of ocean, with surface winds blowing over
them that have swept the face of the
waters for many thousands of miles, and
which must at their temperature be heavily
loaded with vapour. Yet these winds furnish
no supplies of rain sufficient to form any
rivers of magnitude. Those lands are almost
riverless.

On the other hand the winds which blow
over the gigantic rivers of the northern
hemisphere—the mighty streams of America,
Russia, India and China—have all traversed
but little of ocean, their way from the
equator has chiefly been over dry land,
whence they could raise up little if any
moisture. Whence then is it that countries
with comparatively so little water about
them should recieve so copiously of rain,
whilst those in the very heart of the seas are
devoid of any such supply?

To take up surface water and hold it in
suspense the air must be at a high temperature;
to part with it again in the shape of
rain its temperature must be considerably
lowered. The only winds which, by reason
of the temperature, can perform this lifting
process, are the Trades on either side of the
equatorial region. In their course over the
vast body of waters, they become highly
charged with vapour. On their meeting at
the zone of equatorial calms they rise, reach
a cooler atmosphere, and consequently become
expanded and part with some of their moisture;
and hence we hear of such extraordinary
falls of rain in these regions as that sailors
have actually taken up buckets of fresh water
from the surface of the ocean during one of
these down-pourings. But the winds only
part with a portion of their load; the south-east
trade lifts itself and its load of aqueous
vapour high above the surface, and coursing
on towards the north in the contrary direction
of the north-east trade below, becomes
gradually cooled on its way, and as it cools
parts as gradually with its vapours in the
shape of rain.

In like manner the north-east trade that
rose as an upper current at the equator to
take its way to the south, performed also its
task of evaporation, but to a far less degree.
Coming from the regions of the north, it is a
cold wind, and therefore not in a condition to
raise up vapour until it be near the equator,
consequently it has but little to precipitate in
the shape of rain, and hence we find the lands
of the south so devoid of rivers. Were it to
be otherwise than thus, were the south-east
vapour-loaded winds to traverse the surface
of the earth in their northerly career, they
would not part with their moisture where
most needed by reason of their high
temperature, but would deposit the whole
when arrived in the frigid zone, where least
needed.

Again, if this south-east wind when it rose
up was turned back in its course, and instead
of passing over to the northern hemisphere
to water these vast regions of dry earth,
pursued a southerly career, its stores of rain
would be spent over very small tracks of earth
and over immense regions of water. It is
clear, therefore, that no other system than that
which it is now believed is the course of the
winds could be productive of the great
benefits which we receive from them. The
southern hemisphere may be likened to an
enormous boiler, the northern to a huge
condenser, by means of which all the
moisture in the world is dealt with for
distribution.

The one exception of the Rio de la Plate
to the absence of large rivers in the south,
serves equally to prove the theory. If the
reader will refer to a map of the world,
he will perceive that the north-east trade-wind
which is lifted at the equator, passes
as an upper current of precipitation over the
sources of the Plata, must have crossed the
equatorial region in about one hundred
degrees west longitude, and, therefore, having
come from the north-east, must have traversed
some thousands of miles across the
Atlantic, and then meeting in its southerly
career with the lofty Andes, become forced
up by them into still higher regions of cold,
draining in its ascent the last drop of
moisture from those mountains to supply the
solitary river of the south.

In like manner, a reference to the map will
show that the north-east wind which traverses
the great Sahara of central Africa, is
flung up at the equator, and thence passes
over South Africa in a south-westerly direction,
leaving no rain in that riverless country.
Again, the same trade which sweeps
the sterile, rainless steppes of Chinese
Tartary, crosses the line to the southward of
Ceylon, and thence takes its vapourless way
over the great Australian continent, where
also there are no rivers of any size.

There is a remarkable circumstance
connected with whirlwinds at sea, or cyclones
as they are termed, which goes far to confirm
this theory of our Air Map. In the northern
hemisphere, all these circular storms revolve
from right to left; in the south they revolve
from left to right; and these are precisely
the courses indicated by the present
theory, which the various currents of
atmosphere take at the two poles in their return
circuits.

We have thus given the main features of
the great wind-roads of this earth, as laid
down by Lieutenant Maury. There are,
however, many lesser tracts—small footways,
as it were—diverging from the main trunk
roads of the atmosphere, which taking their
course and strength from the varied surface
of the land follow irregular, and, as yet, but
little known directions. It is to these, and
to the confirmation of what is already
believed to be the case, that the attention of
nautical observers is wished to be directed,
so that, in the course of time, by the united