when the south-east monsoons are at their height. At the same season
the African south-westerly monsoons are blowing. Accordingly there are
two sets of winds, both blowing heavily and steadily from the Atlantic,
disturbing the atmospheric equilibrium, and thus in all probability
generating the great West Indian hurricanes. The storms thus arising
show their force first at a distance of about six or seven hundred
miles from the equator, and far to the east of the region in which they
attain their greatest fury. They sweep with a north-westerly course to
the Gulf of Mexico, pass thence northwards, and so to the north-east,
sweeping in a wide curve (resembling the letter ∪ placed thus ⊂) around
the West Indian seas, and thence travelling across the Atlantic,
generally expending their fury before they reach the shores of Western
Europe. This course is the storm-track (or storm-⊂ as I shall call it).
Of the behaviour of the winds as they traverse this track, I shall
have to speak when I come to consider the peculiarity from which these
storms derive their names of ‘cyclones’ and tornadoes.
The hurricanes of the Indian Ocean occur at the ‘changing of the
monsoons.’ ‘During the interregnum,‘ writes Maury, ‘the fiends of the
storm hold their terrific sway.’ Becalmed often for a day or two,
seamen hear moaning sounds in the air, forewarning them of the coming
storm. Then, suddenly, the winds break loose from the forces which have
for a while controlled them, and ‘seem to rage with a fury that would
break up the fountains of the deep.’
In the North Indian seas hurricanes rage at the same season as in the
West Indies.
In the China seas occur those fearful gales known among sailors as
‘typhoons’ or ‘white squalls.’ These take place at the changing of the
monsoons. Generated, like the West Indian hurricanes, at a distance of
some ten or twelve degrees from the equator, typhoons sweep—in a curve
similar to that followed by the Atlantic storms—around the East Indian
Archipelago, and the shores of China, to the Japanese Islands.
There occur land-storms, also, of a cyclonic character in the valley
of the Mississippi. ‘I have often observed the paths of such storms,’
says Maury, ‘through the forests of the Mississippi. There the track
of these tornadoes is called a “wind-road,” because they make an
avenue through the wood straight along, and as clear of trees as if
the old denizens of the forest had been cleared with an axe. I have
seen trees three or four feet in diameter torn up by the roots, and
the top, with its limbs, lying next the hole whence the root came.‘
Another writer, who was an eye-witness to the progress of one of these
American land-storms, thus speaks of its destructive effects. ‘I saw,
to my great astonishment, that the noblest trees of the forest were
falling into pieces. A mass of branches, twigs, foliage, and dust moved
through the air, whirled onward like a cloud of feathers, and passing,
disclosed a wide space filled with broken trees, naked stumps, and
heaps of shapeless ruins, which marked the path of the tempest.’
If it appeared, on a careful comparison of observations made in
different places, that these winds swept directly along those tracks
which they appear to follow, a comparatively simple problem would be
presented to the meteorologist. But this is not found to be the case.
At one part of a hurricane’s course the storm appears to be travelling
with fearful fury along the true storm-⊂; at another less furiously
directed across the storm-track; at another, but with yet diminished
force, though still fiercely, in a direction exactly opposite to that
of the storm-track.
All these motions appear to be fairly accounted for by the theory that
the true path of the storm is a spiral—or rather, that while the centre
of disturbance continually travels onwards in a widely extended curve,
the storm-wind sweeps continually around the centre of disturbance, as
a whirlpool around its vortex.
And here a remarkable circumstance attracts our notice, the
consideration of which points to the mode in which cyclones may be
conceived to be generated. It is found, by a careful study of different
observations made upon the same storm, that cyclones in the northern
hemisphere _invariably_ sweep round the onward travelling vortex of
disturbance in _one_ direction, and southern cyclones in the contrary
direction. If we place a watch, face upwards, upon one of the northern
cyclone regions in a Mercator’s chart, then the motion of the hands is
_contrary_ to the direction in which the cyclone whirls; when the watch
is shifted to a southern cyclone region, the motion of the hands is in
the same direction as the cyclone motion. This peculiarity is converted
into the following rule-of-thumb for sailors who encounter a cyclone,
and seek to escape from the region of fiercest storm:—_Facing the wind,
the centre or vortex of the storm lies to the right in the northern,
to the left in the southern hemisphere_. Safety lies in flying from
the centre in every case save one—that is, when the sailor lies in
the direct track of the advancing vortex. In this case, to fly from
the centre would be to keep in the storm-track; the proper course for
the sailor when thus situated is to steer for the calmer side of the
storm-track. This is always the outside of the ⊃, as will appear from
a moment’s consideration of the spiral curve traced out by a cyclone.
Thus, if the seaman _scud before the wind_—in all other cases a
dangerous expedient in a cyclone[13]—he will probably escape unscathed.
There is, however, this danger, that the storm-track may extend to or
even slightly overlap the land, in which case scudding before the gale
would bring the ship upon a lee-shore. And in this way many gallant
ships have, doubtless, suffered wreck.
The danger of the sailor is obviously greater, however, when he is
overtaken by the storm on the inner side of the storm-⊂. Here he has to
encounter the double force of the cyclonic whirl and of the advancing
storm-system, instead of the difference of the two motions, as on the
outer side of the storm-track. His chance of escape will depend on his
distance from the central path of the cyclone. If near to this, it is
equally dangerous for him to attempt to scud to the safer side of the
track, or to beat against the wind by the shorter course, which would
lead him out of the storm-⊂ on its inner side. It has been shown by
Colonel Sir W. Reid that this is the quarter in which vessels have been
most frequently lost.
But even the danger of this most dangerous quarter admits of degrees.
It is greatest where the storm is sweeping round the most curved part
of its track, which happens in about latitude twenty-five or thirty
degrees. In this case a ship may pass twice through the vortex of the
storm. Here hurricanes have worked their most destructive effects. And
hence it is that sailors dread, most of all, that part of the Atlantic
near Florida and the Bahamas, and the region of the Indian Ocean which
lies south of Bourbon and Mauritius.
To show how important it is that captains should understand the theory
of cyclones in both hemispheres, we shall here relate the manner in
which Captain J. V. Hall escaped from a typhoon of the China seas.
About noon, when three days out from Macao, Captain Hall saw ‘a most
wild and uncommon-looking halo round the sun.’ On the afternoon of the
next day, the barometer had commenced to fall rapidly; and though, as
yet, the weather was fine, orders were at once given to prepare for a
heavy gale. Towards evening a bank of cloud was seen in the south-east,
but when night closed the weather was still calm and the water
smooth, though the sky looked wild and a scud was coming on from the
north-east. ‘I was much interested,’ says Captain Hall, ‘in watching
for the commencement of the gale, which I now felt sure was coming.
That bank to the south-east was the meteor (cyclone) approaching us,
the north-east scud, the outer north-west portion of it; and when at
night a strong gale came on about north, or north-north-west, I felt
certain we were on its western and south-western verge. It rapidly
increased in violence; but I was pleased to see the wind veering
to the north-west, as it convinced me that I had put the ship on
the right track—namely, on the starboard tack, standing, of course,
to the south-west. From ten A.M. to three P.M. it blew with great
violence, but the ship being well prepared, rode comparatively easy.
The barometer was now very low, the centre of the storm passing to the
northward of us, to which we might have been very near had we in the
first place put the ship on the larboard tack.
But the most remarkable point of Captain Hall’s account remains to
be mentioned. He had gone out of his course to avoid the storm, but
when the wind fell to a moderate gale he thought it a pity to lie so
far from his proper course, and made sail to the north-west. ‘In less
than two hours the barometer again began to fall and the storm to rage
in heavy gusts.’ He bore again to the south-east, and the weather
rapidly improved. There can be little doubt that but for Captain Hall’s
knowledge of the law of cyclones, his ship and crew would have been
placed in serious jeopardy, since in the heart of a Chinese typhoon a
ship has been known to be thrown on her beam-ends when not showing a
yard of canvas.
If we consider the regions in which cyclones appear, the paths they
follow, and the direction in which they whirl, we shall be able to form
an opinion as to their origin. In the open Pacific Ocean (as its name,
indeed, implies) storms are uncommon; they are infrequent also in the
South Atlantic and South Indian Oceans. Around Cape Horn and the Cape
of Good Hope heavy storms prevail, but they are not cyclonic, nor are
they equal in fury and frequency, Maury tells us, to the true tornado.
Along the equator, and for several degrees on either side of it,
cyclones are also unknown. If we turn to a map in which ocean-currents
are laid down, we shall see that in every ‘cyclone region’ there is
a strongly marked current, and that each current follows closely the
track which I have denominated the storm-⊂. In the North Atlantic we
have the great Gulf Stream, which sweeps from equatorial regions
into the Gulf of Mexico, and thence across the Atlantic to the shores
of Western Europe. In the South Indian Ocean there is the ‘south
equatorial current,’ which sweeps past Mauritius and Bourbon, and
thence returns towards the east. In the Chinese Sea there is the north
equatorial current, which sweeps round the East Indian Archipelago, and
then merges into the Japanese current. There is also the current in the
Bay of Bengal, flowing through the region in which, as we have seen,
cyclones are commonly met with. There are other sea-currents besides
these which yet breed no cyclones. But I may notice two peculiarities
in the currents I have named. They all flow from equatorial to
temperate regions, and, secondly, they are all ‘horse-shoe currents.’
So far as I am aware, there is but one other current which presents
both these peculiarities—namely, the great Australian current between
New Zealand and the eastern shores of Australia. I have not yet met
with any record of cyclones occurring over the Australian current,
but heavy storms are known to prevail in that region, and I believe
that when these storms have been studied as closely as the storms in
better-known regions, they will be found to present the true cyclonic
character.
Now, if we inquire why an ocean current travelling from the equator
should be a ‘storm-breeder,’ we shall find a ready answer. Such a
current, carrying the warmth of intertropical regions to the temperate
zones, produces, in the first place, by the mere difference of
temperature, important atmospheric disturbances. The difference is so
great, that Franklin suggested the use of the thermometer in the North
Atlantic Ocean as a ready means of determining the longitude, since the
position of the Gulf Stream at any given season is almost constant.
But the warmth of the stream itself is not the only cause of
atmospheric disturbance. Over the warm water vapour is continually
rising; and, as it rises, is continually condensed (like the steam
from a locomotive) by the colder air round. ‘An observer on the moon,’
says Captain Maury, ‘would, on a winter’s day, be able to trace out by
the mist in the air the path of the Gulf Stream through the sea.’ But
what must happen when vapour is condensed? We know that to turn water
into vapour is a process requiring—that is, _using up_—a large amount
of heat; and, conversely, the return of vapour to the state of water
_sets free_ an equivalent quantity of heat. The amount of heat thus
set free over the Gulf Stream is thousands of times greater than that
which would be generated by the whole coal supply annually raised in
Great Britain. Here, then, we have an efficient cause for the wildest
hurricanes. For, along the whole of the Gulf Stream, from Bemini to
the Grand Banks, there is a channel of heated—that is, _rarefied air_.
Into this channel, the denser atmosphere on both sides is continually
pouring, with greater or less strength. When a storm begins in the
Atlantic, it always makes for this channel, ‘and, reaching it, turns
and follows it in its course, sometimes entirely across the Atlantic.’
‘The southern points of America and Africa have won for themselves,’
says Maury, ‘the name of “the stormy capes,” but there is not a
storm-find in the wide ocean can out-top that which rages along the
Atlantic coasts of North America. The China seas and the North Pacific
may vie in the fury of their gales with this part of the Atlantic, but
Cape Horn and the Cape of Good Hope cannot equal them, certainly in
frequency, nor do I believe, in fury.’ We read of a West Indian storm
so violent, that ‘it forced the Gulf Stream back to its sources, and
piled up the water to a height of thirty feet in the Gulf of Mexico.
The ship “Ledbury Snow” attempted to ride out the storm. When it abated
she found herself high up on the dry land, and discovered that she had
let go her anchor among the tree-tops on Elliot’s Key.‘
By a like reasoning, we can account for the cyclonic storms prevailing
in the North Pacific Ocean. Nor do the tornadoes which rage in parts
of the United States present any serious difficulty. The region along
which these storms travel is the valley of the great Mississippi. This
river at certain seasons is considerably warmer than the surrounding
lands. From its surface, also, aqueous vapour is continually being
raised. When the surrounding air is colder, this vapour is presently
condensed, generating in the change a vast amount of heat. We have thus
a channel of rarefied air over the Mississippi valley, and this channel
becomes a storm-track, like the corresponding channels over the warm
ocean-currents. The extreme violence of land-storms is probably due to
the narrowness of the track within which they are compelled to travel.
For it has been noticed that the fury of a sea-cyclone increases as the
range of the ‘whirl’ diminishes, and _vice versâ_.
There seems, however, no special reason why cyclones should follow
the storm-⊂ in one direction rather than in the other. We must, to
understand this, recall the fact that under the torrid zones the
conditions necessary for the generation of storms prevail far more
intensely than in temperate regions. Thus the probability is far
greater that cyclones should be generated at the tropical than at the
temperate end of the storm-⊂. Still, it is worthy of notice, that in
the land-locked North Pacific Ocean, true typhoons _have_ been noticed
to follow the storm-track in a direction contrary to that commonly
noticed.
The direction in which a true tornado _whirls_ is _invariably_ that I
have mentioned. The explanation of this peculiarity would occupy more
space than I can here afford. Those readers who may wish to understand
the origin of the law of cyclonic rotation should study Herschel’s
interesting work on Meteorology.
The suddenness with which a true tornado works destruction was
strikingly exemplified in the wreck of the steamship ‘San Francisco.’
She was assailed by an extra-tropical tornado when about 300 miles
from Sandy Hook, on December 24, 1853. In a few moments she was a
complete wreck! The wide range of a tornado’s destructiveness is shown
by this, that Colonel Reid tells us of one along whose track no less
than 110 ships were wrecked, crippled, or dismasted.
(From _Temple Bar_, December 1867.)
FOOTNOTES:
[12] I remember to have read that in this hurricane guns which had long
lain under water were washed up like mere drift upon the beach. Perhaps
this circumstance grew gradually into the incredible story above
recorded.
[13] A ship by scudding before the gale may—if the captain is not
familiar with the laws of cyclones—go _round and round_ without
escaping. The ship ‘Charles Heddle’ did this in the East Indies, going
round no less than _five times_.
_VESUVIUS._
The numerous and violent eruptions from Mount Vesuvius during the two
last centuries seem to afford an answer to those who think there are
traces of a gradually diminishing activity in the earth’s internal
forces. That such a diminution is taking place, we may admit; but
that its rate of progress is perceptible—that we can point to a time
within the historical epoch, nay, even within the limits of geological
evidence, at which the earth’s internal forces were _certainly_ more
active than they are at the present time—may, I think, be denied
absolutely.
When the science of geology was but young, and its professors sought
to compress within a few years (at the outside) a series of events
which (we now know) must have occupied many centuries, there was room,
indeed, for the supposition that modern volcanic eruptions, as compared
with ancient outbursts, are but as the efforts of children compared
with the work of giants. And accordingly, we find a distinguished
French geologist writing, even so late as 1829, that in ancient times
‘tous les phénomènes géologiques se passaient dans des dimensions
_centuples_ de celles qu’ils présentent aujourd’hui.’ But now we have
such certain evidence of the enormous length of the intervals within
which volcanic regions assumed their present appearance—we have such
satisfactory means of determining which of the events occurring within
those intervals were or were not contemporary—that we are safe from
the error of assuming that Nature at a single effort fashioned widely
extended districts just as we now see them. And accordingly, we have
the evidence of the distinguished geologist, Sir Charles Lyell, that
there is no volcanic mass ‘of ancient date, distinctly referable to a
single eruption, which can even _rival_ in volume the matter poured out
from Skaptâr Jokul in 1783.’
In the volcanic region of which Vesuvius or Somma is the principal
vent, we have a remarkable instance of the deceptive nature of that
state of rest into which some of the principal volcanoes frequently
fall for many centuries together. For how many centuries before the
Christian era Vesuvius had been at rest is not known; but this is
certain, that from the landing of the first Greek colony in Southern
Italy, Vesuvius gave no signs of internal activity. It was recognised
by Strabo as a volcanic mountain, but Pliny did not include it in the
list of active volcanoes. In those days, the mountain presented a very
different appearance from that which it now exhibits. In place of the
two peaks now seen, there was a single, somewhat flattish summit,
on which a slight depression marked the place of an ancient crater.
The fertile slopes of the mountain were covered with well-cultivated
fields, and the thriving cities Herculaneum, Pompeii, and Stabiæ stood
near the base of the sleeping mountain. So little did any thought
of danger suggest itself in those times, that the bands of slaves,
murderers, and pirates which flocked to the standards of Spartacus
found a refuge, to the number of many thousands, within the very crater
itself.
But though Vesuvius was at rest, the region of which Vesuvius is the
main vent was far from being so. The island of Pithecusa (the modern
Ischia) was shaken by frequent and terrible convulsions. It is even
related that Prochyta (the modern Procida) was rent from Pithecusa in
the course of a tremendous upheaval, though Pliny derives the name
Prochyta (or ‘poured forth’) from the supposed fact of this island
having been poured forth by an eruption from Ischia. Far more probably,
Prochyta was formed independently by submarine eruptions, as the
volcanic islands near Santorin have been produced in more recent times.
So fierce were the eruptions from Pithecusa, that several Greek
colonies which attempted to settle on this island were compelled to
leave it. About 380 years before the Christian era, colonists under
King Hiero of Syracuse, who had built a fortress on Pithecusa, were
driven away by an eruption. Nor were eruptions the sole cause of
danger. Poisonous vapours, such as are emitted by volcanic craters
after eruption, appear to have exhaled, at times, from extensive tracts
on Pithecusa, and thus to have rendered the island uninhabitable.
Still nearer to Vesuvius lay the celebrated Lake Avernus. The name
Avernus is said to be a corruption of the Greek word _Aornos_,
signifying ‘without birds,’ the poisonous exhalations from the waters
of the lake destroying all birds which attempted to fly over its
surface. Doubt has been thrown on the destructive properties assigned
by the ancients to the vapours ascending from Avernus. The lake is now
a healthy and agreeable neighbourhood, frequented, says Humboldt, by
many kinds of birds, which suffer no injury whatever even when they
skim the very surface of the water. Yet there can be little doubt that
Avernus hides the outlet of an extinct volcano; and long after this
volcano had become inactive, the lake which concealed its site ‘may
have deserved the appellation of “atri janua Ditis,” emitting, perhaps,
gases as destructive of animal life as those suffocating vapours given
out by Lake Quilotoa, in Quito, in 1797, by which whole herds of cattle
were killed on its shores, or as those deleterious emanations which
annihilated all the cattle in the island of Lancerote, one of the
Canaries, in 1730.‘
While Ischia was in full activity, not only was Vesuvius quiescent, but
even Etna seemed to be gradually expiring, so that Seneca ranks this
volcano among the number of nearly extinguished craters. At a later
epoch, Ælian asserted that the mountain itself was sinking, so that
seamen lost sight of the summit at a less distance across the seas than
of old. Yet within the last two hundred years there have been eruptions
from Etna rivalling, if not surpassing, in intensity the convulsions
recorded by ancient historians.
I shall not here attempt to show that Vesuvius and Etna belong to the
same volcanic system, though there is reason not only for supposing
this to be the case, but for the belief that all the subterranean
regions whose effects have been shown from time to time over the
district extending from the Canaries and Azores, across the whole of
the Mediterranean, and into Syria itself, belong to but one great
centre of internal action. But it is quite certain that Ischia and
Vesuvius are outlets from a single source.
While Vesuvius was dormant, resigning for a while its pretensions to
be the principal vent of the great Neapolitan volcanic system, Ischia,
we have seen, was rent by frequent convulsions. But the time was
approaching when Vesuvius was to resume its natural functions, and with
all the more energy that they had been for a while suspended.
In the year 63 (after Christ) there occurred a violent convulsion
of the earth around Vesuvius, during which much injury was done to
neighbouring cities, and many lives were lost. From this period shocks
of earthquake were felt from time to time for sixteen years. These grew
gradually more and more violent, until it began to be evident that the
volcanic fires were about to return to their main vent. The obstruction
which had so long impeded the exit of the confined matter was not,
however, readily removed, and it was only in August in the year 79,
after numerous and violent internal throes, that the superincumbent
mass was at length hurled forth. Rocks and cinders, lava, sand, and
scoriæ, were propelled from the crater, and spread many miles on every
side of Vesuvius.