THE TRADE WINDS.
There are few things more curious in the history of human knowledge
than the establishment of extensive errors as to matters of fact, and
the perverse tenacity with which they retain their hold on the public
mind. In some cases it would almost seem that the pleasure which
springs from genuine philosophical inquiry is subordinate to that
which arises from the indolent process of taking things for granted.
This applies peculiarly to the phenomena of the Trade-winds,
respecting which many erroneous ideas are generally entertained. To
professional men these fallacies are calculated to prove extremely
mischievous; while even to persons not directly connected with the
sea, the existence of error may often be injurious: and, although it
is not very easy to explain these things in a popular way, I shall
attempt to give a description of the facts as they really exist.
The main characteristics may easily be described.
The great belt of the earth's surface, nearly three thousand miles in
width, lying between the tropics (from 23-1/2° north to 23-1/2° south
latitude), is the chief region of the Trade-winds; though in some
parts of the world they extend to the latitude of 28° both north and
south of the equator; while at other places well within the tropics,
and even close to the line, totally different winds prevail. It is
only in the open parts of the Pacific and Atlantic oceans that the
true Trade-winds blow. In the Indian and China seas, and in many other
portions of the great tropical belt, periodical winds, called
Monsoons, are found. These shifting Trades exact the closest study
from the practical navigator, in consequence of their extensive
variety and seeming complication. But they are not less deserving the
attention of merely curious inquirers, from the beautiful manner in
which these modifications of the regular breezes obey the same general
laws which direct the grand phenomena of the Trades. Indeed, the most
extensive observation serves only to link the whole into one
harmonious chain or series of explanations, exhibiting the uniformity
as well as the exquisite adaptability of Nature, even in those
departments called "inconstant," where she is supposed to be most
capricious.
The only general assertion that can safely be made with respect to the
Trade-winds is, that they blow more or less from the eastern half of
the compass towards the western. On the north side of the equator, the
north-east Trade-winds blow; and on the south side, the south-east
Trade-winds. These two names have undoubtedly contributed to mystify
the subject by naturally suggesting to the imagination currents of air
blowing respectively from the north-east and the south-east, or at an
angle of 45° with the meridian. And I have even seen sailors (old
sailors too) quite surprised, and rather provoked, when they have
encountered very different winds in those parts of their voyage,
where, being misled by the force of names alone, they had taught
themselves to expect a regular breeze from a particular quarter. But,
in point of fact, the Trade-winds do very seldom blow directly from
north-east and south-east; neither are they uniform in their direction
on the same spot at different seasons of the year, nor is their
strength uniform from month to month. I may add, that the equatorial
limits, or bounding lines, of the trades, are not steadily confined to
the same latitude. In short, so far from these winds being perfectly
fixed in direction, force, and position, they are subject to very
considerable mutations, dependent on the position of the sun. Their
vast nautical value, in fact, as well as philosophical curiosity,
turns mainly on their uniformity, which, in spite of all the
fluctuations alluded to, gives them a very distinctive character.
Dr. Young and Hadley, the great authorities on the subject, are both
wrong in their conclusions.[3] Where Hadley obtains his "experience"
he does not tell; but certain it is, that no sailor who ever crossed
the equinoctial line could possibly have furnished him with two of his
principal statements. The Trades are not strongest near the equator,
as he states, nor when they reach that district do they blow along it,
or in a parallel direction, but almost always at right angles to it.
If the earth had no motion on its axis, but were surrounded as at
present with an atmosphere, and if the sun moved round and round it
exactly above the equator, without varying his declination, the
following effects would ensue: That portion of the earth lying, say
thirty degrees, on each side of the equator, being more exposed to the
action of the sun than those further from it, would become much
warmer; while the superincumbent air, being greatly heated by the
contact, would expand, or become specifically lighter, and would
consequently rise. The adjacent air, both on the north and south,
being cooler, and, of course, heavier, would rush in to supply the
place of the heated air. This air coming from the regions beyond the
tropics would, in its turn, be heated, and rise on reaching the warmer
equatorial regions, giving place to a fresh supply, which, it is easy
to see, must be furnished by the descent of that portion of air
formerly heated at the equator, raised into the cold regions of the
sky, and forced into a regular circuit by fresh elevations of heated
air. All these and many other interesting results are clearly
developed in Daniell's Meteorological Essays, a book which every one
at all interested in such inquiries will find it advantageous to
study. The first edition of this work was published in 1823, some
years after these speculations had been forced upon my notice by a
long course of service between the tropics.
It will be understood, that, as long as we imagine the globe at rest
while this circulation is going on, the course of the lower air along
the surface would be directly towards the equator, from due north in
one hemisphere, and from due south in the other; while in the upper
regions the currents would follow the opposite directions, and stream
towards the poles. But the instant we conceive the earth put into
rotatory motion from west to east, a change would take place in the
course of these aërial currents, both above and below. It must be
recollected that a volume of air, when once put in motion, will move
on, like any other body, by the mere force of its own momentum, till
that motion is destroyed by its friction against the substances along
or through which it is impelled. Any one who has observed the ring of
smoke sometimes projected from the mouth of a cannon will be sensible
that this is true.
It may likewise be of use, before going further, to consider, that, if
the globe, instead of being unequally heated; were equally heated at
all parts, from pole to pole, and being surrounded by an atmosphere,
were then made to revolve on its axis, it would carry the atmosphere
round with it exactly at the rate at which it was itself going. That
portion of the air in contact with the equator would move about 1000
miles in one hour, while that in latitude 90° would be as motionless
as the poles themselves.
From this it will be seen, that, while the equator moves at the rate
of 1000 miles an hour, the district about the latitude 30° moves only
860, or 140 miles slower. The average whirling velocity of the earth's
easterly motion, in the space between the equator and latitude 30°,
may be stated at 950 miles an hour; and that of the belt lying between
30° and 40°, at about 800 miles.
In the hypothetical case, above suggested, of the whole surface being
equally heated, and consequently the whole atmosphere at the same
temperature, there would be a universal calm, whatever might be the
rotatory motion impressed upon the earth. If, however, we next
suppose, what really is the case, that the air over the tropical
region is more heated than that which is farther from the equator,
this rarefied air will instantly ascend, and occupy a place above the
colder and denser air, which will flow in from the belts lying beyond
the tropics.
When the comparatively slow-moving air of the temperate zone, lying
beyond the tropics, first comes in contact with those quicker-moving
parts of the earth forming the tropical edges of the torrid zone, the
apparent motion of the air from the east, caused by the relative
difference of the rotatory velocity between the air and earth, is
great, compared to the other motion of the air, caused by its being
drawn directly towards the equator, to supply the place of the heated
portions raised into the sky. Consequently, at the tropical borders of
both Trades the wind is found to blow very nearly from the east point.
Since the cool air of the temperate and comparatively slow-moving
zones beyond the tropics is thus drawn towards the equator, and comes
successively in contact with parallels of latitude moving faster and
faster towards the east, there must be gradually imparted to it, by
the increased friction, a considerable degree of the increased
rotatory velocity belonging to the low latitudes it has now reached;
that is to say, there will be less and less difference of velocity
between the easterly motion of this temperate air and the easterly
motion of the earth; and, consequently, the wind, as it approaches the
equator, will appear to blow less and less directly from the
eastward. But, while the earth's rotation within the tropics is thus
acting on the slower-moving air which has travelled to it from beyond
the tropics, with increased friction at every successive moment, there
has been no such powerful counteracting influence in operation to
diminish the meridional motion impressed on the air in question; for,
although in proceeding from the tropics towards the equator, the wind
might, at first sight, be supposed to have its speed somewhat lessened
by friction along the earth's surface, the retardation due to this
cause, if there be any at all, must be inconsiderable, compared to
that which affects the motion caused by the difference in the rotatory
velocity of the earth at the different parallels. It must be
recollected, also, that there is a constant demand for fresh air from
the north and south, to occupy the place of the heated and rarefied
air which is raised up in the torrid zone; and this demand being
pretty equal, the motion it produces on the air in the direction of
the meridian must likewise be uniform.
If it be admitted that all the easterly character of the Trade-winds
is due to the difference of velocity between the rotation of the
torrid zone of the earth from west to east, and that of the air
impressed only with the slower rotatory motion to the east of the
temperate zone, it will follow, that, if this difference of velocities
between the earth and the air in contact with it be diminished or
annihilated, the easterly character of these winds will be diminished
or annihilated likewise. At the same time, there is no cause in
operation, that I can discover, to alter the direction of the
meridional motion, as it may be called, of the Trade-winds, or that
by which they are impelled directly towards the equator.
At first starting from the temperate zone, on its voyage to the
equator, the cold air of that slow-moving region is impressed with a
rotatory velocity of only 800 miles per hour to the eastward, but it
soon comes over parts of the earth moving more than 100 miles per hour
faster to the eastward than itself. The difference of velocity in the
earth's rotation between latitudes 30° and 20° is 74 miles an hour,
while between 20° and 10° it is only 45 miles, and in the next ten
degrees the difference in rate per hour is reduced to 15 miles.
The velocity with which the air drawn from beyond the tropics travels
along the sea towards the equator is probably not above twenty miles
an hour, a rate slow enough to allow time for the
constantly-increasing friction of the earth's rotation to act upon it,
and draw it more and more entirely to the east. By the time it has
reached the equatorial regions, the friction of the earth's surface
has operated long enough to carry the air completely along with it;
and, of course, all relative motion being done away with, everything
easterly in the character of the Trade-winds will be at an end.
But, although this constantly-increasing friction of the earth's
rotation has thus annihilated all relative easterly motion between the
air and earth, that air still retains its motion towards the equator;
and accordingly we do find the Trade-winds, at their equatorial
limits, blowing, not from the east, as Hadley, Dr. Young, and others,
conceived, but directly from the north and from the south
respectively. The strength and velocity of the Trades at these places
is, in general, considerably diminished, chiefly, perhaps, by the air
becoming heated, and rising up rather than flowing along; and also, no
doubt, by the meeting of the two opposite currents of air--one from
the north, the other from the south--which produces the intermediate
space called the Calms, or the Variables.
In strict conformity with these theoretical views, the clouds above
the Trades are almost invariably observed to proceed in the contrary
direction to the winds below. On the top of the Peak of Teneriffe I
found a gentle breeze blowing from the south-westward, directly
opposite to the course of the Trade-wind.
The more detailed circumstances usually met with in that part of a
voyage to India which lies between 30° north and 30° south, and which
I am about to describe, will now, I imagine, be readily understood.
Before setting out, however, I must strongly recommend any one wishing
to see these matters clearly, to have them fixed in his mind to useful
purpose, to follow both the theoretical and the practical parts of
this explanation with the assistance of a terrestrial globe.
Most ships touch at Madeira, either to take in a stock of wine, to get
fruit and vegetables, or to form a pleasant break in the early and
most disagreeable part of the voyage. Some ships pass barely in sight
of the high mountain which rises above the town of Funchal, and
satisfy themselves with taking sights for verifying the rates of their
chronometers when on the meridian of the island; while others
tantalise their passengers still more by sweeping through the roads,
without anchoring, or communicating with the shore. The captains by
such ships are pretty deeply, if not very loudly, abused by all hands,
passengers especially, who are perhaps the most dissatisfied, because
the most idle, of mortals. Shortly after leaving Madeira, which is in
32-1/2° north latitude, a ship may expect to meet the Trades; but she
cannot calculate with any certainty upon catching them till she
arrives at the parallel of 28°. On first reaching the Trade-wind it
will be found to blow very nearly from due east, and with this a
course is easily steered past or amongst the Canaries, and thence for
the Cape de Verdes. Some navigators pass within this group, others
keep so far out as barely to make San Antonio; and this, I think, is
considered the best route. As the ship proceeds to the southward, the
wind draws gradually round from the east to north-east, and eventually
to north-north-east, and even to north, at the southern margin of the
north-east Trade-wind.
The position of this margin or southern edge, which in technical
language is called the equatorial limit of the Trade, varies
considerably with the season of the year. From December to May
inclusive it frequently reaches as far as the 3rd degree of north
latitude, though it ranges about 5° and 6° north. From June to
November it is shifted back as far, sometimes, as 13° north, but it
seldom extends as far south as 8° north. Subjects which are treated of
in a series of tables showing the equatorial limits of both
Trade-winds, deduced by the late Captain James Horsburgh, hydrographer
of the East India Company, from the observations of 238 ships. These
tables show very clearly the effect of the absence or presence of the
sun in shifting the limits of the Trades, drawing them after him, as
it were. The presence of the sun in either hemisphere obstructs
considerably the regularity and strength of the Trade-winds in that
hemisphere, and _vice versâ_.
The great difficulty experienced in making the outward-bound voyage
commences after the ship has been deserted by the north-east Trade,
for she has then to fight her way to the southward across the region
of Calms and Variables. But as these Variables blow generally from the
southward and westward, from a cause afterwards to be explained, it is
obvious enough why this part of the homeward voyage is always more
easily made than the outward passage. These southerly breezes, which
are met with in the Variables, blow at times with considerable force,
and greatly perplex the young navigator, who, trusting perhaps to some
of the erroneous published accounts, not unnaturally reckons upon
meeting the regular Trade-wind, blowing, as he supposes, from the east
near the equator, not from the south; still less is he prepared or
pleased to find it blowing from the south-westward.
This troublesome range, intervening between the two Trades, varies in
width from 150 to more than 500 miles. It is widest in September, and
narrowest in December or January. I now speak more particularly of
what happens in the Atlantic. In the wide Pacific, far from land,
fewer modifying circumstances interfere with the regular course of the
phenomena, than in the comparatively narrow sea formed by the opposite
shoulders of Africa and South America.
Calms, also, are met with in this intermediate region, or purgatory of
the outward-bound voyage, and occasionally violent tornados or
squalls, which in a moment tear away every rag of canvas from a ship's
yards. For several hours at a time, also, rain falls down in absolute
torrents. Even when the weather clears up, and a fresh breeze comes,
it is generally from the southward, directly in the outward-bound
navigator's teeth. He must have patience, however, and strive to make
the most of it by keeping on that tack by which most southing is to be
gained. It is now, I believe, generally held to be the best practice
to place the ship between 18° and 23° of west longitude on losing the
north-east Trade; and likewise to endeavour, if possible, to cross the
equator somewhere between these two longitudes. Before reaching the
line, however, the navigator will almost always be met by the
south-east Trade-wind. From January to May he may expect to meet it in
1° or 2° north latitude; but in summer and autumn he will find the
northern or equatorial limit of the south-east Trade a degree or two
still further to the northwards of the lines.
On first encountering the south-east Trade an outward-bound ship is
obliged to steer much more to the westward than she wishes to do, in
consequence of the wind blowing so directly towards the equator, and
not along it, as some of the books will insist on, in spite of Nature.
So that if she be a dull sailer she may have some difficulty in
weathering the coast of Brazil about Cape St. Roque. As she proceeds
onwards, however, and makes a little more southing, the wind will haul
more and more round from the south to the south-east, then
east-south-east, and eventually to east at the southern limit of the
Trade-wind. An inexperienced sailor, on first entering the south-east
Trade, is very apt to be too solicitous about making southing, and
hugs the wind much too close; whereas he ought rather to keep his ship
off a little, give her a fathom or two of the fore and main sheets,
and take a small pull of the weather topsail and top-gallant braces,
to ensure making good way through the water. Indeed, many officers go
so far as to recommend flanking across the south-east Trade with a
fore-topmast studding-sail set. Although, I think, there can be no
doubt of the soundness of this advice, I confess that it does require
no inconsiderable degree of faith to adopt a course, which,
apparently, takes the ship not directly away from her object, but very
much out of the straight road. In this respect, it may be remarked
that the scale of navigation on every Indian voyage is so great, and
the importance of getting into those parallels where favourable
breezes are certain to be met with, of so much more consequence than
the gain of mere distance, that two or three hundred miles to the
right or left, or even twice that space, is often not to be regarded.
Accordingly, in cutting or flanking across the south-east Trade-wind,
the object, it should be remembered, is not to shorten the distance,
but to reach those latitudes where strong westerly gales are to be met
with, by help of which five hundred or a thousand miles of lost
distance are speedily made up, and the rest of the passage secured.
In those regions lying beyond the southern tropic westerly winds
prevail during the greater part of the year, exactly as we find on
this side of the northern tropic. In the southern hemisphere, and far
from the land, the wind may be said to blow from the westward almost
as steadily as the Trades do from the eastward. The great object,
therefore, for an outward-bound ship is to get far enough south to
ensure this fair wind. Beyond the latitude of 30°, and as far as 40°,
this purpose will generally be answered.
We are sufficiently familiar in England with the fact of westerly
winds prevailing in the Atlantic. From a list of the passages made by
the New York sailing packets across the Atlantic, during a period of
six years, it is shown that the average length of the voyage from
Liverpool to America, that is, towards the west, was forty days; while
the average length of the homeward passage, or that from west to east,
was only twenty-three days. And it may fix these facts more strongly
in the recollection, to mention that the passage-money from England to
America (in the days of sailing packets) was five guineas more than
that paid on the return voyage.
This prevalence of westerly winds beyond the tropics is readily
explained by the same reasoning which has been applied to the Trades
blowing within them. The swift moving air of the torrid zone, on being
rarefied and raised up, flows along towards the poles, and in a
direction from the equator, above the cooler and slower-moving air,
which, as I have already described, is drawn along the surface of the
earth from the temperate regions beyond the tropics. When the rarefied
equatorial air has travelled some thirty or forty degrees of latitude
along the upper regions of the atmosphere towards the poles it becomes
cooled, and is ready to descend again, between the latitudes of 30°
and 60°, to supply the place of the lower air, drawn off towards the
equator by the Trade-winds. But this partially-cooled air falls on a
part of the earth's surface which is moving much more slowly towards
the east, in its diurnal rotation, than the air which has descended
upon it, and which is still impressed with a great proportion of its
eastern velocity due to the equatorial parallels of latitude, where it
was heated and raised up. The necessary consequence of this is, to
produce a rapid motion in the air from the west over the earth's
surface; and this, combined with the other motion of the same portion
of air, or that which has driven it from the equatorial regions,
produces this remarkable prevalence of south-westerly winds in the
northern hemisphere, and north-westerly winds in the southern
hemisphere, in those districts lying between the latitudes of 30° and
60°.
In all that has been said above it has been assumed that the
quickest-moving or equatorial belt of the earth is also the hottest,
and consequently that over which the air has the greatest tendency to
rise. But, although this is generally true, it is not, by any means,
universally so. The variations, however, which are observed to occur
in those places where the circumstances form an exception to the
general rule, tend strongly to confirm the theory of Hadley. The
monsoons of India, as I shall presently show, are examples of this;
but the most striking instance with which I am personally acquainted
occurs in the Pacific Ocean, between the Bay of Panama and the
Peninsula of California, from latitude 8° to 22° north. If the huge
continent of Mexico were taken away, and only sea left in its place,
there can be no doubt but the ordinary phenomena of the Trade-winds
would be observable in that part of the Pacific above mentioned. Cool
air would then be drawn from the slow moving parallels lying to the
northward, towards the swift moving latitudes, near the equator, in
order to supply the place of the rarefied air removed to the higher
regions of the atmosphere, and, of course, north-easterly breezes
would be produced. But when the sun comes over Mexico, that vast
district of country is made to act the part of an enormous heater, and
becomes a far more powerful cause of rarefaction to the superincumbent
air than the ocean which lies between it and the equator. Accordingly,
the air over Mexico, between the latitudes of 10° and 30°, is more
heated than that which lies over the sea between the line and latitude
20°; and as the coolest, or least heated, that is, the most dense
fluid, always rushes towards the place lately occupied by the hottest
and most buoyant, the air from the equator will be drawn towards the
coast of Mexico, the great local source of heat and rarefaction.
But as this equatorial air is of course impressed with a more rapid
eastern velocity than those parts of the earth which form the southern
shores of Mexico, a westerly wind must be produced by the relative
difference in these two motions. At that particular season of the year
when the sun is in high southern declination, Mexico is not exposed to
his perpendicular rays. The equatorial regions are then more heated
than Mexico, and accordingly we actually find north-easterly breezes
nearly as they would be if Mexico were out of the way, and quite in
accordance with our theory.
In like manner, in the Atlantic, when the sun is far to the north,
the great deserts of the western angle, or shoulder of Africa, become
as vehemently heated, or more so, perhaps, than Mexico, and this draws
the air from the equator, so as to produce the south-westerly winds I
have already spoken of in the troublesome range called the Variables.
Finally, the great monsoons of the Indian ocean and China sea
contribute to establish this theory of Hadley, though I am not aware
that he ever brought it to bear on these very interesting phenomena.
They are eminently deserving of such notice, however, from being
periodical Trade-winds of the highest order of utility in one of the
busiest commercial regions of the world. Their periodical or shifting
character is the circumstance upon which their extensive utility in a
great measure depends, amongst nations where the complicated science
of navigation is but in a rude state. Myriads of vessels sail from
their homes during one monsoon before the wind, or so nearly before
it, that there is no great skill required in reaching all the ports at
which they wish to touch; and when the wind shifts to the opposite
quarter, they steer back again, in like manner, with a flowing sheet.
Thus, with an exceedingly small portion of nautical skill, they
contrive to make their passages by means of what we blue-jackets call
"a soldier's wind, there and back again." It will sometimes happen
that these rude navigators miscalculate their time, or meet with
accidents to retard them till the period of change has gone past, and
then they have no resource but to wait for half-a-year till the
monsoon shifts.
Experienced sailors, in like circumstances, acquainted with the
varieties of winds prevailing in those seas, would speedily get their
vessels out of this scrape, into which the lubberly Chinese junks
sometimes fall. They might, and certainly would, lose time in making a
roundabout of some two or three hundred miles in searching for a wind;
but, if they really knew what they were about, they would be sure to
catch it at last, and to turn it to their purpose.
From April to October, when the sun's rays fall with greatest effect
on Arabia, India, and China, and the several interjacent seas to which
these immense countries give their name, the air in contact with them,
becoming heated, rises, and gives place to fresh supplies drawn from
the equator. But this equatorial mass of air has had imparted to it by
the earth's rotation a greater degree of velocity in the direction
from west to east than belongs to the countries and seas just
mentioned; and this additional velocity, combined with its motion from
the equator, in rushing to fill up the vacuum caused by the
rarefaction of the air over those regions intersected by the tropic,
causes the south-west monsoon. "This wind," says Horsburgh, "prevails
from April to October, between the equator and the tropic of Cancer,
and it reaches from the east coast of Africa to the coasts of India,
China, and the Philippine Islands; its influence extends sometimes
into the Pacific Ocean as far as the Marian Islands, on to longitude
about 145° east, and it reaches as far north as the Japan Islands."
The late Captain Horsburgh thus describes what takes place in the
winter months:--"The north-east monsoon," he says, "prevails from
October to May, throughout nearly the same space that the south-west
monsoon prevails in the opposite season mentioned above. But the
monsoons are subject to great obstructions by land; and in contracted
places, such as Malacca Strait, they are changed into variable winds.
Their limits are not everywhere the same, nor do they always shift
exactly at the same period."
During this last named period, when the north-east monsoon is blowing,
viz. from October to May, the sun is acting with its greatest energy
on the regions about the equator, and the seas lying between it and
the southern tropic, while the countries formerly mentioned (Arabia,
India, and China), lying under the northern tropic, become
comparatively cool. The air over these regions becomes relatively more
dense than the rarefied air near the line; consequently the cool air
rushes to the southward to interchange places with that which has been
heated; and as the cool air comes from slower-moving to quicker-moving
parallels of latitude, that is, from the tropical to the equatorial
regions, the north-easterly monsoon is produced, very much resembling
in its effect, as it strictly does in its cause, the ordinary
trade-wind of the Atlantic and Pacific oceans.
This is a very general view of what may be called the great monsoons
of India; but there are many variations in different places, all of
which are so readily explained by the foregoing theory, that they form
by no means the least interesting branch of the subject, or the least
satisfactory of its illustrations.
One of the most extensive of these varieties, though of a less general
and sweeping character than those which blow in the Arabian sea and
bay of Bengal, is found in a very remote part of the world. "From
October to April this north-west monsoon prevails between the
north-east part of Madagascar and the west coast of New Holland; and
it is generally confined between the equator and 10° or 11° south
latitude, but subject to irregularities." This westerly wind is
evidently produced by the air drawn actually from the equator towards
the slower moving latitudes of the earth, by the rarefaction of the
air to the southward when the sun is near the tropic of Capricorn.
"The south-east monsoon predominates from April to October in the
space last mentioned, and in some places reaches to the equator." In
this case, the slow moving air near the southern tropic is brought, as
in the ordinary case of the south-east Trade wind, to the quick-moving
parts of the earth's surface.
The following remark of Horsburgh's, in describing the monsoons, is
extremely valuable, and assists to explain Hadley's theory of these
matters:--"The parts where the north-west and the south-east monsoons
prevail with greatest strength and regularity are in the Java sea, and
from thence eastward to Timor, amongst the Molucca and Banda islands,
and onward to New Guinea;" for it will be obvious to any one who
inspects the globe, on reading this passage, that there occurs in the
neighbourhood of the spots alluded to a powerful cause for the
strength and regularity of the monsoons. The enormous island, or
continent, as it might almost be called, of Australia, may well be
supposed to act the part of a heater from October to April, when the
sun is so nearly over it. During that period the equatorial air is
drawn to the south, along the intermediate seas, amongst the Moluccas
and other Spice islands, so as to produce a strong and steady
north-westerly monsoon. Of course, the opposite effect will be
produced when the sun retreats to the north, and leaves Australia to
cool.
These instances are quite enough, I should imagine, to satisfy
ordinary curiosity on this point; but professional men ought not to be
contented till they have investigated all branches of this important
topic; including that elegant and very useful episode, the land and
sea breezes of all hot climates, and Horsburgh's East India Directory,
which I have quoted above so frequently, is by far the best authority
with which I am acquainted on these subjects. At the same time, I must
not omit to do justice to a beautifully-written and accurate Essay on
Winds and Currents, by that Prince of all Voyagers, Old Dampier; who,
with means far more circumscribed than most of his successors, has
contrived to arrange and condense his information in such a way as not
only to render it available to practical men, but to make it
intelligible and interesting to every class of readers.[4]
FOOTNOTES:
[3] It is necessary to note here that these questions have been
examined since Captain Hall wrote, by Commander Maury, late secretary
to the American navy, in the true analytical spirit, and immense
progress made in our knowledge of these winds by the mass of practical
observations on the subject made by practical navigators, and
published under his directions.--ED.
[4] The principle of "Great Circle Sailing," which now guides the
navigator to the Indian Ocean, must be studied in connection with this
chapter. "For every degree the ship changes her longitude south of the
Line she sails a shorter distance along the great circle than on any
other curve; for on the parallel of 60° thirty miles corresponds to a
distance of sixty at the equator."--ROBERTSON'S _Theory of Great
Circle Sailing_: Bell and Daldy.