148 
explain the fact, are always on the look-out for these 
furious gales when sailing on the axial lines of the Gulf 
Stream, on the hot Mozambique current (the Gulf Stream 
of the Indian Ocean), and on the dark superheated waters 
of the Kuro Siwo of the Pacific. 
So dangerous and disastrous are the storms which 
course along the Gulf Stream that sailors avoid it, and 
the American Sailing Directions and those of the British 
Admiralty advise all vessels, sailing from the West Indies 
to New York or Liverpool, to beware of taking advantage 
of its current, although it would help them along from 
three to four miles an hour. Close observation has traced 
these storms continuously from the Florida coast to New 
York, through Redfield’s labours, and thence to England, 
through the records of the Cunard steamships, and thou- 
sands of detached observations. 
We have now reached a point where we can properly 
and intelligently consider a question that has always 
baffled meteorologists—the origin of cyclones. The 
diagnosis of the phenomenon necessarily precedes its 
explanation. This subject has engrossed many minds, 
and various have been the ingenious devices for unravel- 
ling its mystery. Mr. Ree fel d—the father of storm 
physics—in his modesty and diffidence, so distrusted him- 
self and in his day so keenly felt the need of a more 
enlarged induction of a at he has scarcely left us 
his opinion. The theories of other writers have all long 
since been abandoned by Enemelves or suffered to drop 
from the notice of the scientific world as evidently inca- 
pable of explaining the phenomena of cyclones. This 
has been the fate of them all, unless possibly we except 
the theory advanced by the great meteorologist, M. Dové, 
of Berlin. Briefly stated, the latter hypothesis is this (at 
least in its application to West Indian hurricanes), viz., 
that “ they owe their origin to the intrusion of the upper 
counter trade-wind into the lower trade-wind current” 
(Dové's “ Law of Storms,” p. 264). 
Without pausing here to examine this theory upon its 
merits and upon the facts, we hasten to mention a diffe- 
rent hypothesis advanced, nearly two years ago, as a sub- 
stitute for that of M. Doyé, and as affording an entirely 
original and satisfactory explanation of the origin of 
cyclones. 
The hypothesis was likewise based upon the agency of 
the trade-winds, but in a manner wholly different from 
that elaborated by the German meteorologist. In the 
original paper in which my view were published, the fol- 
lowing statement was made eet can be demonstrated 
that the origin of cyclones is 1 in the tendency of the 
south-east trade-winds to iny e territory of the north- 
east trades, by sweeping over the equator into our hemi- 
sphere.” Ais 
The hypothesis advanced, in lieu of another seemingly 
less satisfactory, claimed to rest upon observations con- 
ducted in the very region most notorious for the genera- 
tion of cyclones. 
To test this, we need only to examine the Atlantic trade 
winds. 
Theoretically, physical geography has generally repre- 
sented the motions of the atmosphere somewhat as is repre- 
sented inthe accompanying diagram (Fig. 3) of the winds, 
as projected by Prof. William Ferrel, of Cambridge. The 
elaborate pages of Prof. Coffin, in his invaluable volume 
on the “ Winds of the Northern Hemisphere,” as deduced 
from myriads of observations, show that the graphic 
illustration furnished by the diagram is approximately 
correct. 
The region of the trade winds, it will be seen, more 
than covers the torrid zone of the earth, and at all the | 
seasons of the year overlaps both the northern and | 
southern tropics. While this is theoretically true, and is | 
usually put forth as a fact, it must be accompanied with | 
one or two important qualifications and additions. 
Let us see what these are. The well-known oscillation 
NATURE 
PLT gy Ue es ee ee ee 
[ Fune 19, 1873 
or swinging of the belts of winds to and fro on the meri- 
dians, which is kept up in never-ceasing response to the 
apparent annual motion of the sun as he crosses and 
recrosses the equator, must ever underlie the conception we 
form of the trade winds and be perpetually present to the 
mind’s eye. This oscillation has never yet received the 
popular attention it needs. The sun traverses (apparently) 
an arc of 233° on either side ofthe line ; and we might, 
a priori, suppose that the thermal or meteorological 
equator, the thermal or meteorological Tropics of Cancer 
and Capricorn, and all those phenomena which lie be- 
tween them and beyond them, move over an arc of as 
many degrees as they traverse. Such an inference, how- 
ever, is not borne out by observation, and we pro- 
pose to confine ourselves strictly to what may be 
proved by observation. It is clear that the trade- 
wind belt does traverse or vibrate over a wider zone 
than any physicist has yet assigned to it, which is not 
more than ten degrees of latitude north and south 
respectively of the Tropic of Cancer and that of 
Capricorn. These winds, when first experienced by 
Spanish sailors, gave, to that portion of the Atlan- 
tic over which they blew, the name ¢/ Golfo de las 
Damas (the Ladies’ Sea) because they rendered navigation 
so easy that a girl might take the helm. But, “gentle” 
as they are, they havea wide sweep, and, in the summer 
of the Northern Hemisphere, extend far beyond the 
Tropic of Cancer. They have often been distinctly felt 
at Madeira and the Azores (near the 4oth parallel) in 
summer, and it is highly reasonable to suppose that they 
then fully reach the latitude of 40° N. The equatorial 
side of the north-east trade-wind belt, of course, vibrates 
with the sun. In summer it stretches along between the 
1oth and 12th parallels of north latitude, verging in 
August on the 13th parallel, and, according to one writer, 
occasionally the north-east trades at that season do not 
extend south of the 15th parallel of north latitude. 
Dampier, “the prince of navigation,” as the English call 
him, gives the cirection of the wind in the summer 
months, between the equatorand 12° north, as south- 
south-east, south-south-west, and south-west. 
The equatorial side of the north-east trade-wind belt 
in winter approaches very nearly to the equator, and may 
be located in,January at least as far south as the latitude 
of 2° north. 
The freshest-trade-winds in the North Atlantic are 
generally found between the parallels of 10° and 25°, and 
by long protracted experiment in seamanship they have 
been found to have an average propelling power, when 
the wind is taken just abaft the beam, of about six knots 
an hour. But, of course, the northern boundary of the 
south-east trade-wind likewise varies and vibrates with 
the seasons. So also, and under the same condition, 
does the southern boundary of this trade vary and vibrate 
with the seasons. Its normal and mean position is a little 
south of the parallel of 25° south, but in the winter of our 
hemisphere it is pushed much farther south, and in the 
vicinity of 35° south latitude. The charts of Captain 
Wilkes give easterly winds for the east coast of Australia, 
and also for the south coast of Africa. Sir John Herschel, 
speaking from knowledge gained by his long residence at 
the Cape of Good Hope, tells us that there “the south- 
easterly winds which sweep over the Southern Ocean, 
infringing upon the long range of rocks which terminates 
inthe Table Mountain, is thrown up by them, makes a 
clean sweep over the flat table-land which forms the 
summit of that mountain (about 3,85oft. high), and 
thence plunges down with the violence of a cataract” 
(“ Meteorology,” p. 96). 
From these high southern latitudes, we must conceive 
the motion of the south-east trades, extending northward 
in summer to the neighbourhood of the parallel of 10°. 
T. B, MAURY 
(Zo be continued.) 
