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Sune 26, 1873) 
Between the island of Tristan da Cunha and the Cape, 
and northward and westward to the island of Fernando 
Noronha, this streak of powerful winds, with which 
nothing in the trade-wind region of the North Atlantic 
can compare, has its atmospheric current as sharply 
marked asthe dark blue and rapid current of the Gulf 
Stream in the Narrows of Bemini. It is, doubtless, the 
region or band of most intensely acting south-east trades, 
and is probably due to the peculiar configuration of the 
shores of the South Atlantic, and to the wall of the 
South American Andes. It is a well-known fact that 
the volcanic cone of Teneriffe, which lies in the zone of 
north-east trades, intercepts the wind and gives it a lateral 
deflection ; so that, while the trades are blowing strongly 
on the north-east side of the island, on the opposite side 
there is a distinctly-marked and carefully-measured calm 
shadow. Now, the chain of the Andes endeavours to 
exert on the south-eastern trades just such an influence 
as is exerted by the Canary Islands on the north-east 
trades. This influence, in the former case, suffices to 
throw off from the Continent of South America a large 
body of the south-east trades, and to deflect it to the 
eastward, giving it the character of a south-south-west 
wind, and, at the same time, by forcing a greater or 
more concentrated body of air into the regions north- 
east of Brazil, imparting an increased velocity and violence 
to the air-current. It is, therefore, in the air-current that 
the homeward-bound vessel from the Cape of Good Hope 
aims to steer, because she is sure of being wafted happily 
and swiftly to her destination. 
It has long been demonstrated by meteorologic observa- 
tions, taken both at sea and on land, that there is very 
much /ess atmosphere in the Southern Hemisphere than 
in the northern, and for a long time physicists were at a 
loss to account for the difference. It has been, however, 
very satisfactorily explained by the eminent American 
mathematician, Ferrel, in his work on the “ Motions of 
Fluids and Solids, relative to the Earth’s Surface,” where 
he proves at length, and states in detail (p. 39): “‘ As there 
is much more Jand, with higher mountain ranges, in the 
Northern Hemisphere than in the southern, the resistances 
are greater, and consequently the eastward motion of the 
air, upon which the deflecting force depends, is much 
less ; and the consequence is, that the more rapid motions 
of the Southern Hemisphere cause a greater depression 
there, and a greater part of the atmosphere to be thrown 
into the Northern Hemisphere.” It is, doubtless, to this 
tendency of the Southern Hemisphere to throw off much 
of its atmosphere north of the equator that we may attri- 
bute in part the superior force and power of the south- 
east trades, and their well-known ability to battle with the 
north-east trades, and drive them from their own territory, 
at least all summer, and even in winter, as far back across 
the line as 3° or 4° north latitude. Mr. Ferrel, speaking 
of the principle just enunciated, well says: “This also 
accounts for the mean position of the equatorial calm-belt 
being, in general, a little north of the equator. But, in 
the Pacific Ocean, where there is nearly as much water 
north of the equator as south (and the resistances are 
usually equal), its position nearly comcides with the equa- 
tor.” In other words, just as a bucket full of water 
revolving on a perpendicular axis would show a depression 
in the centre, and the fluid be thrown from all sides of its 
rim, the Southern Hemisphere throws its water and its 
atmosphere into the Northern hemisphere, all along the 
equator. 
It is, therefore, a mathematical and mechanical cer- 
tainty that there is an invasion of the north-east trade- 
wind belt from the south-east trades, and observation 
powerfully bears out the deduction of the mathematician. 
Ansted states in his cautiously-written “ Physical Geo- 
graphy :”—“The southern trade-wind region ismuch larger 
than the northern in the Atlantic Ocean. In this sea, 
the south-east trades are fresher, and blow stronger, than 
{ 
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NATURE 
165 
the others, and often reach to the roth or 15th parallel of 
north latitude ; whereas the northern trade-wind seldom 
gets south of the equator, and usually ranges from 9° 
to 29° north latitude” (p. 253). It is not difficult to see 
how easily it happens that a very small atmospheric eddy 
found in the tropical Atlantic by the conflictory north- 
east and overleaping south-east trade-winds may soon be- 
come a hurricane of wide extent and of tremendous 
energy. All that is necessary, as we have before seen, is 
that an initial impulse of gyration be given to a body of 
air. The moment that this takes place by mechanical 
influence, and centrifugal force creates the smallest eddy 
cr vortex, the surrounding air, already highly charged 
with moisture, begins the process of convergence and 
Soe cae motion, followed rapidly by condensation 
aloft. 
The storm-cylinder—the nucleus of the hurricane— 
originally very small, is instantly enlarged and expanded 
by the evolution of latent heat stored away in the vesicles 
of aqueous vapour. For some hours, as all observations 
show to be actually the case, the incipient cyclone scarcely 
moves, while gathering in its energies and laying tributes 
upon all contiguous regions. The process continues with 
momentarily increasing intensity, and, before the sun has 
made his daily circuit, the meteor is formed. 
If it be asked along what parallels of latitude in our he- 
misphere this formation takes place, the intelligent reader 
will at once answer, Near the terrestrial circle of trade- 
wind interference. This, we have already seen, is in 
summer, from the roth to the 12th parallels of north 
latitude, 
This slender zone of debatable ground is the battle- 
field of the two opposing bands of the trades. There 
is really no need of observations to tell us as much, But 
millions of observations attest the fact. Every seaman 
knows it. Every meteorological writer tells the same 
story. You have only to examine physical charts from 
the time of Columbus and Magellan to this, to see the 
absolute unanimity of testimony, and to discover that the 
hypothesis now advanced, and the known facts of the 
case, are in perfect and minute accord, 
If it be asked whether the origin and interest of the 
West-Indian gales is solely due to mechanical inter- 
ference, the proper reply, it would appear, should be in 
the negative. As the south-east trade-wind comes laden 
with the vapour of the southern or water hemisphere, 
which Dové well called “the boiler” of the globe, it is 
met by the cold north-east trade from the northern, or 
land hemisphere. There must be a great difference in 
their temperatures, and consequently extensive condensa- 
tion, which, by the reasoning of Mr. Clement Ley, would, 
of itself, explain the formation of the storm, That con- 
d-nsation greatly assists in producing or intensifying it, 
cannot be doubted. In the high Jatitudes, where the 
polar air-current is sometimes forced by barometric pres- 
sure into the southerly or equatorial current moving over 
the warm waters of the ocean, and thus heavily vapour- 
laden, the consequence is illustrated by such terrific and 
sudden tempests as that of the Royal Charter, distinctly 
proved by Admiral Fitzroy to have been generated be- 
tween the opposite polar and equatorial currents off the 
coast of Wales. 
But that the origin of great depression-systems is solely 
due to condensation can hardly be sustained, and seems 
entirely overthrown if we regard the single fact that, on 
the great equatorial belt—the belt of perennial precipita- 
tion—no hurricane or typhoon has ever been experienced 
by the mariner. It has long been, and is now, the 
almost universally accepted theory of meteorologists, that 
the reason no cyclones have ever been known to occur on 
the equator is, that there the earth’s rotation exerts a de- 
flecting influence on the winds, amounting to zero, and 
hence the formation of a whirl is impossible. This view 
is not satisfactory, because the nucleus of a depression 
. 
