried with the individuals who had laid them up.
Moreover, the life of an individual was too short, and his experience
too limited, to enable him to discover any of the grand laws of Nature;
and as there was no gathering together of information from all quarters,
and all sorts of men, and all seasons (as there is now), the knowledge
acquired by individuals was almost always lost to the world. Thus men
were ever learning, but never arriving at a knowledge of the truth.
May we not here remark, that this evil was owing to another evil--
namely, man's ignorance of, or indifference to, the duty of what we may
term human communication? As surely as gravitation is an appointed law
of God, so surely is it an appointed duty that men shall communicate
their individual knowledge to each other, in order that the general
knowledge of the species may advance and just in proportion to the
fidelity with which men obey this duty--the care and ability with which
they collate and systematise and investigate their knowledge--will be
the result of their efforts.
In order to make the above remarks more clear as regards atmospheric
phenomena, let us suppose the case of a sailor who makes the same voyage
every year, but not precisely at the same time each year (and it must be
remembered that the rigid punctuality at starting which now holds good
did not exist in former times). In his first voyage he had to cross,
say, four of the wind-belts. While crossing belt number one, he
experiences south-west winds chiefly, and, being an observant man, notes
the fact. In belt number two he encounters westerly winds. In number
three he is in a region of variable winds and calms. In this region the
winds blow all round the compass, averaging about three months from each
quarter. But our sailor does not know that; he does not stay there all
the year to make notes; he passes on, having recorded his experience.
In crossing belt number four, he finds the prevailing winds to be
easterly.
Next year he sets forth again but merchants are not always punctual.
The lading cannot be completed in time, or adverse winds render the
setting sail unadvisable. At length, after a month or six weeks' delay,
he proceeds on his voyage, and finds belt number one perhaps much the
same as last year. He congratulates himself on his good fortune, and
notes his observations; but in belt number two, the wind is somewhat
modified, owing to its being later in the season,--it is rather against
him. In number three it is right in his teeth, whereas last year it was
quite in his favour. In number four, which we will suppose is the
trade-wind belt (of which more hereafter), he finds the wind still
easterly. Here, then, is the groundwork of confusion in our sailor's
mind. He has not the remotest idea that in belt number one the wind
blows chiefly, but not always, in one particular direction; that in
number four it blows invariably in one way; and that in number three it
is regularly irregular. In fact, he does not know that such belts exist
at all, and his opportunities of observing are not sufficiently frequent
or prolonged to enable him to ascertain anything with certainty.
Now, when we remember that in this imperfect experience of his he is
still further misled by his frequently encountering _local_
vicissitudes--such as storms and calms resulting from local and
temporary causes--we see how confusion becomes worse confounded. No
doubt he does gather some few crumbs of knowledge; but he is called on,
perhaps, to change his scene of action. Another ship is given to him,
another route entered on, and he ceases altogether to prosecute his
inquiries in the old region. Or old age comes on; and even although he
may have been beginning to have a few faint glimmerings as to laws and
systems in his mind, he has not the power to make much of these. He
dies; his knowledge is, to a very large extent, lost, and his log-books
disappear, as all such books do, nobody knows or cares where.
Now this state of things has been changing during the last few years.
Log-books are collected in thousands. The experiences of many men, in
reference to the same spots in the same years, months, and even hours,
are gathered, collated, and compared; and the result is, that although
there are conflicting elements and contradictory appearances, order has
been discovered in the midst of apparent confusion, and scientific men
have been enabled to pierce through the chaos of littlenesses by which
the world's vision has been hitherto obscured, and to lay bare many of
those grand progressions of nature which move unvaryingly with stately
step through space and time, as the river, with all its minor eddies and
counter-currents, flows with unvarying regularity to the ocean.