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Title: The Eruption of Vesuvius in 1872
Author: Palmieri, Luigi, 1807-1896
Language: English
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THE
ERUPTION OF VESUVIUS
IN 1872,


BY
PROFESSOR LUIGI PALMIERI,
_Of the University of Naples; Director of the Vesuvian Observatory._


WITH NOTES, AND AN
_INTRODUCTORY SKETCH OF THE PRESENT STATE OF KNOWLEDGE_
OF
TERRESTRIAL VULCANICITY,
_The Cosmical Nature and Relations of
Volcanoes and Earthquakes._


BY
ROBERT MALLET,
_Mem. Inst. C.E., F.R.S., F.G.S., M.R.I.A., &c., &c._


WITH ILLUSTRATIONS.


LONDON:

_ASHER & CO._,
13, BEDFORD STREET, COVENT GARDEN, W.C.

1873.


W. S. Johnson, Nassau Steam Press, 60, St. Martin's Lane,
Charing Cross, W.C.


"The Translator should look upon himself as a Merchant in the
Intellectual Exchange of the world, whose business it is to promote the
interchange of the produce of the mind."

                              GŒTHE, "_Kunst und Alterthum_."



INTRODUCTORY SKETCH, &c.


The publishers of this little volume, in requesting me to undertake a
translation of the "Incendio Vesuviano," of Professor Palmieri, and to
accompany it with some introductory remarks, have felt justified by the
facts that Signor Palmieri's position as a physicist, the great
advantages which his long residence in Naples as a Professor of the
University, and for many years past Director of the Meteorological
Observatory--established upon Vesuvius itself, prior to the expulsion of
the late dynasty--have naturally caused much weight to attach to
anything emanating from his pen in reference to that volcano.

Nearly forty memoirs on various branches of physics--chiefly
electricity, magnetism and meteorology--produced since 1842, are to be
found under Palmieri's name in the "Universal Catalogue of Scientific
Papers of the Royal Society," and of these nine refer to Vesuvius, the
earliest being entitled "Primi Studii Meteorologici fatti sul R.
Osservatorio Vesuviano," published in 1853. He was also author, in
conjunction with Professor A. Scacchi, of an elaborate report upon the
Volcanic Region of Monte Vulture, and on the Earthquake (commonly called
of Melfi) of 1851. These, however, by no means exhaust the stock of
Palmieri's labours.

The following Memoir of Signor Palmieri on the eruption of Vesuvius in
April of this year (1872), brief as it is, embraces two distinct
subjects, viz., his narrative as an eye-witness of the actual events of
the eruption as they occurred upon the cone and slopes of the mountain,
and his observations as to pulses emanating from its interior, as
indicated by his Seismograph, and as to the electric conditions of the
overhanging cloud of smoke (so called) and ashes, as indicated by his
bifilar electrometer, both established at the Observatory. The two last
have but an indirect bearing upon Vulcanology. The narrative of the
events of the eruption is characterised by exactness of observation and
a sobriety of language--so widely different from the exaggerated style
of sensational writing that is found in almost all such accounts--that I
do the author no more than justice in thus expressing my view of its
merits.

Nor should a special narration, such as this, become less important or
suffer even in popular estimation by the fact that so recently my
friend, Professor J. Phillips, has given to the world the best general
account of Vesuvius, in its historical and some of its scientific
aspects, which has yet appeared. That monograph--with its sparkling
style, and scholarly digressions, as well as for its more direct
merits--will, no doubt, become the manual for many a future visitor to
the volcanic region of Naples; but it, like the following Memoir of
Palmieri, and in common with almost every work that has appeared on the
subject of Volcanoes, contains a good deal which, however interesting,
and remotely related to Vulcanology, does not properly belong to the
body of that branch of cosmical science, as I understand its nature and
limits.

It tends but little, for example, to clear our views, or enlarge our
knowledge of the vast mechanism in which the Volcano originates, and
that by which its visible mass is formed, that we should ascertain the
electric condition of the atmosphere above its eruptive cone, or into
what crystallographic classes the mineral species found about it may be
divided: it will help us but little to know Pliny's notions of how
Pompeii was overwhelmed, or to re-engrave pictures, assumed to give the
exact shape of the Vesuvian or other cone at different periods, or its
precise altitude, which are ever varying, above the sea. Even much more
time and labour may be spent upon analysing the vapours and gases of
fumaroles and salfatares than the results can now justify.

Nothing, perhaps, tends more to the effective progress of any branch of
observational and inductive science, than that we should endeavour to
discern clearly the scope and boundary of our subject.

To do so is but to accord with Bacon's maxim, "_Prudens questio dimidium
scientiæ_." That once shaped, the roads or methods of approach become
clearer; and every foothold attained upon these direct paths enables us
to look back upon such collateral or subordinate questions as at first
perplexed us, and find them so illuminated that they are already
probably solved, and, by solution, again prove to us that we _are_ in
the right paths.

I believe, therefore, that I shall not do disservice to the grand
portion of cosmical physics to which volcanic phenomena belong, by
devoting the few pages accorded to me for this Introduction to sketching
what seems to me to be the present position of terrestrial
_Vulcanicity_, and tracing the outlines and relations of the two
branches of scientific investigation--_Vulcanology_ and _Seismology_--by
which its true nature and part in the Cosmos are chiefly to be
ascertained.

The general term, _Vulcanicity_, properly comprehends all that we see or
know of actions taking place upon and modifying the surface of our
globe, which are referable not to forces of origin above the surface,
and acting superficially, but to causes that have been or are in
operation beneath it. It embraces all that Humboldt has somewhat vaguely
called "the reactions of the interior of a planet upon its exterior."

These reactions show themselves principally and mainly in the marking
out and configuration of the great continents and ocean beds, in the
forcing up of mountain chains, and in the varied phenomena consequent
thereon, as seen in more or less adjacent formations.

These constitute the mechanism which has moulded and fashioned the
surface of our globe from the period when it first became superficially
solid, and prepared it as the theatre for the action of all those
superficial actions--such as those of tides, waves, rain, rivers, solar
heat, frost, vitality, vegetable and animal (passing by many others less
obvious)--which perpetually modify, alter or renew the surface of our
world, and maintain the existing regimen of the great machine, and of
its inhabitants. These last are the domain of Geology, properly so
called. No geological system can be well founded, or can completely
explain the working of the world's system as we now see it, that does
not start from Vulcanicity as thus defined; and this is equally true,
whether, as do most geologists, we include within the term Geology
everything we can know about our world as a whole, exclusive of what
Astronomy teaches as to it, dividing Geology in general into Physical
Geology--the boundaries of which are very indistinct--and
Stratigraphical Geology, whose limits are equally so.

It has been often said that Geology in this widest sense begins where
Astronomy or Cosmogony ends its information as to our globe, but this is
scarcely true.

Vulcanicity--or Geology, if we choose to make it comprehend that--must
commence its survey of our world as a nebula upon which, for unknown
ages, thermic, gravitant and chemical forces were operative, and to the
final play of which, the form, density and volume, as well as order of
deposition of the different elements in the order of their chemical
combination and deposition was due, when first our globe became a liquid
or partly liquid spheroid, and which have equally determined the
chemical nature of the materials of the outward rind of the earth that
now is, and with these some of the primary conditions that have fixed
the characters, nature and interdependence of the vegetables and animals
that inhabit it. Physical Astronomy and Physical Geology, through
Vulcanicity, thus overlap each other; the first does not end where the
second begins; and in every sure attempt to bring Geology to that
pinnacle which is the proper ideal of its completed design--namely, the
interpretation of our world's machine, as part of the universal Cosmos
(so far as that can ever become known to our limited observation and
intelligence)--we must carry with us astronomic considerations, we must
keep in view events anterior to the "_status consistentior_" of
Leibnitz, nor lose sight of the fact that the chain of causation is one
endless and unbroken; that forces first set moving, we know not when or
how, the dim remoteness of which imagination tries to sound in shadowy
thought, like those of the grand old Eastern poem, "When the morning
stars first sang together," are, however changed in form, operative
still. The light and fragile butterfly, whose glorious garb irradiates
the summer zephyr in which it floats, has had its power of flight--which
is its power to live--determined by results of that same chain of causes
that lifted from the depths the mountain on whose sunny side he floats,
that has determined the seasons and the colour of the flower whose
nectar he sucks, and that discharges or dissipates the storm above, that
may crush the insect and the blossom in which it basked. And thus, as
has been said, it was not all a myth, that in older days affirmed that
in some mysterious way the actions and the lives of men were linked to
the stars in their courses.

Whatever may have been the manifestations of Vulcanicity at former and
far remoter epochs of our planet, and to which I shall return, in the
existing state of regimen of and upon our globe it shows itself chiefly
in the phenomena of Volcanoes and of Earthquakes, which are the
subjects of Vulcanology and of Seismology respectively, and in principal
part, also, of this Introduction.

The phenomena of hot springs, geysers, etc., which might be included
under the title of Thermopægology, have certain relations to both, but
more immediately to Vulcanology.

Let us now glance at the history and progress of knowledge in these two
chief domains of Vulcanicity, preparatory to a sketch of its existing
stage as to both, and, by the way, attempt to extract a lesson as to the
methods by which such success as has attended our labours has been
achieved.

It will be most convenient to treat of Seismology first in order.

Aristotle--who devotes a larger space of his Fourth Book, Περἱ Κοσμου,
to Earthquakes--Seneca, Pliny, Strabo, in the so-called classic days,
and thence no end of writers down to about the end of the seventeenth
century--amongst whom Fromondi (1527) and Travagini (1679) are, perhaps,
the most important now--have filled volumes with records of facts, or
what they took to be such, of Earthquakes, as handed down to or observed
by themselves, and with plenty of hypotheses as to their nature and
origin, but sterile of much real knowledge.

Hooke's "Discourses of Earthquakes," read before the Royal Society about
1690, afford a curious example of how abuse of words once given by
authority clings as a hindrance to progress. He had formed no distinct
idea of what he meant by an Earthquake, and so confusedly mixes up all
elevations or depressions of a permanent character with "subversions,
conversions and transpositions of parts of the earth," however sudden or
transitory, under the name of Earthquakes.

A like confusion is far from uncommon amongst geological writers, even
at the present day, and examples might be quoted from very late writings
of even some of the great leaders of English Geology.

From the seventeenth to the middle of the eighteenth century one finds
floods of hypotheses from Flamsteed, Höttinger, Amontons, Stukeley,
Beccaria, Percival, Priestly, and a crowd of others, in which
electricity, then attracting so much attention, is often called upon to
supply causation for a something of which no clear idea had been formed.
Count Bylandt's singular work, published in 1835, though showing a
curious _partial_ insight in point of advancement, might be put back
into that preceding period.

In 1760 appeared the very remarkable Paper, in the fifty-first volume of
the "Philosophical Transactions," of the Rev. John Mitchell, of
Cambridge, in which he views an Earthquake as a sudden lifting up, by a
rapid evolution of steam or gas beneath, of a portion of the earth's
crust, and the lateral transfer of this gaseous bubble beneath the
earth's crust, bent to follow its shape and motion, or that of a wave of
liquid rock beneath, like a carpet shaken on air. Great as are certain
collateral merits of Mitchell's Paper, showing observation of various
sorts much in advance of his time, this notion of an Earthquake is such
as, had he applied to it even the imperfect knowledge of mechanics and
physics then possessed in a definite manner, he could scarcely have
failed to see its untenable nature. That the same notion, and in a far
more extravagant form, should have been reproduced in 1843 by Messrs.
Rogers, by whom the gigantic parallel anticlinals, flanks and valleys of
the whole Appalachian chain of mountains are taken for nothing more than
the indurated foldings and wrinkles of Mitchell's carpet, is one of the
most salient examples of the abuse of hypothesis untested by exact
science.

Neither Humboldt nor Darwin, great as were the opportunities of
observation enjoyed by both, can be supposed to have formed any definite
idea of _what_ an Earthquake is; and the latter, who had observed well
the effects of great sea-waves rolling in-shore after the shock, did not
establish any clear relation between the two.[A]

Hitherto no one appears to have formed any clear notion as to what an
Earthquake is--that is to say, any clear idea of what is the nature of
the movement constituting the shock, no matter what may be the nature or
origin of the movement itself. The first glimmering of such an idea, so
far as my reading has enabled me to ascertain, is due to the penetrating
genius of Dr. Thomas Young, who, in his "Lectures on Natural
Philosophy," published in 1807, casually suggests the probability that
earthquake motions are vibratory, and are analogous to those of
sound.[B] This was rendered somewhat more definite by Gay Lussac, who,
in an able paper "On the Chemical Theories of Volcanoes," in the
twenty-second volume of the "Annales de Chémie," in 1823, says: "En un
mot, les tremblements de terre ne sont que la propagation d'une
commotion à travers la masse de la terre, tellement indépendante des
cavités souterraines qu'elle s'entendrait, d'autant plus loin que la
terre serait plus homogène."

These suggestions of Young and of Gay Lussac, as may be seen, only refer
to the movement in the more or less solid crust of the earth. But two,
if not three, other great movements were long known to frequently
accompany earthquake shocks--the recession of the sea from the shore
just about the moment of shock--the terrible sounds or subterraneous
growlings which sometimes preceded, sometimes accompanied, and sometimes
followed the shock--and the great sea-wave which rolls in-shore more or
less long after it, remained still unknown as to their nature. They had
been recognised only as concomitant but unconnected phenomena--the more
inexplicable, because sometimes present, sometimes absent, and wholly
without any known mutual bearing or community of cause.

On the 9th February, 1846, I communicated to the Royal Irish Academy my
Paper, "On the Dynamics of Earthquakes," printed in Vol. XXI., Part I.,
of the Transactions of that Academy, and published the same year in
which it was my good fortune to have been able to colligate the observed
facts, and bringing them together under the light of the known laws of
production and propagation of vibratory waves in elastic, solid, liquid
and gaseous bodies, and of the production and propagation of liquid
waves of translation in water varying in depth, to prove that all the
phenomena of earthquake shocks could be accounted for by a single
impulse given at a single centre. The definition given by me in that
Paper is that an earthquake is "_The transit of a wave or waves of
elastic compression in any direction, from vertically upwards to
horizontally, in any azimuth, through the crust and surface of the
earth, from any centre of impulse or from more than one, and which may
be attended with sound and tidal waves dependent upon the impulse and
upon circumstances of position as to sea and land_."

Thus, for example, if the impulse (whatever may be its cause) be
delivered somewhere beneath the bed of the sea, all four classes of
earthquake waves may reach an observer on shore in succession. The
elastic wave of shock passing through the earth _generally_ reaches him
first: its velocity of propagation depending upon the specific
elasticity and the degree of continuity of the rocky or the incoherent
formations or materials through which it passes.

Under conditions pointed out by me, this elastic wave may cause an
aqueous wave, producing recession of the sea, just as it reaches the
margin of sea and land.

If the impulse be attended by fractures of the earth's crust, or other
sufficient causes for the impulse to be communicated to the air directly
or through the intervening sea, ordinary sound-waves will reach the
observer through the air, propagated at the rate of 1,140 feet per
second, or thereabouts; and may also reach him before or with or soon
after the shock itself, through the solid material of the earth; and
lastly, if the impulse be sufficient to disturb the sea-bottom above the
centre of impulse, or otherwise to generate an aqueous wave of
translation, that reaches the observer last, rolling in-shore as the
terrible "great sea-wave," which has ended so many of the great
earthquakes, its dimensions and its rate of propagation depending upon
the magnitude of the originating impulse and upon the variable depth of
the water. It is not my purpose, nor would it be possible within my
limits here, to give any complete account of the matter contained in
that Paper, which, in the words of the President of the Academy upon a
later occasion, "fixed upon an immutable basis the true theory of
Earthquakes."[C] I should state, however, that in it I proved the
fallacy of the notion of vorticose shocks, which had been held from the
days of Aristotle, and showed that the effects (such as the twisting on
their bases of the Calabrian Obelisks) which had been supposed due to
such, were but resolved motions, due to the transit rectilinearly of the
shock.

This removed one apparent stumbling block to the true theory.

Incidentally also it was shown that from the observed elements of the
movement of the elastic wave of shock at certain points--by suitable
instruments--the position and depth of the _focus_, or centre of
impulse, might be inferred.

In the same volume ("Transactions of the Royal I. Academy," XXI.) I gave
account, with a design to scale, for the first self-registering and
recording seismometer ever, to my knowledge, proposed. In some respects
in principle it resembles that of Professor Palmieri, of which he has
made such extended use at the Vesuvian Observatory, though it differs
much from the latter in detail. In June, 1847, Mr. Hopkins, of
Cambridge, read his Report, "On the Geological Theories of Elevation and
Earthquakes," to the British Association--requested by that body the
year before--and printed in its Reports for that year.

The chief features of this document are a digest of Mr. Hopkins's
previously published "Mathematical Papers" on the formations of
fissures, etc., by elevations and depressions, and those on the
thickness of the earth's crust, based on precession, etc., which he
discusses in some relations to volcanic action.

This extends to forty-one pages, the remaining eighteen pages of the
Report being devoted to "Vibratory Motions of the Earth's Crust produced
by Subterranean Forces--Earthquakes."

The latter consists mainly of a _résumé_ of the acknowledged laws, as
delivered principally by Poisson, of formation and propagation of
elastic waves and of liquid waves, by Webers, S. Russel and others--the
original matter in this Report is small--and as respects the latter
portion consists mainly in some problems for finding analytically the
position or depth of the centre of disturbance when certain elements of
the wave of shock are given, or have been supposed registered by
seismometric instruments, such as that described by myself, and above
referred to.[D] At the time my original Paper "On the Dynamics of
Earthquakes" was published, there was little or no _experimental_
knowledge as to the actual velocity of transit of waves--analogous to
those of sound, but of greater amplitude--through elastic solids. The
velocity as deduced from theory, the solid being assumed quite
_homogeneous_ and _continuous_, was very great, and might be taken for
some of the harder and denser rock formations at 11,000 or 12,000 feet
per second. That these enormous velocities of wave transit would be
something near those of actual earthquake shock seemed probable to me,
and was so accepted by Hopkins.

Thus, he says (Report, p. 88): "The velocity of the sea-wave, for any
probable depth of the sea, will be so small as compared with that of the
vibratory wave, that we may consider the time of the arrival of the
latter at the place of observation as coincident with that of the
departure of the sea-wave from the centre of divergence."

In my original Paper (Dynamic, &c.), I had suggested, as an important
object, to ascertain by actual experiment what might be the wave's
transit rate in various rocky and incoherent formations; and having
proposed this in my first "Report upon the Facts of Earthquake" to the
British Association, I was enabled by its liberality to commence those
experiments, in which I was ably assisted by my eldest son, then quite a
lad--Dr. Jno. William Mallet, now Professor of Chemistry at the
University of Virginia, U.S.; and to give account of the results, in my
second Report ("Report, British Association for 1851") to that body.

Those experiments were made by producing an impulse at one end of an
accurately measured base line, by the explosion of gunpowder in the
formation experimented upon, and noting the time the elastic wave
generated required to pass over that distance, upon a nearly level
surface. Special instruments were devised and employed, by which the
powder was fired and the time registered, by touching a lever which
completed certain galvanic contacts. The media or formations in which
these experiments were conducted were, damp sand--as likely to give the
minimum rate--and crystalline rock (granite), as likely to give the
maximum. The results were received, not with doubt, but with much
surprise, for it at once appeared that the actual velocity of transit
was vastly below what theory had indicated as derivable from the density
and modulus of elasticity of the material, taken as homogeneous, etc.
The actual velocities in feet per second found were:

  In sand                          824·915 feet per second.
  In discontinuous and
    much shattered granite       1,306·425      "     "
  In more solid granite          1,664·574      "     "

This I at once attributed, and as it has since been proved correctly, to
the loss of _vis viva_, and consequently of speed, by the _discontinuity
of the materials_.

And some indication of the general truth of the fact was derivable from
comparing the rude previous approximations to the transit rate of some
great Earthquakes. In the case of that of Lisbon, estimated by Mitchell
at 1,760 feet per second. It was still desirable to extend similar
experiments to the harder classes of stratified and of contorted rocks.
This I was enabled to carry into effect, at the great Quarries at
Holyhead (whence the slate and quartz rocks have been obtained for the
construction of the Asylum Harbour there), taking advantage of the
impulses generated at that period by the great mines of powder exploded
in these rocks.

The results have been published in the "Philosophical Transactions for
1861 and 1862 (Appendix)." They show that the mean lowest rate of wave
transit in those rocks, through measured ranges of from 5,038 to 6,582
feet, was 1,089 feet per second; and the mean highest, 1,352 feet per
second; and the general mean 1,320 feet per second.

By a separate train of experiments on the compressibility of solid cubes
of these rocks, I obtained the mean modulus of elasticity of the
material when perfectly continuous and unshattered, with this
remarkable result--that in these rocks, as they exist at Holyhead,
_nearly seven-eighths of the full velocity of wave transmission due to
the material, if solid and continuous, is lost by reason of the
heterogeneity and discontinuity_ of the rocky masses as they are found
piled together in Nature.

I also proved that the wave-transit period of the unshattered material
of these rocks was greatest in a direction _transverse_ to the bedding,
and least in line parallel with that; but the effect of this in the
rocky mass itself may be _more_ than counterbalanced by the
discontinuity and imperfect contact of the adjacent beds.

These results indicate, therefore, that the superficial rate of
translation of the solitary sea-wave of earthquakes may, when over very
deep water, equal or even exceed the transit rate (in some cases) of the
elastic wave of shock itself.

These results have since received general confirmation by the careful
determinations of the transit rates of actual earthquake waves, in the
rocks of the Rhine Country and in Hungary, by Nöggerath and Schmidt
respectively, and by those made since by myself in those of Southern
Italy, to which I shall again refer. In an elastic wave propagated from
a centre of impulse in an infinitely extended volume of a perfect gas,
normal vibrations are alone propagated--as is the case with sound in
air.

In the case of like movements propagated in elastic and perfectly
homogeneous and isotropic solids, the wave possesses both normal and
transversal vibrations, and is, in so far, analogous to the case of
light. Mr. Hopkins, in his Report above referred to, has based certain
speculations upon the assumed necessary co-existence of both orders of
vibration in actual earthquake shocks in the materials of which our
earthy crust is actually composed.

The existence of transversal vibration in those materials has not been
yet proved experimentally, though there is sufficient ground to preclude
our denying their probable existence.

That if they do exist they play but a very subordinate part in the
observable phenomena of actual Earthquake is highly probable. This is
the view, supported not only by observations of the effects of such
shocks in Nature, but by the theoretic consideration of the effects of
discontinuity of formations in planes or beds more or less transverse to
the wave path (or line joining the centre of impulse with the mean
centre of wave disturbance at any point of its transit). If we suppose,
for illustration sake, such an elastic wave transmitted perpendicularly
through a mass of glass plates, each indefinitely thin, and all in
absolute contact with each other, but without adhesion or friction, more
or less of the transversal vibration of the wave would be cut off and
lost at each transit from plate to plate, as the elastic compression
can, by the conditions, be transmitted only normally or by direct push
perpendicularly from plate to plate. This must take place in Nature, and
to a very great extent, and the consideration, with others, enabled me
generally to apply the normal wave motion of shock alone to my
investigation as to the depth of the centre of impulse of the great
Neapolitan Earthquake of 1857, an account of which was published in
1862, and to be presently further referred to.

Hitherto the multitudinous facts, or supposed facts, recorded in
numberless accounts of Earthquakes had remained almost wholly
unclassified, and so far as they had been discussed--in a very partial
manner, as incidental portions of geological treatises--with little
attempt to sift the fabulous from the real, or to connect the phenomena
admitted by reference to any general mechanical or physical causes. In
1850 my first "Report upon the Facts of Earthquakes," called for by the
British Association in 1847, was read and published in the Reports of
that body for that year. In this, for the first time, the many recorded
phenomena of Earthquakes are classified, and the important division of
the phenomena into primary and secondary effects of the shock was
established. Several facts or phenomena, previously held as marvellous
or inexplicable, were either, on sufficient grounds, rejected, or were,
for the first time, shown susceptible of explanation. Amongst the more
noticeable results were the pointing out that fissures and fractures of
rock or of incoherent formations were but secondary effects, and, in the
latter, were, in fact, generally of the nature of inceptive landslips.
This last was not accepted, I believe, by geologists at the time; but
the correctness of the views then propounded as to earth fissures--the
nature of the spouting from them of water or mud--the appearances taken
for smoke issuing from them, etc.--have since been fully confirmed,
first, by my own observations upon the effects of the Great Neapolitan
Earthquake of 1857, and more lately by those of Dr. Oldham upon the
Earthquake of Cachar (India), where he was enabled to observe fissures
of immense magnitude, the nature of the production of which he has well
described and explained in the "Proceedings, Geological Society, London,
1872."

The relations between meteorological phenomena proper and Earthquakes
have always been a subject of popular belief and superstition.

This was here carefully discussed, and with the result of disproving any
connection, or, if any, but of an indirect nature. I also, to some
extent, towards the end of this Report, discussed the question of the
possible nature of the _impulse itself_ which originates the shock; I
showed that it must be of the nature of a blow, and ventured to offer
_conjecturally_ five possible causes of the impulse:

     1. Sudden fractures of rock, resulting from the steady and slow
     increase of elevatory pressure.

     2. Sudden evolution (under special conditions) of steam.

     3. Sudden condensation of steam, also under special conditions.

     4. Sudden dislocations in the rocky crust of the earth, through
     pressure acting in any direction.

     5. Occasionally through the recoil due to explosive effects at
     volcanic foci (p. 79-80).

The first and last of these I am, through subsequent light, disposed now
to withdraw or greatly to modify.

The first, the supposed "_snap and jar_, occasioned by the sudden and
violent rupture of solid rock masses," to which Mr. Scrope, in his very
admirable work on Volcanoes, is disposed to refer the impulse of
earthquake shocks (Scrope, 2nd edit., p. 294), I believe may be proved
on acknowledged physical principles--when applied to the known
elasticities and extensibilities of rocks, and keeping in view the small
thicknesses fractured _at the same instant_--to be capable of only the
most insignificant impulsive effects; and if we also take into
consideration that strata, if so fractured, are necessarily not _free_,
but surrounded by others above and below, any such impulsive effect
emanating from fracture may be held as non-existent or impossible. In
the statement of his views which follows, and in objecting to my second
and third possible causes (p. 295-296, headed "Objections to Mallet's
Theory"), Mr. Scrope appears to me to have fallen into the error of
assuming that the nature of the _impulse_, or the cause producing it,
forms any part of "my theory of earthquake movement," or in anywise
affects it. I carefully guarded against this in the original Paper
("Transactions, Royal Irish Academy," Vol. XXI., p. 60, and again, p.
97), when I stated "it is quite immaterial to the truth of my theory of
earthquake motion what view be adopted, or what mechanism be assigned,
to account for the original impulse."

As regards the fifth conjecture suggested by me, I am now, with better
knowledge and larger observation of volcanic phenomena, not prepared to
admit any single explosion at volcanic vents of a magnitude sufficient
to produce by its recoil an earthquake wave of any importance, or
extending to any great distance in the earth's crust. The rock of 200
tons weight, said to have been projected nine miles from the crater of
Cotopaxi, which I quoted from Humboldt as an example,[E] I believe to be
as purely mythical as the rock (_bloc rejetté_) of perhaps one-sixth of
that weight which, previous to the late eruption, lay in the middle of
the Atria dell Cavallo, and which it was roundly affirmed had been
_blown_ out of the crater, but which in reality had at some time rolled
down from near the top of the cone, after having been dislodged from
some part of the upper lip of the crater walls, where, as its wonderful
hardness and texture and its enamel-like surface showed, it had been
roasted for years probably.

Nor do I believe in the _sudden_ blowing away of one-half the crater and
cone of Vesuvius, or of any other volcano, at one effort, however
affirmed.

Nothing more than conjecture as to the nature of the impulse producing
great or small Earthquakes can, I believe, as yet be produced. That
there is some one master mechanism productive of most of the impulses of
great shocks is highly probable, but that more causes than one may
produce these impulses, and that the causes operative in small and long
repeated shocks, like those of Visp-Comrie and East Haddam, differ much
from those producing great Earthquakes, is almost certain.

We shall be better prepared to assign all of these when we have admitted
a true theory of volcanic action, and so are better able to see the
intimate relations in mechanism between seismic and volcanic actions.

It is not difficult meanwhile to assign the very probable mechanism of
those comparatively petty repercussions which are experienced in close
proximity to volcanic vents when in eruption, and which, though
certainly seismic in their nature, and powerful enough, as upon the
flanks of Etna, to crack and fissure well-built church-towers, can
scarcely be termed Earthquakes.

In my First Report I stated that almost nothing was known then of the
distribution of recorded Earthquakes in time or in space over our
globe's surface, and I proposed the formation and discussion of a
complete catalogue of all recorded Earthquakes, with this in view.

This was approved by the Council of the British Association and at once
undertaken by me, with the zealous and efficient co-operation of my
eldest son, Dr. J. W. Mallet. Nearly the whole of the Second British
Association Report, of 1851, is occupied with the account of the
experiments as to the transit rate of artificially made shocks in sand
and granite, as already referred to.

The Third Report, of 1852-1854, contains the whole of this, "The
Earthquake Catalogue of the British Association" (of which, through the
liberality of that body, more than one hundred copies were distributed
freely), in which are given, in columnar form, the following
particulars, from the earliest known dates to the end of 1842:

     1. The date and time of day, as nearly as recorded.

     2. The locality or place of occurrence.

     3. The direction, duration, and number of shocks so far
     recorded.

     4. Phenomena connected with the sea--great sea-waves, tides,
     etc.

     5. Phenomena connected with the land--meteorological phenomena
     preceding and succeeding. Secondary phenomena--all minor or
     remarkable phenomena recorded.

     6. The authority for the record.

Though most materially assisted by the previous labours and partial
catalogues of Von Hoff, Cotte, Hoffman, Merrian, and, above all, of
Perrey, the preparation of this catalogue--which demanded visits to the
chief libraries of Europe, and the collating of some thousands of
authors in various languages and of all time--was a work of great and
sustained labour, which, except for my dear son's help, I should never
have found time and power to complete. Professor Perrey, formerly of the
Faculté des Sciences of Dijon, now _en retrait_, who has devoted a long
and useful life to assiduous labours in connection with Seismology, was
our great ally; and his catalogues are so large and complete for most
known parts of the world after 1842, that we were able to arrest our own
catalogue at that date, and take M. Perrey's as their continuation up to
1850.

The whole British Association Catalogue thus embraces the long historic
period of from 1606 B.C. of vulgar chronology, when the first known
Earthquake is recorded, to A.D. 1850; and the base of induction which it
presents as to the facts recorded extends to between 6,000 and 7,000
separate Earthquakes. My Fourth Report ("Reports, British Association,
1858,") is occupied principally with the discussion of this great
catalogue, and with that of several special catalogues produced by other
authors with limited areas or objects.

The discussion of M. Perrey's local catalogues with those of others, in
reference to a supposed prevalent apparent horizontal direction of shock
in certain regions--as to distribution, as to season, months, time of
day or night, relation to state of tide--the bearings of the views of
Zantedeschi and others as to the probable existence of a terrane
tide--the supposed relations of the occurrence of Earthquakes upon the
age of the moon, as deduced by Perrey, viz.: that 1st, Earthquakes occur
most frequently at the syzygies; 2nd, that their frequency increases at
the perigee and diminishes at the apogee; 3rd, that they are more
frequent when the moon is on the meridian than when she is 90° away from
it--and the views of several authorities as to the distribution of
Earthquakes in time and in space--occupy the first 46 pages of this
Report.

It then proceeds to discuss the distribution in time and in space as
deduced from the full base of the great catalogue.

The results as to time are reduced to curves, and those as to space (or
distribution over our globe's surface) to the great seismic map
(Mercator's projection), upon which and in accordance with certain
principles and conventional laws, which admit of the indication of both
intensity and frequency, all recorded Earthquakes have been so laid down
as to present a real indication of the distribution of seismic energy
for the whole historic period and all over the world.

The original of this map, which also shows the Volcano (size, about 7
feet by 5 feet), remains for reference in the custody of the Royal
Society. A reduced copy was published with the Report, and to a still
more reduced scale has been reproduced in other places. It is impossible
here to do more than refer to a few of the more salient points.

As regards distribution in time, durational seismic energy may be
considered as probably constant during historic time, though it is
probably a decaying energy viewed in reference to much longer periods.
It does not appear of the nature of a distinctly periodic force.

     1. Whilst the minimum paroxysmal interval may be a year or two,
     the average interval is from five to ten years of comparative
     repose.

     2. The shorter intervals are in connection with periods of
     fewer Earthquakes, not always with those of least intensity,
     but usually so.

     3. The alternations of paroxysm and of repose appear to follow
     no absolute law deducible from these causes.

     4. Two marked periods of extreme paroxysm are observable in
     each century (for the last three centuries), one greater than
     the other--that of greatest number and intensity occurring
     about the middle of each century, and the other towards the end
     of each.

As respects season, there appear distinct indications of a maximum about
the winter solstice, and equally so of a minimum rather before the
autumnal equinox. It is not improbable that there is a remote relation
between Earthquakes and the annual march of barometric pressure.

We may expect, at present, one great Earthquake about every eight
months, and were we possessed of a sufficient report from all parts of
our globe, we should probably find scarcely a day pass without a very
sensible Earthquake occurring somewhere, whilst, as regards still
smaller tremors, it might almost be said that our globe, as a whole, is
scarcely ever free from them.

As respects the distribution of seismic energy in space of our earth's
surface, it is that of bands of variable and of great breadth, with
sensible seismic influence extending to from 5° to 15° transversely,
which very generally follow:

     1. The lines of elevated tracts which mark and divide the great
     oceanic or terra-oceanic basins (or _saucers_, as I have called
     them, from their shallowness in relation to surface, in this
     discussion) of the earth's surface.

     2. And in so far as these are frequently the lines of mountain
     chains, and these latter those of volcanic vents, so the
     seismic bands are found to follow these likewise. Isolated
     Volcanoes are found in these bands also.

     3. While sensible seismic influence is generally limited to
     the average width of the band, paroxysmal efforts are
     occasionally propagated to great distances transversely beyond
     that.

     4. The sensible width of the band depends upon the energy
     developed at each point of the length, and upon the accidental
     geologic and topographic conditions along the same.

     5. Seismic energy _may_ become sensible at any point of the
     earth's surface, its efforts being, however, greater and more
     frequent as the great lines of elevation and of volcanic
     activity are approached; yet not in the inverse ratio of
     distance, for many of the most frequently and terribly shaken
     regions of the earth, as the east shore of the Adriatic, Syria,
     Asia Minor, Northern India, etc., are at great distances from
     active Volcanoes.

     6. The surfaces of minimum or of no known disturbance are the
     central areas of great oceanic or of terra-oceanic basins or
     saucers, and the greater islands existing in shallow seas.

Space obliges me to pass unnoticed here many minor but not unimportant
deductions. The discussions as to distribution in time and space occupy
seventy-two pages of this fourth and last Report, the remainder of which
(thirty-one pages) embraces the description and mathematical discussion
as to seismometers, to which I may refer, as comprising the most
complete account of these instruments that has, I believe, been anywhere
given.

The appendix to the Report comprises the entire bibliography of
Earthquakes collected during those researches, and a concluding chapter
on desiderata, and inquiries as to ill-understood phenomena supposed to
be connected with Earthquakes.

       *       *       *       *       *

In 1849-50, I was honoured by the request to draw up the article
"Earthquake Phenomena," which has appeared in the first and subsequent
editions of the "Admiralty Manual of Scientific Inquiry." Originally the
subject was intended to have formed part of the article on Geology,
entrusted to Mr. Darwin, who consulted me upon the subject; and upon my
representing how much Earthquakes had, within a short time, become
matter for the mathematician and physicist, he, with a singleness of eye
to science which it is but just to place on record, took the necessary
steps with the Admiralty authorities that Earthquakes should form a
separate article, and advised its being placed, as it was, in my hands.
To record this will, I believe, be sufficient justification for my
reference to this article, in which a good deal of information as to
Seismometry is to be found.

       *       *       *       *       *

By recurring to Mr. Hopkins's Report on Earthquake Theory, before
remarked upon ("Report of British Association, 1847"), it will be seen
that the solutions of the problems which he there gives for finding the
depth of focus of shock are founded upon the _velocity of propagation_
of the wave in the interior of the mass, the _apparent horizontal
velocity_ and the _horizontal direction of propagation_ at any proposed
point being known (p. 82).

By this it appears plainly that at that time Mr. Hopkins supposed that
it was the _velocity of translation_ of the wave of shock that did the
mischief, and not the _velocity of the wave particle_, or wave itself.
And, further, that the former might be obtained by reference simply to
the modulus of elasticity of the rock of any given formation, as,
indeed, was my own earliest view when I produced my "Dynamics of
Earthquake" in 1846. From the remarks already made as to the vast
difference between the actual transit velocity in more or less
discontinuous rocks--such as they occur in Nature--it will be equally
obvious that Mr. Hopkins's methods, as above mentioned, are
impracticable, even were there no confusion between the velocity of
translation of the wave and that of the wave particle or wave itself.

This applies also to the demonstration and diagram (taken from Hopkins)
given by Professor Phillips ("Vesuvius," pp. 258-259).

In December, 1857, occurred the great Neapolitan Earthquake, which
desolated a large portion of that kingdom; and an opportunity then arose
for practically applying to the problems of finding the directions of
earthquake shock at a given point through which it has passed, and
ultimately the position and depth of focus, other methods, which I had
seen, from soon after the date of publication of my original Paper
(1846), were easily practicable, and the details of which I had
gradually matured.

Bearing in mind that, in the case of the normal vibration in any elastic
solid of indefinite dimensions, the direction of motion in space of the
_wave particle_ coincides in the first semiphase of the wave, and at the
instant of its _maximum velocity_ with the right line joining the
particle and the focus or centre of disturbance, it follows that, in the
case of earthquakes, the normal vibration of the wave of shock is always
in a vertical plane passing through the focus and any point on the
earth's surface through which the shock passes (assuming for the present
no disturbing causes after the impulse has been given), and that at such
a point the movement of the wave particle in the first semiphase of the
wave is in the same direction or sense as that of translation; and at
the moment of maximum velocity the direction in space of the motion of
the wave particle is that of the right line joining the point through
which the wave has passed with the focus or centre of impulse.

If, therefore, we can determine the direction of motion of the wave
particle in the first semiphase, and its maximum velocity, we can
obtain, from any selected point, a line (that of emergence of the shock)
_somewhere in which_, if prolonged beneath the earth, the focus must
have been; and if we can obtain like results for two or more selected
points, we decide the position and the depth of the focus, which must be
in the intersection of the several lines of direction of the wave
particle motion at each point, when prolonged downwards.

Now, as I have said, it is the _vibration of the wave itself_, _i.e._,
the motion of the wave particle that does the mischief--_not_ the
transit of the wave from place to place on the surface; just as in the
analogous (but _not_ similar) case of a tidal wave of translation
running up an estuary and passing a ship anchored there, it is not the
transit up the channel, but the wave form itself--_i.e._, the motion of
the wave particles--that lifts the ship, sends her a little way higher
up channel, drops her to her former level, and sends her down channel
again to the spot she lay in just before the arrival of the wave.

Everything, therefore, that has been permanently disturbed by an
earthquake shock has been thus moved in the direction and with the
maximum velocity impressed upon it by the wave particle in the first
semiphase of the wave; and thus almost everything that has been so
disturbed may, by the application of established dynamical principles,
be made to give us more or less information as to the velocity of the
wave particle (or as we, for shortness, say, the velocity of shock), the
direction of its normal vibration, and the position and depth beneath
the earth's surface, from which came the generating impulse. We thus
arrive at these as simply and as surely as we can infer from the
position taken by a billiard ball, on which certain forces are known to
have acted, the forces themselves and their direction; or, from a broken
beam, the pressure or the blow which fractured it.

It is obvious, then, that nearly every object disturbed, dislocated,
fractured or overthrown by an earthquake shock is a sort of natural
seismometer, and the best and surest of all seismometers, if we only
make a judicious choice of the objects which being found after such a
shock, we shall employ for our purpose. This was the principle which I
proposed to the Royal Society at once to apply to the effects of the
then quite recent great Neapolitan Earthquake of 1857, and which,
through the liberality and aid of that body, I was enabled to employ
with the result I had pretty confidently anticipated, namely, the
ascertainment of the approximate depth of the focus.

_Every_ shock-disturbed object in an earthquake-shaken country is
capable of giving _some_ information as to the shock that acted upon it;
but it needs a careful choice, and some mechanical νους, to select
_proper_ and the best objects, so as to avoid the needless perplexity of
disturbing forces _not_ proper to the shock, or other complications.

When properly chosen, these natural seismometers, or evidences fitted
for observation after the shock, are of two great classes, by which the
conditions of the earthquake motion are discoverable:

     1. Fractures or dislocations (chiefly in the masonry of
     buildings), which afford two principal sources and sorts of
     information, namely:

     _a._ From the observed _directions of fractures or fissures_,
     by which the _wave path_, and frequently the _angle of
     emergence_, may be immediately inferred.

     _b._ Information from the preceding, united with known
     conditions as to the strength of materials to resist
     _fracture_, by which the _velocity_ of the fracturing impulse
     may be calculated.

     2. The overthrow or the projection, or both, of bodies large or
     small, simple or complex. From these we are enabled to infer:

     _c._ By direct observation, the _direction in azimuth_ of the
     wave path.

     _d._ By measurements of the horizontal and vertical distances
     of overthrow or of projection, to infer either the _velocity_
     of projection, or _angle of emergence_.

Fractures by shock present their planes always nearly in directions
transverse to the wave path. Projections or overthrow take place (unless
secondarily disturbed) in the line of the wave path, or in the vertical
plane passing through it: but the direction of fall or overthrow may be
either in the same direction as the wave transit (_i.e._, as the motion
of the wave particle in the first semiphase), or contrary to it.

It is thus obvious that the principal phenomena presented by the effects
of earthquake shock upon the objects usually occurring upon the surface
of the inhabited parts of the earth, resolve themselves into problems of
three orders, and are all amenable to mechanical treatment, viz.:

     1. Problems relating to the direction and amount of velocity
     producing fracture or fissures.

     2. Problems relating to the single or multiplied oscillations
     of bodies, considered as compound pendulums.

     3. Problems referable to the theory of projectiles.

These three may combine in several cases, and on the part of the
observer must combine with measurements, angular and linear, and with
geodetic operations to be conducted in the shaken country.

The methods of application in detail are described fully, as well as
their actual application and results, in my work published in 1862 (2
vols.), entitled "The First Principles of Observational Seismology, as
developed in the Report to the Royal Society of London of the Expedition
made by Command of the Society into the Interior of the Kingdom of
Naples, to investigate the Circumstances of the Great Earthquake of
December, 1857," to the many illustrations of which the pecuniary grant,
in aid, of £300 was most liberally made to the publishers (Messrs.
Chapman and Hall) by the Society.

It is not my intention here, nor would space allow, of my going into the
details of observation, nor of the deductions and conclusions I have
recorded in those volumes. I have referred to their contents as marking
the advent of a new method. I have ventured to call it a new _organon_
in the investigation of Earthquakes, and, through them, of the deep
interior of our earth; and will only add that the method, on this its
very first trial, proved fertile and successful. The depth of focus for
this shock of December, 1857, was about seven to eight geographical
miles below sea level, roughly stated. It gives me great pleasure to add
that my friend, Dr. Oldham, Director-General of the Geological Survey of
India, has since applied these same methods to the phenomena of the
great Cachar Earthquake of the 10th January, 1869, and with success. The
pressure of official duties has, he informs me, as yet prevented his
fully working out his results, but they appear so far to indicate, as we
should expect, a depth of focus or origin considerably greater than in
the European case of 1857. Some account of Dr. Oldham's results were
this year communicated to the Geological Society of London through
myself, they are of great interest and importance.

Such, briefly and imperfectly sketched, is the existing state of
Seismology. As a branch of exact science it is, as it were, an affair of
yesterday. It is with reluctance that I have been compelled, in this
review, to refer to my own work so prominently. The harvest has been and
still is plenteous, but in this field of intellectual work the labourers
are few. This must continue to be so as long as Geology shall continue
to be viewed in public estimation (in England at least) as a fashionable
toy, that everyone who has been to school is supposed capable of
handling; and until all who profess to be geologists shall have learnt
that, to make sound progress, they must first become mathematicians,
physicists and chemists.

It is to the general imperfect knowledge of these sciences amongst
geologists that speculative errors show such vitality, and that Geology
makes such poor progress towards becoming the interpretation of the
world as a machine (_Erdkunde_).

It is for the same reason that Seismology and Vulcanology make little
progress; the first cannot be pursued beyond its present boundaries, nor
can even its present position be understood or explained by anyone
unfamiliar with the laws of wave motion, of all classes of waves; and it
would be easy to show, by quoting from various British or foreign
text-books on Geology, how extremely imperfect is the grasp of some of
the authors upon the subject of earthquake-wave motion, even such as
they admit and endeavour to explain and apply: in fact, many geologists
appear never to have framed to themselves any clear idea of what _is_ a
wave of any sort, liquid or elastic. The general silence as to seismic
theory of French geological writers is remarkable, to whatever cause
attributable. It has been said that French philosophers show themselves
little disposed to acknowledge or to follow the lead of their foreign
compeers in any branch of science. If this be true, or in so far as it
may be so, it is unworthy of French science, which has such boundless
claims upon our homage. I am disposed to attribute the fact in this case
to other circumstances; and, amongst these, to the small extent to which
our language is known amongst French scientific men.

Germany has shown more desire to cultivate this branch of science.
Although, as yet, the distinct enunciation of its fundamental principles
has but sparsely found its way into her text-books, several able
monographs, such as those of Schmidt and of Höttinger, prove how
completely some of her philosophers have mastered and how well applied
them. The men of science of Northern Italy, amongst whom so many
glorious names are to be found on the roll of discovery, have shown
themselves quite alive to the importance of Seismology; and I know of no
more clear, exact and popular exposition of its principles and
application, and of its cosmical relations, than is to be found in a
small volume by Professor Gerolamo Boccardo, published at Genoa in 1869,
entitled _Sismopirologia Terremoti, Vulcani e lente oscillazione del
suolo, saggio di una teoria di Geographia Fisica_.

My object, so far, has been to mark the progress of ascertained
theoretic notions as to Seismology. I have, therefore, passed without
notice many speculative monographs, and the treatment upon Earthquakes,
whether speculative or historical, and however able, that constitutes a
prominent feature of nearly all systematic works on Geology.

That which may be at present viewed as achieved and certainly
ascertained in theoretic Seismology is the clear conception of the
nature of earthquake motion; the relations to it of great sea or other
water wave commotions; the relations to it of sound waves--as to which,
however, more remains to be known; and the relations of all these to
secondary effects, tending in various ways to modify more or less the
topographic and other conditions of the land or sea bottom. And in
descriptive Seismology the present distribution of the earthquake bands
or regions of greatest seismic prevalence and activity are tolerably
ascertained, and their connection with volcanic lines and those of
elevation rendered more evident. Viewed alone, nothing can yet be said
to be absolutely ascertained as to the immediately antecedent cause or
causes of the impulse. The function of Earthquake, as part of the
cosmical machine, has become more clear, as the distinctive boundaries
between Earthquake and permanent elevation of the earth have been made
evident; and it has been seen that Earthquake, however contemporaneous
occasionally with permanent elevation, is not the cause, though it may
be one of the consequences of the same forces which produce elevation;
and thus, that an infinite number of Earthquakes, however violent, and
acting through however prolonged a time, can never act as an agent of
permanent elevation, unless, indeed, on that minute scale in which
surface elevation may arise from secondary effects, like that of the
Ullah Bund.

Much remains to be done, and much may be expected even from the
continuation, if done in a systematic and organised manner, of the
statistic record of Earthquakes in connection with those other branches
of cosmical statistics, Climatology, Meteorology, Terrestrial Magnetism,
etc., the observation of which is already, to a certain extent,
organised over a large portion of the globe.

And now let us look back for a moment to ask, How, by what mental path
of discovery, have we arrived at what we have passed in review?

The facts of Earthquakes have been before men for unknown ages "open
secrets," as Nature's facts have been well called; "but eyes had they
and saw not." Facts viewed through the haze of superstition, or of
foregone notions of what Nature _ought_ to do, cease to be facts. When,
after the great Calabrian Earthquake of 1783, the Royal Academy of
Naples sent forth its commission of its learned members to examine into
the effects, they had spread around them in sad profusion all that was
necessary to have enabled them to arrive at a true notion of the nature
of the shock, and thence a sound explanation of the varied and great
secondary effects they witnessed, and of which they have left us the
records in their Report, and the engravings illustrative of it. But we
look in vain for any light; the things seen, often with distortion or
exaggeration, are heaped together as in the phantasmagoria of a wild
and terrible dream, from which neither order nor conclusion follow.

Why was this? Why were these eminent _savants_ no more successful in
explaining what they saw than the ignorant peasants they found in the
Calabrian mountains?

Because physical science itself was not sufficiently advanced, no doubt;
but also because they had no notion of applying such science as they
had, to the very central point itself of the main problem before them,
freed from all possible adventitious conditions, and so, as it were,
attacking it in the rear. How different might have been the result of
their labours, had they begun by asking themselves, What is an
earthquake? Can we not try to find out what it _is_ by observing and
_measuring_ what it has done? We see the converse mode of dealing with
Nature in Torricelli. "Nature abhors a vacuum," was told him, as the
wisdom of his day. Possibly: but her abhorrence is limited, for I find
it is _measured_ by the pressure of a column of water of thirty-four
feet in height. We need not pursue the story with Pascal, up to the top
of the Puy de Dôme.

This lesson is instructive generally to all investigators, and
particularly here; for Vulcanology, to which we are about now to turn,
has occupied until almost to-day much the same position that Seismology
did in those of the Neapolitan Commissioners.

Whole libraries have been written with respect to it dealing with
_quality_, but _measure_ and _quantity_ remain to be applied to it.

To a very preponderant class in the civilised world no knowledge is of
much interest or value that does not point to what is called a
"practical result," one measurable into utility or coin. I do not stop
to remark as to the bad or as to certain good results of this tendency
of mind; but I may venture to point out to all, that the exact knowledge
of the nature of earthquake motion, even during the short time that it
has become known, has not been barren in results absolutely practical
and utilitarian. The minute investigation of the destruction of
buildings, etc., and the deductions that have been made as to the
relations between the form, height, materials, methods of building,
combination of timber and of masonry, and many other architectural or
constructive conditions, have made it certain now that earthquake-proof
houses and other edifices can be constructed with facility, and at no
great increase, if any at all, of cost. I can affirm that there is no
physical necessity why in frequently and violently shaken countries,
such as Southern Italy or the Oriental end generally of the
Mediterranean, victims should hereafter continue by thousands to be
sacrificed by the fall of their ill-designed and badly built houses.

Were a "Building Act" properly framed, put in force by the Italian
Government in the Basilicatas and Capitanata, etc., so that new houses
or existing ones, when rebuilt, should be so in accordance with certain
simple rules, a not very distant time can be foreseen when Earthquakes,
passing through these rich and fertile but now frequently sorely
afflicted regions, should come and go, having left but little trace of
ruin or death behind. Some disasters there must always be, for we
cannot make the flanks of mountains, nor the beds of torrents, etc.,
always secure; but the main mortality of all Earthquakes is in the
houses or other inhabited buildings. Make these proof, and the wholesale
slaughter is at an end.

The principles we have established have been thus practically applied in
another direction. The Japanese Government, with the keen and rapid
perception of the powers inherent in European science which
characterises now that wonderful people, has commenced to illuminate its
coasts by lighthouses constructed after the best European models. But
Japan is greatly convulsed by earthquakes, and lighthouses, as being
lofty buildings, are peculiarly liable to be destroyed by them.

The engineer of the Japanese Government for these lights, Mr. Thomas
Stevenson, C.E. (one of the engineers to the Commissioners of Northern
Lights), was instructed to have regard, in the design of those
lighthouses, to their exposure to shock. I was consulted by Mr.
Stevenson as to the general principles to be observed; and those
edifices have been constructed so that they are presumedly proof against
the most violent shocks likely to visit Japan; not, perhaps, upon the
best possible plan, but upon such as is truly based upon the principles
I have developed. Mr. Stevenson has published some account of their
construction.

The earthquake regions of South America might with incalculable benefit
apply those ideas; and, indeed, they have been, to some extent, already
applied by my friend, Mr. William Lloyd, Member of the Institution of
Civil Engineers, to the New Custom Houses constructed from his designs
at Valparaiso.

As one of these utilitarian views, and an important one, it will occur
to many to ask--Can the moment of the occurrence or the degree of
intensity of earthquake shock be predicted, or is it probable that at a
future day we may be able to predict them? At present, any prediction,
either of the one or the other, is impossible; and those few who have
professed themselves in possession of sufficient grounds for such
prediction are deceivers or deceived. Nor is it likely that, for very
many years to come, if ever, science shall have advanced so as to render
any such prediction possible; but it is neither impossible nor
improbable that the time shall arrive when, within certain, perhaps
wide, limits as to space, previous time, and instant of occurrence, such
forewarnings may be obtainable.

Earthquakes, like storms and tempests, and nearly all changes of
weather, are not periodic phenomena, nor yet absolutely uncertain or, so
to say, accidental as to recurrence.

They are quasi-periodic, that is to say, some of their conditions as to
causation rest upon a really periodic basis, as, for example, the
recurrence of storms upon the periodic march of the earth, and sun and
moon, etc., and the recurrence of Earthquakes upon the secular cooling
of our earth; but the conditions in both are so numerous and complicated
with particulars, that we cannot fully analyse them--hence, cannot
reduce the phenomena to law, and so cannot predict recurrence. Yet
storms and tempests--which were, along with pestilences and Earthquakes,
amongst the natural phenomena which Bishop Butler deemed in his own day
impossible of human prediction--have already, through the persistent and
systematised efforts of meteorological observers, become to a certain
extent foreseeable; and medical science assures us that it has rendered
that, though to a much less degree of probability, true of pestilences.

We may, therefore, give the utilitarian some hope, that if he will help
us along--who value our accessions of knowledge primarily upon a
different standard to his--in our talk of discovery, our posterity, in a
century or two hence, may not improbably possess the advantage of being
able, in some degree, to predict their Earthquakes. I fear the
inducement will go but a small way with the utilitarian generation,
whose bent tends much towards asking, "What has posterity ever done for
them?"

But though we cannot as yet predict the time when an Earthquake may take
place in any locality, we can, on mixed statistic and dynamic grounds,
in many cases state the limits of probable violence of the next that may
recur. For example, the three shafts of marble columns of the Temple of
Serapis, at Pozzuoli, each of about 41-1/2 feet in height, and 4 feet 10
inches in diameter at the base, remain standing alone, since they were
uncovered, in the year 1750.

Now, as we can calculate exactly what velocity of earthquake-wave motion
would be required to overset these, we are certain that, during the last
one hundred and twenty-two years, the site of the Temple, and we may
say Naples and the Phlegræan fields generally, have never experienced a
shock as great as the very moderate one that would overset these
columns. A shock whose wave particle had a horizontal velocity of only
about 3-1/2 feet (British) per second would overturn these columns;
which is only about one-fourth the velocity (within the meizoseismic
area) of the great shock of 1857, that produced wide-spread destruction
in the Basilicatas, and not enough to throw down any reasonably
well-built house of moderate height.

Naples, so far as Earthquake is concerned, whether coming from the
throes of Vesuvius or elsewhere, has a pretty good chance of safety. She
may possibly (though not probably) be some day smothered in ashes; but
is in little danger of being shaken to the earth. During this time there
have been taking place, larger eruptions of Vesuvius and earthquake
shocks from other centres, together probably about the same number of
times as the numbers of those years, when those columns have been more
or less shaken.

We may therefore affirm that the probability (on the basis of this
experience _only_) is, say 120 to 1, that the next shock, whether
derived from Vesuvius, or elsewhere, that may shake Pozzuoli, will be
one less in power than would be needed to overturn the shafts of the
Temple of Serapis there.

       *       *       *       *       *

Let us now turn to the second branch of our subject--viz.,
Vulcanology--upon which, as yet, we have secured less firm standing
ground than we have seen we possess in Seismology, for which reason we
took that first into consideration.

It is the part of Vulcanology to co-ordinate and explain all the
phenomena of past or present times visible on our globe which are
evidences of the existence and action, whether local or general, of
temperatures within our globe greatly in excess of those of the surface,
and which reach the fusing points of various mineral compounds as found
arriving, heated or fused, at the surface.

The stratigraphic geologist sees that such heated or fused masses have
come up from beneath, throughout every epoch that he can trace; but he
cannot fail to discern more or less a change in the order or character
of those outcomings, as he traces them from the lowest and oldest
formations to those of the present day. He sees immense outpourings of
granitoid or porphyrytic rocks that have welled up and overflowed the
oldest strata--huge dykes filling miles of fissures that had been
previously opened for the reception of the molten matter that has filled
them, and often passing through those masses of previously outpoured
rock; later he sees huge tables of basaltic rock poured forth over all.
One grand characteristic common to all these--commonly called plutonic
products--being that, whether they were poured forth over the surface or
injected into cavities in other rocks, the movements of the fused
material were, on the whole, hydrostatic and _not explosive_.

At the present day, whatever other evidences we have of high temperature
below our globe's surface, that which primarily fixes the eye of the
geologist is the Volcano, whose characteristic, as we see it in
activity, _is explosive_. But though there is this great characteristic
difference between the plutonic and the volcanic actions and their
products, the two, when looked at largely, are seen so to inosculate,
that it is impossible not to refer them to an agency common to both,
however changed the modes of its action have been between the earliest
epochs of which traces are presented to us and the present day.

To us little men, who, as Herschell has well said, in referring to the
methods of measuring the size of our globe, "can never see it all at
once, but must creep like mites about its surface," the Volcano, in the
stupendous grandeur of its effects, tends to fix itself in our minds in
exaggerated proportions to its true place in the cosmic machine; and, in
fact, nearly all who have sought to expound its nature and mode of
origination have occupied themselves far too exclusively with describing
and theorising upon the strange and varied phenomena which the volcanic
cone itself and its eruptions present, and too often, in the splendour
and variety of these, have very much lost sight of what ought to be the
centre-point of all such studies, namely, to arrive at some sound
knowledge of what is the _primum mobile_ of all these wonderful efforts.
Nor has the distinction been very clearly seen between the main
phenomena presented at and about volcanic active mouths, which can be
employed to elucidate the nature of the causation at work far below, and
those most varied and curious, and in other respects most pregnant and
instructive phenomena, mechanical and chemical, which are called into
action in and by the ejected matter of the volcanic cone after its
ejection. It can help us but little or very indirectly, in getting at a
true conception of the nature and source of the heat itself of the
Volcano, to examine, for example, all the curious circumstances that are
seen in the movements and changes in the lava that has already flowed
from its mouth; but it would be of great importance if we can ascertain,
by any form of observation around the cone, from what depth it has come,
or at what depth the igneous origin lies.

The physician, endeavouring to ascertain the real nature of small-pox or
measles, will scarcely make much progress who, however curiously or
minutely, confines his attention to the pustules that he sees upon the
skin.

Yet the Volcano, or rather all volcanic activity as now operative upon
our globe, is, as it were, an experiment of Nature's own perpetually
going on before us, the results of which, if well chosen--that is, as
Bacon says, by keeping to the main and neglecting the accidents--can,
when colligated and correctly reasoned upon, in relation to our planet
as a whole, give us the key to the enigma of terrestrial Vulcanicity in
its most general sense, and at every epoch of our world's geognostic
history, and show us its true place and use in the cosmical machine. Let
us glance at the history of past speculation on this subject, from which
so little real knowledge is to be derived, and then at the salient facts
of Vulcanology as now seen upon our earth, and finally see if we can
connect these with other great cosmical conditions, so as to arrive at
a consistent explanation in harmony with all.

We gain nothing absolutely from the knowledge of the so-called
"ancients" as to Volcanoes in Europe at least, where alone historic
records likely to refer to them exist. The Volcanoes of Europe are few
and widely scattered. The Greeks saw but little of them, and the Romans
were all and at all times most singularly unobservant of natural
phenomena.

Cæsar never mentions the existence in France of the Volcanoes of
Auvergne, so much like those he must have seen in Italy and Sicily; and
Roman writers pass in silence that great volcanic region, though
inhabited by them, and their language impressed upon the places, as
Volvic (_volcano-vicus_) seems with others to indicate; and though there
is some reason to believe that one or other of the Puys was in activity
within the first five hundred years of our epoch, the notices which
Humboldt and others have collected as from Plato, Pausanius, Pliny,
Ovid, etc., teach nothing.

Whatever of mere speculation there may have been, volcanic theory, or
what has passed for such, there was none before 1700, when Léméry
brought forward a trivial experiment, the acceptance of which, even for
a moment, as a sufficient cause for volcanic heat (and it retarded other
or truer views for years), we can now only wonder at. Breislak's origin,
in the burning of subterranean petroleum or like combustibles, was
scarcely less absurd than Léméry's sulphur and iron filings.

Davy, in the plenitude of his fame, and full of the intense chemical
activities of the metals of the alkalies which he had just isolated,
threw a new but transient verisimilitude upon the so-called chemical
theory of Volcanoes, by ascribing the source of heat to the oxidation of
those metals assumed to exist in vast, unproved and unindicated masses
in the interior of the earth. But Davy had too clear an intellect not to
see the baseless nature of his own hypothesis, which in his last work,
the "Consolations in Travel," he formally recanted; and it only survived
him in the long-continued though unconvincing advocacy of Dr. Daubeny.
So far, the origin of the heat had been sought always, in the crude
notion of some sort of _fuel consumed_, whether that were petroleum or
potassium and sodium; but as no fuel was to be found, nor any indicated
by the products, so far as known, of the volcanic heat, so what has been
called the mechanical theory, in a variety of shapes, took its place.

This, in whatever form, takes its lava and other heated products of the
volcano ready made from a universal ocean of liquid material, which it
supposes constitutes the interior or nucleus of our globe, and which is
only skinned over by a thin, solid crust of cooled and consolidated
rock, which was variably estimated at from fourteen to perhaps fifty
miles in thickness. Here was a boundless supply of more than heat, of
hot lava ready made, the existence of which at these moderate depths the
then state of knowledge of hypogeal temperature, which was supposed to
go on increasing with depth at the rate of about 1° Fahrenheit, for
every thirty or forty feet, seemed quite to sustain.

The difficulty remained, how was this fiery ocean brought to the
surface or far above it? To account for this two main notions prevailed,
and, indeed, have not ceased to prevail. Some unknown elastic gases or
vapour forced it up through fissures or rents pre-existent, or produced
by the tension of the elastic and liquid pressure below.

The form in which this view took most consistency, and approaching most
nearly to truth, finds the elastic vapour in steam generated from water
passed down through fissures from the sea or from the land surface. But
to this the difficulty was started, that fissures that could let down
water would pass up steam. The objection, when all the conditions are
adequately considered, has really no weight; and it has been completely
disposed of, since within a few years it has been proved that capillary
infiltration goes on in all porous rocks to enormous depths, and that
the capillary passages in such media, though giving free vent to
water--and the more as the water is warmer--are, when once filled with
liquid, proof against the return through them of gases or vapours. So
that the deeply seated walls of the ducts leading to the crater, if of
such material, may be red hot and yet continue to pass water from every
pore (like the walls of a well in chalk), which is flushed off into
steam that cannot return by the way the water came down, and must reach
the surface again, if at all, by the duct and crater, overcoming in its
way whatever obstructions they may be filled with.

And this remarkable property of capillarity sufficiently shows how the
lava--fused below or even at or above the level of infiltration--may
become interpenetrated throughout its mass by steam bubbles, as it
usually but not invariably is found to be.

Nor is it difficult to see such a mechanism between volcanic ducts and
fissures conveying down water, as large and open pipes, for a large part
of their depth, as shall bring down water to foci of volcanic heat,
without the power of the water flowing back except as steam and through
the crater.

Indeed, the facts known as to geysers, and those of half-drowned-out
Volcanoes such as Stromboli--whose action is intermittent just as much
as that of a geyser--show that this is not merely probable. There is,
therefore, no need for the hypothesis of those who have supposed all the
huge volumes of steam blown off from Volcanoes in eruption to come from
vesicular water pre-existent in the minute cavities of crystalline or
other rocks before their fusion into lava: a fact not proved for many
classes of rock, and for none in sufficient quantity to account for the
vast volume of steam required and for the irregularity of its issue.

It is rather to anticipate, but I may state at once that, so far as the
admission of superficial waters to the interior, and to any depth to
which fissures or dislocation can extend, I believe no valid physical or
mechanical difficulties exist, taking into account _all_ the conditions
that may come into play together.

Another set of views has been suggested and supported by various
writers, which proposes to account for the rise of lava on purely
hydrostatic principles. The solid crust, fractured into isolated
fragments by tensions due to its own contraction, is supposed to sink
into the sea of lava on which it floats; and much ingenuity has been
expended in imagining the mechanism by which, in places, the liquid
matter is supposed to rise _above_ the surface of the crust.

I have no space for discussing these views further than to assert that,
in the existing state of our globe, and even admitting a solid crust of
only 60,000 metres thick, dislocation of the crust by _tension_ is not
possible. The solid crust of our globe, as I hope we shall see further
on, is not in a state of tension, and has not been so since it was
extremely thin, a mere pellicle as compared with the liquid nucleus, but
is, on the contrary, in a state of _tangential compression_.

However tenable, in other respects, may be the volcanic theory which
rests upon the assumption of a very _thin_ crust and a universal ocean
of fused rock beneath, it fails wholly to explain many of the most
important circumstances observable as to the distribution and movements
of existing Volcanoes on our globe.

It affords no adequate explanation of the configuration of the lines of
Volcanoes, nor of their occurrence in the ocean bed, nor of their
existence in high latitudes, near the Poles, where, no matter how or at
what rate our globe cooled from liquidity, the crust must be thickest;
nor of the independence of eruptive action of closely adjacent volcanic
vents; nor of the non-periodicity, the sudden awakening-up to activity,
the as sudden exhaustion, the long repose, the gradual decay of action
at particular vents, and of much more that might be stated and
sustained as difficulties left by that theory unexplained, or that are
of a nature even opposed to it.

The researches of the last few years have, however, as it appears to me,
rendered any theory that demands as its postulates a _very thin crust_,
and a universal liquid nucleus beneath it, absolutely untenable.

Without attaching any importance to the arguments of Mr. Hopkins, based
upon precession and nutation, it appears to me, on various other
grounds, some of which have been urged by Sir William Thompson, that the
earth's solid crust is not a thin one, at least not thin enough to
render it conceivable that water can ever gain admission to a fluid
nucleus, if any such still exist, situated at so great a depth; and
without such access we can have no Volcano. It is not necessary to go to
the extent of a crust of 800 or 1,000 miles thick: with one of half the
minor thickness, I believe it may be proved, on various grounds,
hydraulic amongst others, that neither water could reach the nucleus,
nor the liquid matter of the nucleus reach the surface. Mr. Hopkins
having proved to his own satisfaction an enormous thickness for the
crust, and seeing clearly the difficulties that this involved to the
generally accepted volcanic theory, and having no other to substitute
for it, fell back upon that most vague and weak notion of the existence
of isolated lakes of liquid rock, existing at comparatively small depths
beneath the earth's surface within the solid and relatively cold crust,
each supplying its own Volcano, or more than one, with ready-made lava.
What is to produce these lakes of fused matter in the midst of similar
solidified matter? what is perpetually to maintain their fluidity in the
midst of solid matter continually cooling? what has given them their
local position? why near or less near the surface? what should have
arranged them in directions stretching in some cases nearly from Pole to
Pole?

Surely this creation of imaginary lakes, merely because it happens to
fit the vacant chink that seems needed to wedge up a falling theory, is
an instance of that abuse of hypothesis against which Newton so
vehemently declaims--"_Hypotheses non fingo._"

Hypothesis, to be a philosophic scaffolding to knowledge, must, as
Whewell has said, "be close to the facts, and not merely connected with
them by arbitrary and untried facts." Yet this appears accepted by Lyell
(10th edition, Vol. II., p. 227, and elsewhere); by Phillips
("Vesuvius," pp. 331, 332); by Scrope, if, as I hope, I mistake him not
("Volcanoes," pp. 265, 307-8); though none of these excellent
authorities seem either quite clear or quite satisfied with the notion;
and in the very passage referred to, Lyell _may_ have possibly a much
more philosophic notion in view, where he says: "It is only necessary,
in order to explain the action of Volcanoes, to _discover some cause
which is capable of bringing about such a concentration of heat as may
melt one after the other certain portions of the solid crust_, so as to
form seas, lakes or oceans of subterraneous lava." (Vol. II, pp. 226,
227). If by this is meant, that all that is needed to complete a true
theory of volcanic action is to discover _an adequate cosmical cause for
the heat_--that is to say, a prime mover to which all its phenomena may
be traced back, which shall be at once reconcilable with the conditions
of our planet as a cooling mass in space and with facts of Vulcanology
as they are now seen upon it--then I entirely agree with it.

It has been my own object to endeavour to discover and develope that
adequate cause in a Paper "On Volcanic Energy, an Attempt to develope
its True Nature and Cosmical Relations," read (in abstract) before the
Royal Society of London ("Proceedings, Royal Society," Vol. XX., May,
1872), and now (October, 1872) under consideration of Council with a
view to publication.

I propose concluding this review of the progress of Vulcanology (in
which I have had to limit myself to reviewing merely the chief stages of
advance towards knowledge of the nature and origin of volcanic heat
itself, and have had to pass without notice the vast and important mass
of facts and reasonings collected by so many labourers as to its visible
phenomena and products, and the still greater mass of speculation, good
and bad, on every branch of the subject), by giving a necessarily very
brief and imperfect sketch of my own views as in that Paper in part
developed. It will first be necessary to retrace our steps a little, in
order to gain such a point as shall afford us a fuller view of the whole
problem before us.

It is not necessary to dilate, even did space allow, upon the many
points which bind together Earthquakes and Volcanoes as belonging to the
play of like forces. These are generally admitted; and in various ways,
more or less obscure, geologists generally have supposed some relations
between these and the forces of elevation, which have raised up mountain
chains, etc.

No one, however, that I am aware of, prior to myself, in the Paper just
alluded to, has attempted to show, still less to prove upon an
experimental basis, that all the phenomena of elevation, of volcanic
action, and of Earthquakes, are explicable as parts of one simple
machinery--namely, the play of forces resulting from the secular cooling
of our globe. We have seen that, on the whole, both Earthquakes and
Volcanoes follow along the great lines of elevation of our surface. Any
true solution of the play of forces which has produced any one of those
three classes of phenomena must connect itself with them all, and be
adequate to account for all. And this would have earlier been seen, had
geologists generally framed for themselves any correct notions of the
mechanism of elevation itself, and seen its real relation with the
secular cooling of our planet. But the play of forces resulting from
this secular cooling has never, until very recently, been adequately or
truly stated. The arbitrary assumption and neglect of several essential
conditions by La Place, in his celebrated Paper "On the Cooling of the
Earth," in the fifth volume of the "Mécanique Céleste," and the
arbitrary and unsustainable hypothesis of Poisson upon the same subject,
have tended to retard the progress of physical Geology as to the nature
of elevation: the first, by leaving the geologist in doubt as to whether
our globe were cooling at all; the second, by suggesting distorted
notions as to the mode of its cooling and consolidation. On the other
hand, neither geologists nor mathematicians generally have framed for
themselves any clear notions of the mechanism of elevation. Had a true
conception been formed of the forces and interior movements brought
necessarily into operation by the secular cooling of the globe,
geologists could scarcely have failed to see that their notion as to the
way and direction in which the forces producing elevation have actually
acted could not, if arising from refrigeration, be those which they have
almost universally supposed, namely--some force acting vertically
upwards, _i.e._, radially from the centre of the sphere. Had geologists
only looked at Nature with open eye, they must have seen that mountain
ranges, and elevations generally (exclusive of volcanic cones),
presented circumstances absolutely incompatible with their having been
thrust up by any force _primarily_ acting in the direction of a radius
to the spheroid.

Yet this is the erroneous notion of the mechanism of elevation which to
the present hour prevails amongst geologists, so far as they in general
have framed to themselves any distinct idea of such mechanism at all.

Thus, only to cite two examples from recent authors of justly high
reputation. Lyell says of the probable subterranean sources, whether of
upward or downward movement, when permanently uplifting a country, and
in reference to the crumpling of strata on mountain flanks by lateral
pressure, it would be rash to assume these able to resist a power of
such stupendous energy, "_if its direction, instead of being vertical_,
happened to be oblique or horizontal." This is somewhat vague--and I
trust I do not mistake or misrepresent the illustrious author--yet it
is the most explicit expression I can find in the "Principles of
Geology" as to his notion of the primary direction of elevatory force
(Edit. 10, Vol. I., p. 133). That Mr. Scrope's idea is that only of
primary radial or vertical direction of such forces, is apparent on
inspecting his Diagram No. 64 ("Volcanoes," p. 285), and in the use of
the words, "an axial wedge of granite," which, on the next page, we find
is "liquefied granite;" and if we read on to page 294, and refer also to
pages 50 and 51, I believe there can be no doubt that _vertical_ or
_direct up-thrust_ is the author's notion of the primary direction of
all forces of elevation. The true nature of these forces was, however,
clearly seen and most justly stated by Constant Prevost ("Compt. Rend.,"
Tome XXXI., 1850, and "Bulletin de la Société Géolog. de France," Tome
II., 1840) as consisting, not in forces of some unknown origin acting
primarily in the vertical, but in _tangential pressures acting
horizontally, and resolved by mutual pressures at certain points into
vertical resultants_. These Prevost rightly attributed to the
contraction of the earth's solid crust. The same idea has been adopted
by Elie de Beaumont as the true mechanism of the elevation of mountain
ranges; and although De Beaumont's views as to the thinness he assigns
to the solid and contracting crust, and his strange deduction as to the
parallelism of contemporaneous mountain chains uplifted by its spasmodic
action along certain lines, may be untenable, his notion generally as to
the play of forces producing mountain elevation is much more nearly
correct.

Mr. Hopkins's notion is simply that of the geologists. Anyone who reads
his well-known papers on elevation and the formation of fissures, etc.,
must see that he views all elevatory forces as of liquids or
quasi-liquids forced up and acting primarily _vertically_ upon the
strata above them, and that these strata are not under tangential
compression, but under tension. Hence the mathematical deductions
contained in those papers as to the directions in which elevatory forces
act, and in which fissures are formed by them, are not in any way a
setting forth of such facts as occur in Nature, and, much attention as
they have attracted, can only now be viewed as exercises of mathematical
skill misapplied, because based upon data not to be found in Nature. In
fact, those papers do but misrepresent Nature, and, like many other
mathematical investigations based on untrue or insufficient data, have
tended to retard knowledge.

The views which I have put forward in the Paper I have referred to, read
to the Royal Society, recapitulated in skeleton, so to say, are as
follows. Omitting those portions which treat of our globe from the
period of the first liquefaction out of a nebulous condition, and of the
earliest stages of the cooling by radiation into space, when the crust
was extremely thin, and of the deformation of the spheroid as one of the
first effects of its contraction, and through that the general shaping
out of continents and ocean beds; I have endeavoured to show that the
rate of contraction of the crust, while very thin, exceeded that of the
large fluid nucleus supporting it, and so gave rise to _tangential
tensions_ in the crust, and fracturing it into segments; next, that as
the crust thickened, these _tensions_ were gradually converted into
_tangential pressures_, the contraction of the nucleus now beginning to
exceed (for equal losses of heat) that of the crust through which it
cooled. At this stage these tangential pressures gave rise to the
_chief_ elevations of mountain chains--not by liquid matter by any
process being injected from beneath vertically, but by such pressures,
mutually reacting along certain lines, being resolved into the vertical,
and forcing upwards more or less of the crust itself. The great outlines
of the mountain ranges and the greater elevation of the land were
designated and formed during the long periods that elapsed in which the
continually increasing thickness of crust remained such that it was
still, as a whole, flexible enough, or opposed sufficiently little
resistance to crushing, to admit of this uprise of mountain chains by
resolved tangential pressures. I have shown that the simple mechanism of
such tangential pressures is competent to account for all the complex
phenomena both of the elevations and of the _depressions_ that we now
see on the earth's surface (other than continents and ocean beds),
including the production of gaping fissures (in directions generally
orthogonal to those of tangential pressure). And as our earth is still a
cooling body, and the crust, however now thicker and more rigid, is
still incapable of sustaining the tangential pressures to which it is
now exposed, so I by no means infer that slow and small (relatively)
movements of elevation and depression may not be still and now going on
upon the earth's surface; in fact all the phenomena of elevation and
depression, rending, etc., which at a much remoter epoch acted upon a
much grander and more effective scale. So that, for aught my views say
to the contrary, all the mountain chains in the world may be possibly
increasing in stature year by year, or at times; but in any case at a
rate almost infinitesimally small in its totality over the whole earth
to that with which their ridges were originally upreared.

But the thickness of the earth's crust--thus constantly added to, by
accretion of solidifying matter from the still liquid or pasty nucleus,
as the whole mass has cooled--has now assumed such a thickness as to be
able to offer a too considerable resistance to the tangential pressures,
to admit of its giving way to any large extent by resolution upwards;
yet the cooling of the whole mass is going on, and contraction, though
unequal, both of thick crust and of hotter nucleus beneath also, whether
the latter be _now_ liquid or not. Were the contraction, lineal or
cubical, for equal decrements or losses of heat, or in equal
times--equal both in the material of the solidified crust and in that of
the hotter nucleus--there could be no such tangential pressures as are
here referred to, at any epoch of the earth's cooling. But in accordance
with the facts of experimental physics, we know that the co-efficient of
contraction for all bodies is greater as their actual temperature is
higher, and this both in their solid and liquid states.

Hence for equal decrements of heat, or by the cooling in equal times,
the hotter nucleus contracts more than does its envelope of solid
matter.

The result is now, as at all periods since the signs changed of the
tangential forces thus brought into play--_i.e._, since they became
tangential _pressures_--that the nucleus tends to shrink away as it
were from beneath the crust, and to leave the latter, unsupported or but
partially supported, as a spheroidal dome above it.

Now what happens? If the hollow spheroidal shell were strong enough to
sustain, as a spheric dome, the tangential thrust of its own weight and
the attraction of the nucleus, the shell would be left behind altogether
by the nucleus, and the latter might be conceived as an independent
globe revolving, centrally or excentrically, within a shell outside of
it. This, however, is not what happens.

The question then arises, Can the solid shell support the tangential
thrust to which it would be thus exposed? By the application to this
problem of an elegant theorem of Lagrange, I have proved that it cannot
possibly do so, no matter what may be its thickness nor what its
material, even were we to assume the latter not merely of the hardest
and most resistant rocks we know anything of, but even were it of
tempered cast-steel, the most resistant substance (unless possibly
iridio-osmium exceed it) that we know anything about. Lagrange has shown
that if P be the normal pressure upon any flexible plate curved in both
directions, the radii of these principal curvatures being ρ' and ρ'',
and T the tangential thrust at the point of application and due to the
force P, then:

     P = T (1/ρ' + 1/ρ'')

When the surface is spherical, or may be viewed as such, ρ' = ρ'' and

     P = 2T/ρ  or, T = P × ρ/2

In the present case P is for a unit square (taken relatively small and
so assumed as plane) of the shell, suppose a square mile, equal to the
effect of gravity upon that unit, ρ being the earth's radius, and if we
assume the unit square be also a unit in thickness, P is then the weight
of a cubic mile of its material; and if we take (roughly) the earth's
radius as 4,000 miles, the tangential pressure, T, is, on _each face_ of
the cubic mile, equal to

     (4000/2) P,

or equal to the pressure of a column of the same material of 2,000 times
its weight.

If the cubic mile that we have thus supposed cut out of the earth's
crust at the surface were of the hardest known granite or porphyry, it
would be exposed to a crushing tangential pressure equal to between 400
and 500 times what it could withstand, and so must crush, even though
only left unsupported by the nucleus beneath, to the extent of 1/400 or
1/500 of its entire weight. And what is true here of a mile taken at the
surface, is true (neglecting some minute corrections for difference in
the co-efficient of gravity, etc.) if taken at any other depth within
the thick crust.[F]

The crust of our earth, then, as it now is, must crush, to follow down
after the shrinking nucleus--if so be that the globe be still cooling,
and constituted as it is; even to the limited extent to which we know
anything of its nature--it must crush unequally, both regarded
superficially and as to depth; generally the crushing lines being
confined to the planes or places of greatest weakness; and the crushing
will not be absolutely constant and uniform anywhere, or at any time, or
at any of those places of weakness to which it will be principally
confined, but will be more or less irregular, quasi-periodic, or
paroxysmal: as is, indeed, the way in which all known material
substances (more or less rigid) give way to a slow and constantly
increasing, steady pressure.

We have now to ask, _How much_ of this crushing is going on at present
year by year? And the answer to this depends upon what amount of heat
our world is losing into space year by year.

Geologists who have taken on trust the statement, that La Place has
proved that the world has lost no sensible amount of heat for the last
10,000 years seem generally to suppose that to be a fact; but in reality
La Place has _proved_ nothing of the sort, as those geological teachers
who have echoed the conclusion should have known, had they deciphered
the mathematical argument upon which it has been supposed to rest.

By application of Fourier's theorem (or definition) to the observed rate
of increment of heat in descending from the geothermal _couche_ of
invariable temperature, and the co-efficients of conductivity of the
rocks of our earth's crust, as given by the long-continued observations
made beneath the Observatories of Paris and of Edinburgh, it results
that the annual loss of heat into space of our globe at present is equal
to that which would liquefy into water, at 32° Fahr., about 777 cubic
miles of ice; and this is the measuring unit for the amount of
contraction of our globe now going on. The figures are not probably
exact, for the data are not on a basis sufficiently full or exactly
established as yet; but they are not very widely wrong, and their
precise exactness is not material here. Now, how is this annual loss of
heat (great or small, as we may please to view it) from the interior of
our globe disposed of?

What does it _do_ in the interior? We have already seen that it is
primarily disposed of by conversion into work; into the work of
diminishing the earth's volume as a whole, and in so doing crushing
portions of the solid surrounding shell.

But does the transformation of lost heat into the work of vertical
descent, and of the crush as it follows down after the shrinking
nucleus, end the cycle? No. A very large portion of the mechanical work
thus produced, and resolved, as we have seen, into tangential crushing
pressure, is retransformed into heat again in the very act of crushing
the solid material of the shell. If we see a cartload of granite
paving-stones shot out in the dark, we see fire and light produced by
their collision; if we rub two pieces of quartz together, and crush thus
their surfaces against each other, we find we heat the pieces and evolve
light.

The machinery used for crushing by steam-power, hard rocks into road
metal, gets so hot that the surfaces cannot be touched.

These are familiar instances of one result of what is now taking place
by the crushing of the rocky masses of our cooling and descending
earth's crust, every hour beneath our feet, only upon a vastly greater
scale. It is in this local transformation of work into heat that I find
the true origin of volcanic heat within our globe. But if we are to test
this, so as in the only way possible to decide is it a true solution of
this great problem, we must again ask the question, _How much?_ and to
answer this, we must determine _experimentally_ how much heat can be
developed by the crushing of a given volume, say a cubic mile, of such
rocky materials as we know must constitute the crust of our globe down
to the bottom of the known sedimentary strata, and extending to such
crystalloid rocks as we may presume underlie these. We must also obtain
at least approximately what are the co-efficients of _total contraction_
between fusion and atmospheric temperature of such melted rocks, basic
and acid silicates, as may be deemed representative of that co-efficient
for the range of volcanic fused products, basalts, trachytes, etc.,
which probably sufficiently nearly coincide with that of the whole
non-metallic mass of our globe.

The first I have determined experimentally by two different methods, but
principally by the direct one of the _work_ expended in crushing prisms
of sixteen representative classes of rock; the specific gravities and
specific heats of which I have also determined.

If H be the height of a prism of rock crushed to powder by a pressure,
P, applied to two opposite faces, which, when the prism has been
reduced to its volume in powder, has acted through a range of H - t,
then

     P × (H - t) / 772

is the heat corresponding to the work expended in the crushing,
expressed in British units of heat. The following were the rocks
experimented upon: Caen stone, Portland (both oolites), magnesian
limestone, sandstones of various sorts, carboniferous limestones
(marbles), the older slates (Cambrian and Silurian), basalts, various
granites and porphyries, thus ranging from the newest and least
resistant to the oldest and most resistant rocks. The results have been
tabulated, and are given in detail in my Paper, now in possession of the
Royal Society. The minimum obtained is 331 and the maximum 7,867 British
units of heat developed, by transformation of the work of crushing one
cubic foot of rock. If we apply the results to a thickness of solid
crust of 100 miles (British), of which the upper twenty-one miles
consist of neozoic, newer palæozoic, older palæozoic and azoic rocks in
nearly equal proportion as to thickness, and the remaining eighty miles
of crystalloid rocks (acid and basic magmas of Durocher) of physical
properties which we may assume not very different from those of our
known granites and porphyries--and which, in so far as they may differ,
would give a still _higher_ co-efficient of work transformed into heat
than I have attributed to them by ranging them as only equal to the
granites, etc.--then we obtain a mean co-efficient for the entire
thickness of crust of 100 miles of 6,472 British units of heat,
developable from each cubic foot of its material, if crushed to powder.
It results from this that each cubic mile of the mean material of such
a crust, when crushed to powder, developes sufficient heat to melt 0·876
cubic miles of ice into water at 32°, or to raise 7·600 cubic miles of
water from 32° to 212° Fahr., or to boil off 1·124 cubic miles of water
at 32° into steam of one atmosphere, or, taking the average melting
point of rocky mixtures at 2,000° Fahr., to melt nearly three and a-half
cubic miles of such rock, if of the same specific heat.

Of the heat annually lost by our globe and dissipated into space,
represented by 777 cubic miles of ice melted, as before stated, the
chief part is derived from the actual hypogeal source of a hotter though
not necessarily fused nucleus, and nearly, if not wholly, is quite
independent of the heat of Vulcanicity, which is developed as a
consequence of its loss or dissipation. But were we to take the extreme
case, and suppose it possible that all the heat the globe loses annually
resulted from the transformation of the work of internal crushing of its
shell, we shall find that the total volume of rock needed to be crushed
in order to produce the required amount of lost heat is perfectly
insignificant as compared with the volume of the globe itself, or that
of its shell. For, as 1·270 cubic miles of crushed rock developes heat
equivalent to that required to melt one cubic mile of ice to water at
32°, and if we assume the volume of our globe's _solid_ crust to equal
one-fourth of the total volume of the entire globe, 987 cubic miles of
rock crushed annually would supply the whole of the heat dissipated in
that time. But that is less than the _one sixty-five millionth_ of the
volume of the crust only.

But a very small portion of the total heat annually lost by our globe
is sufficient to account for the whole of the volcanic energy of every
sort, including thermal waters, manifested annually upon our earth. In
the absence of complete data, we can only approximately calculate what
is the annual amount of present volcanic energy of our planet. This
energy shows itself to us in three ways: 1. The heating or fusing of the
ejected solid matters at volcanic vents. 2. The evolution of steam and
other heated elastic fluids by which these are carried. 3. The work of
raising through a certain height all the materials ejected. To which we
must add a large allowance for waste, or thermal mechanical and chemical
energy ineffectually dissipated in and above the vents. All these are
measurable into units of heat.

I have applied this method of calculation to test the adequacy of the
source I have assigned for volcanic heat, in two ways, viz.: 1. To the
phenomena presented during the last two thousand years by Vesuvius, the
best known Volcano in the world; and 2. To the whole of the four hundred
and odd volcanic cones observed so far upon our globe, of which not more
than one-half have ever been known in activity.

It is impossible here to refer to the details of the method or steps of
these calculations. The result however is, that making large allowances
for presumably defective data, _less than one-fourth_ of the total
telluric heat annually dissipated (as already stated in amount) is
sufficient to account for the annual volcanic energy at present expended
by our globe.

It is thus represented by the transformation into heat of the work of
crushing about 247 cubic miles of (mean) rock, a quantity so perfectly
insignificant, as compared with the volume of the globe itself, as to be
absolutely inappreciable in any way but by calculation; and as its
mechanical result is only the vertical transposition transitorily of
material within or upon our globe, the proportion of the mass of which
to the whole is equally insignificant, so not likely in any way to
produce changes recognisable by the astronomer.

Space here forbids my entering at all upon that branch of my
investigation which is based upon the experimental results, above
mentioned, of the total contraction of fused rocks: for these, the
original Paper can, I hope, be hereafter referred to. I am enabled,
however, to prove thus how enormously more than needful has been the
store of energy dissipated since our globe was wholly a melted mass, for
the production, through the contraction of its volume, of all the
phenomena of elevation and of Vulcanicity which its surface presents.
And how very small is the amount of that energy in a unit of time as now
operative, when compared with the same at very remote epochs in our
planet's history.

I have said that if we can find a true cause in Nature for the
origination of volcanic _heat_, all the other known phenomena, at and
about volcanic vents, become simple. Lavas and all other solid ejecta of
Volcanoes, from all parts of the earth's surface, as well as basalts,
present in chemical and physical constitution close resemblance, and may
be all referred to the melting of more or less fusible mixtures of
siliceous crystalloid rocks with aluminous (slates, etc.) and calcareous
rocks. Their general chemical composition, and the higher or lower
temperatures of fusion resulting therefrom, together with the higher or
lower temperatures to which they have been submitted at the different
volcanic foci, determine their difference of flow (under like surface
conditions) and of mineral character after ejection and cooling.

St. Clair de Ville and Fouqué have shown that the gaseous ejections, of
which steam forms probably 99 per cent., are such as arise from water
admitted to a _pre-existent focus of high temperature_.

Whether sea or fresh water is not material, when we bear in mind that
the chemical constituents found in sea water and in natural fresh waters
that have penetrated the soil are, on the whole, alike in kind and only
differ in proportions. But I must pass almost without notice all the
varied and instructive phenomena which are presented by volcanic vents,
for to treat of these at all would be to more than double the size of
this sketch.

In the source that has been pointed out as that from which volcanic heat
itself is derived, viz., the secular cooling of our globe, and the
effects of that upon its solid shell, we are enabled to point to that
which is the surest test of the truth of any theory--that it not only
enables us to account for all the phenomena, near or remote, but to
predict them. We see here linked together as parts of one grand play of
forces, those of contraction by cooling, producing by _direct_
mechanical action the elevation of mountain chains, and by their
_indirect_ action, by transformation of mechanical work into heat, the
production of Volcanoes; and both by direct and by indirect action, of
Earthquakes, never previously shown to have thus the physical connection
of one common cause, but merely supposed, more or less, to be connected
by their distribution upon our earth's surface.

We now discern thus the physical cause _why_ Volcanoes are distributed,
viewed largely, linearly, and follow the lines of elevation; we see
equally why their action is uncertain, non-periodic, fluctuating in
intensity, with longer or shorter periods of repose, shifting in
position, becoming extinct here, appearing in new activity or for the
first time there. We have an adequate solution of the before
inexplicable fact of their propinquity, and yet want of connection. We
have an adequate cause for the fusion of rock at local points without
resorting to the baseless hypothesis of perennial lakes of lava, etc.

For the first time, too, we discern a true physical cause for earthquake
movement, where volcanic energy does not show itself. The crushing of
the world's solid shell, whether thick or thin, goes on _per saltum_ and
at ever-shifting places, however steadily the tangential pressures
producing it may act. Hence crushing _alone_ may be shown to develope
amply sufficient impulse to produce the most violent Earthquakes,
whether they be or be not at a given place or time connected with
volcanic outburst or possible injection, or with tangential pressures,
enough still, in some cases, to produce partial permanent elevation.

When subterraneous crushing takes place, and the circumstances of the
site do not permit the access of water, there may be Earthquake, but can
be no Volcano; where water is admitted, there may be both.

And thus we discern why there are comparatively few submarine
Volcanoes, the floor of the ocean being, on the whole,
water-tight--"puddled," as an engineer would say, by the huge deposit of
incoherent mud, etc., that covers most of it, and probably having a
thicker crust beneath it than beneath the land.

We see, moreover, that the geological doctrine of absolute uniformity
cannot be true as to Vulcanicity, any more than it can for any other
energy in play in our world. Its development was greatest at its
earliest stages, when the great masses of the mountain chains were
elevated. It is even now--though as compared to men's experience, and
even to all historic time, apparently uniform and always the same--a
decaying energy.

The regimen of our planet as part of the Cosmos, which seems to some
absolute (and presented to Playfair no trace of a beginning nor
indication of an end), is not absolute, and only seems to us to be so
because we see so little of it, and of its long perspective in time.
This the now established doctrine of the conservation of energy renders
certain.

With this source for volcanic heat, too, in our possession, we can look
from our own world to others, and predict within certain limits, which
must widen as our knowledge of the facts of their substance and surface
becomes greater, what have been and what are the developments of
Vulcanicity which have taken place or are occurring in or upon them.
Looking to our own satellite, we see for the first time a sufficient
physical cause for the enormous display of volcanic energy there which
the telescope divulges to us; one which is not to be explained alone by
the commonly made statement of the small density of the moon, but by
the fact that as the rate of her cooling from a given temperature, as
compared with that of our earth (apart from questions of the chemical
nature of the two bodies, or of their specific heats, etc.), has been
inversely as their respective masses, and directly as their surfaces, so
has the rate of cooling of the moon been vastly greater than that of the
earth, and the energy due to contraction by cooling more intense and
rapidly developed in our satellite than upon our globe.

We have thus traced, in meagre and broken outline only--because space
admitted no more--the progress of Science to its existing state as
respects Vulcanicity, in its two branches of Vulcanology and of
Seismology, and pointed out their more intimate relations and points of
connection, and been at length able to refer them, on the sure basis of
physical laws, to one common cause, and that one derived from no
hypothesis, but simply from the postulate of our world as a terr-aqueous
globe cooling in space.

What I have here advanced with reference to volcanic energy, which
appertains to my own researches, I do not conceal from myself, nor from
the reader, has yet to await the reception generally and the award of
the true men of science of the world.

That, like every new line of thought which has attempted or succeeded in
supplanting the old, it will meet with opposition, I make no doubt.

My belief, however, is that in the end it will be found to have added a
fragment to the edifice of true knowledge.

The interpretation which I have given of the nature and origin of
volcanic activity points at once to the function in the Cosmos which it
is its destiny to fulfil. It is the instrument provided for the purpose
of continually preserving the earth's solid shell in a state to follow
down after the descending nucleus. It does this by an apparatus or play
of mechanism whereby the material of the solid shell, locally or along
certain lines, is not only crushed, but the crushed material is blown
out as dust, or expelled as liquid rock from between the walls of the
shell, which are thus enabled to approach each other; and thus, by
relief of the tangential thrusts, to permit the shell to descend, which
it is obvious that crushing alone, unless it extended to the whole mass
of the shell, could not accomplish.

It is a wonderful example of Nature's mechanism thus to see how simple
are the means by which this end is accomplished. The same inevitable
crush that dislocates the solid shell along certain lines, produces the
heat necessary to expel to the surface the material crushed.

When attempted to be made the basis for philosophic discovery, "final
causes" are no doubt barren, as Bacon has said; but when we have
independently and by strict methods arrived at a result, we may justly
appeal, as a test of its truth, to its showing itself as plainly
fulfilling a needful end, and, by a distinctly discernible mechanism,
preserving that harmony and conservation which are the obvious law of
the universe.

As has been said, if I mistake not by Daubeny, John Phillips, by
Herschell, and by myself, the function of the Earthquake and the
Volcano is not destructive but, preservative. But we now see that: that
the preservative scope of this function, as respects our earth, is far
wider than what has been previously attributed to it. The Volcano does
not merely throw up new fertile soil, and tend, in some small degree, to
restore to the dry land the waste for ever going on by rain and sea; it
fulfils a far weightier and more imperative task; it--by a mechanism the
power of which is exactly balanced to the variable calls demanded of it,
and which working almost imperceptibly, although in a manner however
terrible its surface-action may at times appear to us little
men[G]--prevents at longer intervals such sudden and unlooked-for
paroxysms in the mass of our subsiding earth's shell as would be
attended with wide-spread destruction to all that it inhabit.

To the popular mind, Volcanoes and Earthquakes are only isolated items
of curiosity amongst "the wonders of the world:" few geologists even
appear to realise how great and important are the relations of
Vulcanicity to their science, viewed as a whole. Yet of Vulcanicity it
is not too much to say, that in proportion as its nature and doctrines
come to be known and understood as parts of the Cosmos, the nearer will
it be seen to lie at the basis of all Physical Geology.


[A] For a fuller account of the literature and history of advancement of
human knowledge as to Earthquakes, here merely glanced at, I must refer
to my First Report on the Facts of Earthquakes, "Reports, British
Association, 1850," and to the works of Daubeny, Lyell, Phillips and
others, its _complete_ history remaining yet to be written.

[B] Yet how indistinctly formed were Young's ideas, and indistinct in
the same direction as those of Humboldt, becomes evident by a single
sentence: "When the agitation produced by an Earthquake extends further
than there is any reason to suspect a subterraneous communication, it is
probably propagated through the earth nearly in the same manner as a
noise is conveyed through the air."--_Lectures, Nat. Phil._, Vol. I.

[C] The Right Rev. Charles Graves, F.R.S., etc., then Fellow of Trinity
College, Pres. R. I. Acad., and now Bishop of Limerick, on presentation
of the Academy's Cunningham Medal.

[D] In this Report, though I have never before referred to it, and do so
now with reluctance, I have always felt that the Author did me some
injustice. The only reference made to my labours, published the
preceding year only, is in the following words: "Many persons have
regarded these phenomena (viz., Earthquakes) as due in a great measure
to vibrations ... and the subject has lately been brought under our
notice, in a Memoir by Mr. Mallet, 'On the Dynamics of Earthquakes,' in
which he has treated it in a more determinate manner, and in more
detail, than any preceding writer" (p. 74). If that Paper of mine be
collated with this Report, it will be, I believe, found that, as
respects the earthquake part, the latter tint parades, in a mathematical
dress, some portions of the general theory of earthquake movements,
previously published by me as above stated. So, also, in the chapter (p.
90) referring to Seismometry, and the important uses to Geology that
might be (and since have been, to some extent) made of it, no mention is
made of those instruments previously proposed by me, nor of my
anticipation of their important uses. This is but too mortifyingly
suggestive of the--

     "Pereant qui mea ante mihi dixerunt."

Having left this unnoticed for so many years, and during which the
Author has preceded me to that bourne where our errors to each other
must be forgotten, I should certainly not have now trespassed on the
good rule, _De mortuis nil nisi bonum_, had I not observed very recently
one amongst other results probably attributable to it. In Professor
Phillips's "Vesuvius," if any one will refer to the passage beginning
"The mechanism of earthquake movement has been investigated by competent
hands. The late eminent mathematician, Mr. Hopkins, explained these
tremors in the solid earth by the general theory of vibratory motion,"
etc. (pages 257-259)--I think he must, in the absence of collateral
information, conclude that, not I, but Mr. Hopkins, was the discoverer
of the Theory of Earthquakes as explained by the general theory of
vibratory motion.

Probably my friend, Professor Phillips, had not recently referred to
those Memoirs and Reports of twenty-four years back, and I am thoroughly
convinced that, if he has here perpetuated an injustice, he has done so
unintentionally and unwittingly.

Still, the facts show how true it is that

     "The ill men do lives after them,
     The good they do is oft interred with their bones."

And I may venture to ask my friend, should his admirable book reach, as
I doubt not it will, another edition, to modify the passage.

[E] Assuming the point of ejection of this block (the crater) to be
8,000 feet above where it landed, and allowing it as high a density as
admissible, and the angle of projection the best for large horizontal
range, it may be proved that this mass, to reach nine miles
horizontally, would require an initial velocity of projection of from
1,500 to 1,600 feet per second, one as great as that of a smooth-bore
cannon-shot at the muzzle, and perfectly inconceivable to be produced by
a volcano.

[F] The Rev. O. Fisher, M.A., F.G.S., in a most interesting and valuable
Paper, "On the Elevation of Mountain Chains by Lateral Pressure, its
Cause, and the Amount of it, with a Speculation on the Origin of
Volcanic Action," read, April, 1868, and published in the Transactions
of the Cambridge Philosophical Society, Vol. XI., Part III., in 1869,
has deduced the necessary crushing of the earth's crust by a different
but closely analogous method. I had not seen this Paper until after my
own was in the hands of the Royal Society. The author's volcanic views
are wholly different from my own, and do not appear to me equally valid
with his notions as to elevation.--R. M.

[G] "Magna ista quia parvi sumus"--SENECA, "Quæs. Nat."


END.



TRANSLATION
OF

PROFESSOR PALMIERI'S

_ACCOUNT OF_
THE ERUPTION OF VESUVIUS
OF
_1871-1872_



I.

ACCOUNT OF THE ERUPTION.


The great and disastrous conflagration of Vesuvius, which took place on
the 26th of April, 1872, was, in my opinion, the last phase of an
eruption which commenced at the end of January, 1871, an account of
which I was unwilling to write, because I was convinced that it would
not really terminate without a more or less violent explosion, such as I
had often predicted. I shall now state the reasons upon which my
prediction was founded.

When the central crater begins to heave, with slight eruptions, one may
always predict a series of slight convulsions of greater or less
duration, which are preparatory to the grand explosion, after which the
Volcano remains for the most part in repose. Thus, when I observed the
cone fissuring in November, 1868, and copious lava streams issuing from
it, and flowing over the beautiful and fertile plains of the Novelle,
through the Fossa della Vetrana, instead of announcing the beginning of
an eruption, I announced the termination of one which had been manifest
for upwards of a year by the constant flow of lava from the summit of
the cone.

From the month of November, 1868, until the end of December, 1870, the
mountain remained quiet, except that the fumaroles at the head of the
fissure showed a degree of activity by which chlorides and sulphides of
copper, sulphide of potash and other products, were engendered.

But in the beginning of 1871 the seismograph was disturbed,[1] and the
crater discharged, with a slight detonation, a few incandescent
projectiles. Then I announced that _a new eruption had commenced, which
might be of long duration, but with phases that could not possibly be
foreseen_; and on the 13th January, on the northern edge of the upper
plain of the Vesuvian cone, an aperture appeared, from which at first a
little lava issued, and then a small cone arose and threw out
incandescent projectiles, with much smoke of a reddish colour, whilst
the central crater continued to detonate more loudly and frequently. The
lava-flow continued to increase until the beginning of March, without
extending much beyond the base of the cone, although it had great
mobility. In March, this little cone appeared not only to subside, but
even partly to give way, as almost happens with eccentric cones when
their activity is at an end. Upon visiting it, I observed that four
prismatic or pillar-like masses remained standing, three of which were
formed of scoriæ which had fallen back again in a pasty condition, and
had become soldered together, the fourth consisting of a pyramidal block
of compact and lithoidal lava, which appeared to have been forced up by
impetus from the ground beneath. A little smoke issued from the small
crater, and a loud hissing from the interior was audible. By lying
along the edge, I could see a cavity of cylindrical form about ten
metres in depth, tapestried with stalactitic scoriæ covered with
sublimations of various colours. The bottom of this crater was level,
but in the centre a small cone of about two metres had formed, pointed
in such a manner that it possessed but a very narrow opening at the
apex, from which smoke issued with a hissing sound, and from which were
spurted a few very small incandescent scoriæ. This little cone increased
in size as well as activity until it filled the crater, and rose four or
five metres above the brim.[A] New and more abundant lavas appeared near
the base of this cone, and, pouring continually into the Atria del
Cavallo, rushed into the Fossa della Vetrana in the direction of the
Observatory and towards the Crocella, where they accumulated to such an
extent as to cover the hill-side for a distance of about 300 metres;
then turning below the Canteroni, they formed a hillock there without
spreading much farther. These very leucitic lavas are capable of great
extension, the pieces which are ejected forming for the most part very
fine filiform masses, which may be collected on the mountain in great
quantities, and specimens of which I presented to the Academy under the
name of _filiform lapilli_. These threads were often of a clear
yellowish colour, and, when observed under the microscope, were found to
consist of very minute crystals of leucite embedded in a homogeneous
paste. The crystals were still smaller as the diameter of the threads
was less, and never formed knots or swellings even in the most hair-like
threads. These observations led me to reject the opinion of those who
hold that crystals of leucite are pre-existent in the lava. The viscous
nature of these lavas prevented their being covered with fragmentary
scoriæ, but caused the formation at first of a skin, which, thickening,
became at last a more or less pliable shell, that, when more solidified,
allowed the still fluid part to run as in a tube formed of this solid
shell. For many months the lava descended thus from the cone and
traversed the Atria del Cavallo, always covered, appearing below the
Canteroni of a lively fluidity, until it could no longer be enveloped in
its skin, which was stretched by the addition of new lava, and finally
rent asunder to give room to the current until, owing to diminished
liquidity, it was constrained to stop. When the lava, having traversed
the covered channel it had made for itself from the top of the mountain
to below the Canteroni, made its appearance still running, it frequently
formed large bubbles on the surface, which mostly burst to give vent to
smoke, and then disappeared.

In October, 1871, near the edge of the central crater, another small
crater was formed by falling in, which, after a few days, gave vent to
smoke and several jets of lava. The principal cone frequently opened in
some point of the slope to give egress to small currents of lava, which
quickly ceased. But towards the end of October the detonations
increased, the smoke from the central crater issued more densely and
mixed with ashes, and the seismograph and accompanying apparatus were
disturbed: for all these reasons, I said in one of my bulletins, _we
have either reached a new phase or the end of the eruption_, not knowing
whether the new phase would be the last. On the 3rd and 4th November
copious and splendid lava streams coursed down the principal cone on its
western side, but were soon exhausted. The cone of 1871 appeared again
at rest, and partly even fell in, but did not cease to emit smoke and to
show fire in the interior.

In the beginning of January, 1872, the little cone again became active,
the crater of the preceding October resumed strength, with frequent
bellowings and projectiles, and soon after lavas of the same kind as
before reappeared. The cone of 1871, formed again by the lava ejected,
became so full that the lava poured from its summit in the most singular
and enchanting manner. So far only an eccentric or ephemeral cone had
risen close to the central crater, which, after exhaustion, regained
vigour and discharged lava from the apex instead of the base, as usually
happens.

In the month of February matters were somewhat moderated; but in March,
with the full moon, the cone opened on the north-west side--the cleavage
being manifest by a line of fumaroles--and a lava stream issued from the
lowest part without any noise and with very little smoke, and poured
down into the Atria del Cavallo as far as the precipices of Monte di
Somma. This lava ceased flowing after a week, but the fumaroles pointed
out the cleft of the cone; and between the small re-made cone, which had
risen to the height of 35 metres, and the central crater, a new crater
of small dimensions and interrupted activity opened.

On the 23rd April (another full moon) the Observatory instruments became
agitated, the activity of the craters increased, and on the evening of
the 24th splendid lavas descended the cone in various directions,
attracting on the same night the visits of a great many strangers. All
these lava streams were nearly exhausted on the morning of the 25th;
only one remained, which issued from the base of the cone, not far from
the spot whence that of the preceding month had issued. Numbers of
visitors, attracted by the splendour of the lava streams of the
preceding night, which they supposed still continued, soon arrived, but,
finding them exhausted, were for the most part conducted by their guides
to see the one still flowing. It was almost inaccessible, and to reach
it one had to walk over the rough inequalities of the scoriæ. It took me
two hours to get there from the Observatory, when I visited it that
morning, and therefore I endeavoured to dissuade those who wished to
visit it at night from the attempt, but set out myself from the
Observatory at 7 p.m., leaving my only assistant there. The instruments
were agitated. After midnight the Observatory was closed, and my
assistant retired to rest. Late and unlucky visitors passed unobserved
with an escort of inexperienced guides; at half-past 3 o'clock in the
morning of the 26th they were in the Atria del Cavallo, when the
Vesuvian cone became rent in a north-westerly direction, the fissure
commencing at the little cone which disappeared, and extending to the
Atria del Cavallo, whence a copious torrent of lava issued. Two large
craters formed at the summit of the mountain, discharging numerous
incandescent projectiles with white ashes, and glittering with particles
of mica, which frequently recurred.

A cloud of smoke enveloped these unfortunates, who were under a hail of
burning projectiles and close to the lava torrent. Some were buried
beneath it[B] and disappeared for ever; two dead bodies were picked up,
and eleven grievously injured, one of whom died close to the
Observatory. He alone revealed his name, Antonio Giannone. I learned
afterwards that he was a fine young fellow, and Assistant-Professor in
one of the Universities.

Assistant-Professors Signor Franco, who is a priest, and Signor
Francesco Cozzolino, a priest also, entrusted with the festive mass for
the Observatory, hastened to assist the dying. On my own return thither,
the sad spectacle of the dead and dying awaited me; the former were
conveyed, through the assistance of the municipal officer of Resina, to
the Cemetery, and the latter to the Hospital. But we must leave this
scene of grief and sorrow, and return to the eruption.

The fissure of the cone on the north-west side was large and deep, and
extended into the Atria del Cavallo, about 300 metres. No mouth opened
along the cleft of the cone itself; all the lava issued from that part
which extended into the Atria. From previous experience I should have
expected to have seen the formation of adventitious cones along the
widest part of the fissure, which is never that most elevated, and these
discharging from their summits æriform matter frequently mixed with
projectiles, and from their base lava; but on this occasion no cone
appeared at the widest part of the fissure, but a long hillock was
formed like a little chain of mountains, one point of which was elevated
about fifty metres above the plain beneath, and bearing no resemblance
to a cone.

Another fissure opened in the cone on the south side, which did not
extend to the base, and lava issued from this and flowed in the
direction of the Camaldoli. Streams of less importance furrowed the cone
in other directions, but the largest quantity of lava proceeded from the
fissure in the Atria del Cavallo, below the hillock or miniature chain
of collines just described. This lava stream was for some time
restrained within the Atria del Cavallo, among the holes and
inequalities of the lavas of 1871, but these being filled up and
overcome, it divided into two branches--the smaller one flowing through
a hollow which separated the lavas of 1867 from those of 1871, and made
its way over the lavas of 1858, threatening Resina, but stopped as soon
as it reached the first cultivated ground; the larger branch
precipitated itself into the Fossa della Vetrana, occupying the whole
width, about 800 metres; and traversing the entire length of 1,300
metres in three hours. It dashed into the Fossa di Faraone; here it
again divided into two streams, one overlying the lava of 1868, on the
Plain of the Novelle, partially covering the cultivated ground and
country-houses; the other flowing on through the Fossa di Faraglione,
over the lava of 1855, reached the villages of Massa and St. Sebastiano,
covering a portion of the houses, and thence continued its course
through the bed of a foss or trench which, contrary to my advice, had
been excavated after the eruption of 1855, in the expectation of
diverting the course of that lava. I did not fail to observe that the
rains which previously descended through these steep channels, would in
future be kept back to filtrate through the scoriæ, without ever
reaching the new channel.

The lava of this eruption, meeting with this said excavation, flowed
into it, instead of pursuing its road over the lava of 1855, and thus
invaded highly cultivated ground and towns of considerable value,
extending to the very walls of a country-house belonging to the
celebrated painter, Luca Giordano. This lava stream, having surmounted
the obstacles which the heaps of scoriæ in the Atria del Cavallo
presented to it, ran with great velocity (notwithstanding its being
greatly widened out in the Fossa del Vetrano), so that between 10 a.m.
and 11 p.m. it traversed about five kilometres of road, occupying a
surface of five to six square kilometres. If it had not greatly
slackened after midnight, from the failure of supply at its source, in
twenty-four hours more, by occupying Ponticelli, it would have reached
Naples, and flowed into the sea.

Although I had often visited the two villages of Massa and St.
Sebastiano, previously greatly injured by the lava of 1855, yet I could
not well estimate, upon now seeing them again, the number of houses
which had disappeared. Massa seemed to me diminished by about one-third,
and St. Sebastiano by somewhat less than a fourth. But the way of escape
was open to the inhabitants of Massa; whilst a great river of lava
occupying the road leading to St. Giorgio a Cremano would have hindered
the flight of the inhabitants of St. Sebastiano, if they had been
dilatory. The lava stream now separating the two villages is little less
than a kilometre in width, and is about six metres in height.

On the night of the 26th April, the Observatory lay between two torrents
of fire, which emitted an insufferable heat. The glass in the
window-frames, especially on the Vetrana side, was hot and cracking, and
a smell of scorching was perceptible in the rooms. The cone, besides
being furrowed by the lava streams just described, was traversed by
several others, which appeared and disappeared. It seemed completely
perforated, and the lava oozed as it were through its whole surface. I
cannot better express this phenomenon, than by saying that _Vesuvius
sweated fire_. In the day-time, the cone appeared momentarily covered
with white steam jets (fumaroles), which looked like flakes of cotton
against the dark mountain-side, appearing and disappearing at brief
intervals.

Simultaneously with the grand fissure of the cone, two large craters
opened at the summit, discharging with a dreadful noise, audible at a
great distance, an immense cloud of smoke and ashes with bombs and
flakes, rising to the height of 1300 metres[C] above the brim of lava
(_sull' orlo de essi_). The white ashes, before described, although they
did not fall beyond the Crocella, were carried by the wind as far as
Cosenza, from whence they were sent to me by Dr. Conti. These ejections
were followed by dark sand, with lapilli and small fragments of scoriæ
of the same colour. The smoke, driven up with violence, assumed the
usual aspect of a pine tree, of so sad a colour that it reminded us of
the shadowy elm of Virgil's dreams ("_ulmus opaca ingens_"). From the
trunk and branches of the pine-tree cloud fell a rain of incandescent
material, which frequently covered all the cone. The lapilli and the
ashes were carried to greater distances.

The victims of the morning of the 26th, the torrents of fire which
threatened Resina, Bosco and Torre Annunziata, and which devastated the
fertile country of the Novelle, of Massa, St. Sebastiano and Cercola,
the two partially buried villages, the continual and threatening
growlings of the craters, caused such terror that numbers fled from
their dwellings near the mountain into Naples, and several in Naples
went to Rome or to other places. Very many delayed from the knowledge
that I was in the Observatory, and held themselves in readiness for
flight whenever I should abandon it.

The rapidity with which the vast torrent of fire assailed the houses
(_i.e._, in these villages), and the great heat which spread to a
distance, scarcely allowed the fugitives to carry away any of their
belongings; many were completely destitute. The authorities vied with
each other in zealous efforts to relieve the distress, and the
municipality of Naples sheltered and fed the wretched beings for many
days.

The igneous period of the eruption was short, for on the morning of the
27th the lava stream, bearing down upon Resina, having covered a few
cultivated fields, stopped; the lava descending from the summit of the
mountain towards the Camaldoli also stopped; and the great lava torrent,
which passed the shoulders of the Observatory through the Fossa della
Vetrana, lowered the level of its surface below those of its two sides,
which appeared like two parallel ramparts above it.

If these streams had continued on the 27th, flowing in the same manner
as they did on the night of the 26th, they would have reached the sea,
bringing destruction to the very walls of Naples.

But before leaving the subject of these lavas I must narrate an
important fact to which I was witness, and which was thrice repeated,
near the banks of the great river of fire that ran close to the
Observatory. At three several points, and at different times, I observed
great balls of black smoke issue from the lava, driven up with continued
violence, as if from a crater; through the smoke I frequently observed
numerous projectiles thrown up into the air, but I could not say whether
with noise or in silence, for the noise of the central crater was
deafening. Each of these little eruptions, which I may call _external
eruptions_, lasted from fifteen to twenty minutes. The first took place
at the most elevated point of the Fossa della Vetrana, on the right bank
of the torrent; the second, under the hill of Apicella, where the lava
divided into the two branches, before described; and the third near to
the Observatory on the left bank of the lava stream. These singular
explosions terminated without leaving little cones or craters, the lava
in its impetuosity carrying every trace away. These eruptions were seen
from Naples, and the Observatory was justly believed to be in danger.
One has been clearly photographed, the one which was the best seen from
Naples, being the nearest and the least darkened by the smoke of the
lava. (Plate 4.) Is this the first time that the phenomenon has been
remarked? I believe that it is at least the first time it has been
authenticated. The authority of Julius Schmidt, quoted by Scrope, has no
weight with me, for I was also a witness of what happened at Vesuvius in
1855; and, although these cones were in the midst of the lava in the
Atria del Cavallo, they originated, according to the opinion of
everyone, from the fissure from which the other and much larger cones
proceeded. The same phenomenon was observed in the Atria del Cavallo in
1858, when I caused two of the little cones to be brought to the
Observatory; but these also might belong to the fissure along which the
other cones were arranged. The same may be said of the little craters
observed, after they had been exhausted, by Professor Scacchi in 1850.
But the discharging mouths now observed in the Fossa della Vetrana,
which existed for twenty minutes and then disappeared, and which were
not at all in a continuous line, and could not be supposed to correspond
with any fissure beneath, constitute a circumstance which, if not new,
is evident for the first time, and cause the recognition of a power in
the lava itself to form eruptive fumaroles.[2]

The igneous period of the eruption having terminated on the evening of
the 27th, the ashes, lapilli, and projectiles became a little more
abundant, whilst the roaring noises of the craters apparently became
greater. The pine-tree cloud was of a darker colour, and was furrowed by
continual lightning, visible by daylight from the Observatory. Many
writers on the subject of Vesuvius affirm that the flashes which appear
through the smoke cloud were lightning unaccompanied with thunder, but
they studied the phenomena from Naples, or some place more or less
distant from the crater, where the report of the thunder was inaudible,
or could not be distinguished from the bellowing and detonation of the
mountain. The fact is that these flashes were constantly followed by
thunder, after an interval of about seven seconds.[D] When the flash was
very short, a simple noise like the report of a gun was heard, but if it
were long, a protracted sound like that from torn paper ensued.

On the 28th the ashes and lapilli, continuing to fall abundantly,
darkened the air, yet without diminishing the terrible noise; at Resina,
Portici, St. Giorgio a Cremano, Naples, etc., terror was universal.

On the 29th, with a strong wind blowing from the east, scoriæ of such a
size fell at the Observatory, that the glass of the windows unprotected
by external blinds was broken. The noise from the crater continued, but
the projectiles rose to a less height, indicating a diminution in the
dynamic power of the eruption. Towards midnight the noise of the craters
was no longer continuous, and recurred with less force and for shorter
intervals. Almost at the same hour a tempest burst over the Campania
with loud thunder and a little rain. The grass, the seeds, the vine
tendrils, the leaves and tops of the trees dried up immediately, and the
country was changed from spring to winter. The storm, although repeated
on the following days, passed away by degrees, and thus the floods,
which I strongly feared, did not occur. Almost always after great
eruptions of Vesuvius, storms of heavy rain have followed, and the
ground being covered with ashes, the water could not filtrate through
into the soil, but descended in muddy torrents over the adjacent
country, occasioning as much damage as the fire itself.

On the 30th, the detonations were very few, and the smoke issued only at
intervals, and by the 1st May the eruption was completely over.

When the smoke had cleared off the figure of the cone was seen to be
changed. (_Vide_ Plate 5a.)

The ground was perpetually disturbed whilst the Volcano raged, so that
the Observatory oscillated continually. Some shocks were felt not only
in the adjacent territory, but at a greater distance, at Montovi and
elsewhere. The oscillations at the Observatory were chiefly undulatory,
from N.E. to S.W. They were observed for some days after the termination
of the eruption, but not continuously, although they maintained some
intensity.

If we refer to January, 1871, we shall find that that eruption was
preceded by several earthquakes, among which were those of the months of
October, November and December, in the previous year, that wrought such
destruction in Calabria, and especially in the province of Cosenza; if
we consider that as only the last phase, we shall find that it was
preceded by great shocks of earthquake that devastated some regions of
Greece.[3]

The great quantity of lapilli which fell buried the scoriæ with which
the Vesuvius cone was covered, so that it became somewhat more difficult
to ascend to the summit, and much less difficult to descend. Having
reached the top of the mountain, I found a large crater divided into two
parts by what seemed a cyclopean wall. The two abysses had vertical
sides, and revealed the internal structure of the cone. Their vertical
depth was 250 metres; and beyond that I observed a sort of tunnel
perforated in the rock, with a covering arch raised above the bottom of
the eastern abyss about 12 metres, judging by the eye. The interior
walls of the crater showed neither the usual stalactitic scoriæ nor
sublimations, nor fumaroles, but alternate beds of scoriæ and of compact
lava. The fumaroles and sublimations abounded, only about the brims of
the craters. Hydrochloric and sulphuric acid and sometimes sulphuretted
hydrogen affected respiration, and the temperature rose sometimes to 150
degrees. Various fissures about the brim of the double crater indicated
prolongations downwards, which allowed me to descend with a rope, in
order to examine the interior of the tunnel to which I have just
alluded. The highest brim of the crater was fissured for a distance of
80 metres, and the greatest depth of fissure was at that place.

By measurement with the barometer, we ascertained approximately (for
only one barometer was used) that the height of the Vesuvian cone was
somewhat diminished.

Not only the Vesuvian cone, but the whole adjacent country appeared
white for many days, as if covered with snow, when exposed to sunlight.
This was due to the sea-salt contained in the ashes with which the
surface was strewn.

A great quantity of coleoptera assembled on the flat roof of the
Observatory, where the ashes and lapilli were heaped up two decimetres
in height. I found the same species on the cone, where many insects were
observed on other occasions, such as the _Cuccinella septempunctata_;
the crysomela populi, etc., were wanting. This phenomenon of the
extraordinary concourse of insects on the top of Vesuvius, in order to
die in some of the fumaroles, especially noted previous to and after
great eruptions, is a circumstance for which I cannot account.[4] The
whole of the lava emitted in this eruption occupies a surface of about
five square kilometres; allowing an average thickness of four metres, we
obtain a mass of twenty millions of cubic metres. About three-fifths of
this lava did no injury, being deposited upon other pre-existing lava.
However, the lava in the Novelle, which was deposited upon the lava of
1858, covered quarries of the best stone which had been worked at the
time, covered many paths that had been cleared, and buried the new
Church of St. Michele, with some houses that surrounded it, which had
been rebuilt on the site of the former church, which was covered by the
lava of 1868. The destruction of land in occupation, of buildings and of
crops, exceeded three million francs in value. Many proposals for
relieving the sufferers have been received. Wishing to aid in this
benevolent work, I gave a public lecture, admission for each person
being one franc; and this lecture, from notes badly taken, was printed
by private speculation, and I was compelled to repudiate the report of
it through the public papers.

The evolutions of carbonic acid (_mofette_), which usually appear at the
end of great Vesuvian eruptions at low-situated spots or hollows, with
very rare exceptions, were observed on this occasion a few days after
the eruption had completely ceased. They appeared in the direction of
Resina. I found the most elevated at Tironi, and the most numerous
between La Favorita and the Bosco Reale di Portici.

The water in wells was on this occasion neither deficient nor scarce
previous to the eruption, but was very acid after the appearance of the
carbonic acid evolutions in those neighbourhoods in which they
abounded. Having stated that the disastrous conflagration of the 26th
April ought, in my opinion, to be regarded as the last phase of a long
period of eruption, which commenced at the beginning of 1871, I consider
it right to discuss the question at somewhat greater length.

Not only from twenty years' personal observations, but from the
attentive study of accounts of previous eruptions, I have found that
when the central crater awakens with small eruptions after a certain
time of previous repose, these almost always have a long duration, and,
after various phases of increase and decrease, terminate in a great
eccentric eruption, that is to say, with the production of an aperture
from which a copious lava stream issues. The eruptions of 1858, 1861,
1868 and 1872, furnish the most recent examples of what I affirm. I
might cite many others of earlier date, but I shall content myself with
recording the greatest conflagration of this century, that of October,
1822.

Before the erection of the Vesuvian Observatory, it was impossible to
obtain a consecutive account of all the phases which the Volcano
presented; but we generally obtained the description of the more
splendid phases of the eruption which arrested the attention of
everyone. Hence, notices of the small phenomena which preceded a great
eruption are frequently wanting. We cannot always ascertain whether the
fumaroles of the craters became active and at what periods, what was
their temperature and what the diverse nature of their emanations, etc.:
whether and when any change in the crater with slight eruptive
manifestations occurred; discharges which sometimes commenced in the
bottom of a crater becoming active, and so are invisible at Naples.

But it may be asked whether the inverse proposition be equally true,
that is, whether all the great eruptions of our Volcano were preceded by
small fiery manifestations of long duration? There have undoubtedly been
great eruptions not preceded by small central eruptions, but these also
had their period of preparation or precursory signs. After the great
eruption of 1850, Vesuvius remained in apparent repose until the end of
May, 1855, when there was an eccentric eruption and a great flow of lava
lasting twenty-seven days. But for a year before the fumaroles on the
top of the mountain had acquired great activity, their temperature
increased, and hydrochloric and sulphuric acid became more abundant, and
generated the usual coloured products on the adjacent scoriæ. Finally,
in the month of January, a crater was formed by falling in of the
ground, and although it did not discharge fire, yet it poured forth
dense smoke. This was the beginning of the fissure manifested four
months afterwards.

Ignazio Sorrentino, who spent a long life in the study of Vesuvius, and
frequently ascended it, considered the increase of those yellow
products--which are chiefly chlorides of iron, but were, at that time,
mistaken for sulphur--as the sign of an approaching eruption.

The only grave objection that can be alleged is that of the memorable
eruption of 1631, which surprised the neighbouring population so
suddenly that many perished miserably, surrounded or covered with lava.
But that terrible conflagration occurred after centuries of repose, so
that trees had grown in the interior of the crater. No one suspected the
possibility of danger. It took place, too, at the end of autumn, when
the cone is usually covered with clouds, and, therefore, no one had an
opportunity of observing any precursory phenomena.

When the Observatory was established, I was able--in the first instance,
at my own expense, and afterwards with some slight assistance from
Government--to undertake studies more assiduous than any previously
made. I had two instruments adjusted to indicate the internal efforts of
the Volcano, viz., M. Lamond's apparatus of variations, which, by means
of finely-balanced needles and methods of amplification proposed by
Gauss, indicates the slightest trepidation of the ground, and my own
electro-magnetic seismograph, a self-registering instrument of exquisite
delicacy. These instruments, when attentively observed, give the most
valuable information with respect to the activity of the adjacent
Volcano.

If the very slightest eruption occurs, these instruments manifest slight
perturbation, increasing with the activity of the mountain. When the
Volcano attains a certain degree of activity, and the instruments are
proportionately disturbed, it is impossible to foresee a new phase of
increase without constantly watching the changes in the intensity of the
perturbations; and to effect this it is requisite to have upon the spot
a staff of assistants sufficiently numerous, scientific and intelligent.
If, therefore, on the night preceding the 26th of April the instruments
had been properly watched, they would have undoubtedly indicated the
great increase in the activity of the Volcano. The perturbations on the
23rd were steadily increasing, and on the evening of the 25th they were
much stronger than on the 24th, but on the morning of the 26th they had
become extraordinarily strong; they must, therefore, have increased
considerably during the night.



II.

NATURE OF THE LAVAS.


When the observer is near the source of the lava, he sees matter in a
state of fusion, which, like a torrent of liquid fire, runs along, with
more or less impetuosity, between two banks formed by itself. But as
soon as the surface of the torrent cools to the point of congelation, it
loses the splendour of its first incandescence. The part which begins to
harden breaks readily in some lavas into fragments which float on the
viscous fluid beneath; these, increasing in number with distance from
the source, conceal the molten matter beneath and retard its progress,
and at last nothing is seen but the more or less red-hot scoriæ moving
along. These lavas I shall call "_Lavas with fragmentary scoriæ_."

On other occasions, a skin forms on the surface of the lava, which,
gradually thickening, keeps flexible for some time, and then wrinkles or
swells or extends and breaks to give egress to the hot fluid within,
which, in its turn, skins over and repeats the same phenomena. This I
shall call "_Lavas with a united surface_."

These, in their course, discharge less smoke than the first, draw out
more easily into threads, and, when cold, have a dark colour, something
like bitumen or pitch. _The lava with fragmentary scoriæ_, when
stretched, breaks easily, discharges smoke copiously, and, when
hardened, has a more bluish tint, like clods of upturned earth (_formato
di zolle_). It is noisy in its course, because the incoherent scoriæ
that it carries along strike and crunch against each other; the other
lava flows silently, except for a sort of crackling arising from the
actual fracturing up of the solid skin by distension from the liquid
matter within. If required to give the mineralogical characteristics of
this lava, I would say that it was rich in leucite and contained little
or no pyroxene; the fragmentary lava, on the contrary, is poor in
leucite and rich in pyroxene. The lavas of 1871 were of the "united
surface" character; those of 1872 were "fragmentary," with some
characteristics which I shall describe:

     1. They were of the clearest tint I have ever seen, when
     regarded superficially, but, when broken, the fracture was
     darker than any other lava.

     2. They had very little leucite and abounded in pyroxene and
     olivine, and sometimes contained a few crystals of amphibole.

     3. Their specific gravity varied with their porosity; the most
     compact attained 2·75.

     4. These lavas carried along in their course a quantity of
     scoriæ which had long been subjected to the action of the acids
     of the fumaroles close to the craters, and also a great many
     bombs (_bombe_)--that is, round masses similar to those ejected
     from craters. These varied in size, some having a diameter of
     four to five meters. They frequently contained a large nucleus
     of very leucitic lava, like that of 1871, with a larger or
     smaller quantity of feroligiste (peroxide of iron). Others
     contained lavas changed by the action of the acid vapours near
     the craters. These bombs must have flowed out with the lava,
     for they are found through its whole course, and they were
     certainly not ejected from the crater; for not only are they
     found on the lava exclusively, but masses so enormous were not
     thrown up from the craters during the eruption; those lying on
     the cone near the craters seldom exceed a decimetre in
     diameter.

As to the qualitative chemical analysis of the lavas, it always presents
the same elements, with the exception of small quantities of some
metals, lead for example, which have escaped the researches of good
chemists, but which I have constantly found in the sublimations of the
fumaroles of the lava. With respect to the quantitative analysis, two
specimens of the same lava appear indeed to have their constituents in
different proportions. To arrive at any conclusion a long and patient
investigation, requiring means and assistance which the Observatory does
not possess, would be necessary.

Professor Fuchs, of Heidelberg, has devoted himself to this work for
years past, and if he continue it with well-selected and sufficiently
large specimens we may hope some day to obtain satisfactory results.

     5. Every specimen of lava which I examined with a very
     sensitive magnetoscope improved by myself, was invariably
     magneto-polar, not excepting the pieces of the bombs, whether
     rejected from the crater or carried along with the lava.



III.

FUMAROLES OF THE LAVAS.


Smoke generally issues from all lava when it cools down to a certain
degree, hence it is more abundant at the edges of the fiery torrent, or
is liberated from the scoriæ that form on its surface. But when the lava
stops, the smoke issues only from certain vent-holes, through which we
can still see the fire, and at the edge of which different amorphous or
crystallized matters collect by sublimation. These centres of heat, of
more or less duration, are the fumaroles of the lavas. I believe I have
on other occasions shown that a fumarole is nothing but a communication
between the more or less cooled and hardened surface of the lava and the
interior, which is still incandescent. Some fumaroles last but a day,
others preserve their activity for weeks, months or years, according to
the depth of lava through which they penetrate; and when they cease to
be active, that is, when the sublimations are formed, or smoke or other
æriform matters issue from them, they still retain a rather elevated
temperature. In the lavas of 1858, in a place where they had a
transverse width of 150 metres, a vent-hole may still be found where the
thermometer registers 60° and the scoriæ are warm. Sometimes, while the
lava is in process of cooling, new fumaroles appear, in which the fire
is visible. This phenomenon, which appeared marvellous and inexplicable
when I first observed it in 1855, is now very easily understood; the
cooled and hardened crust of the lava fractures with noise and suddenly,
and so a new communication is opened with the incandescent lava below,
thus creating a new fumarole.

As the smoke of the fluid lava is perfectly neutral, that is, neither
acid nor alkaline, so the fumaroles at the first period of their
existence with sublimations of sea-salt, mixed frequently with oxide of
copper either in black powder or in shining laminæ, ought also to be
neutral. But if the fumarole continues active, hydrochloric acid issues
with the smoke, and often some time after sulphuric acid. Then the
sublimations turn first yellow, then green, and more rarely azure. The
chemical reactions show that these sublimations are chlorides or
sulpho-chlorides, and sometimes sulphides, and they afford reactions,
indicative of soda, magnesia, copper, lead, and traces of other
substances, not excluding ammonia, which I must speak of separately.
This, I have observed, is the general law with the fumaroles of the
tranquil lavas, which occur with long and moderate eruptions--for
instance, the lavas of 1871, and even those of 1872, preceding the 26th
April.

But in the great lavas of the great conflagrations of Vesuvius, chloride
of iron more or less in combination with all the other substances above
mentioned changes the appearance of the sublimations. The fumaroles in
the lava of the 26th April frequently indicated chloride of iron.
Sulphuretted hydrogen, by reaction of sulphurous acid, is decomposed,
and sulphur sublimed, having a particular aspect, collects on the
scoriæ. This is never found but in fumaroles of the smaller lavas; it
was therefore absent in those of 1871, but frequently occurred in those
of 1872.

Although the sublimations are generally mixtures, yet sometimes distinct
and crystallized chemical or mineral species are found, such as sulphur,
sal ammoniac, _tenorite_, _cotunuite_, etc. Micaceous peroxide of iron
(feroligiste), so common near eruptive cones, is very scarce on lava;
any found in it has been carried down from the craters, and proofs of
this transport are very abundant and striking in the lavas of this last
eruption. Even the iron found in the bombs is evidently transported;
there is a fumarole on the ridge of the lava in the Fossa di Faraone
which contains micaceous peroxide of iron, and this, at first sight,
appears to oppose what I have affirmed; nevertheless, it gives
additional force to my statement. This fumarole is only a bomb or
rounded mass of enormous size, four or five metres in diameter. Smoke
and hydrochloric acid issued from the aperture in its envelope, and
being partly broken it was seen to contain lapilli and pieces of
antecedent lava, covered with micaceous peroxide of iron. The internal
temperature of this mass was very high; the hydrochloric acid which it
discharged had, in some places, covered the micaceous iron with a yellow
coating of chloride of iron. From small apertures, on the lower side of
the mass, white and green stalactites of chloride of calcium were
visible. In one spot only of lava I found a fumarole, with a small
quantity of micaceous peroxide of iron, evidently in a state of
formation; but this was the very spot where the lava became eruptive,
and whence issued the column of smoke which was so well
photographed--the place under the hill of Apicella. (See Plate 4a.)

I have enumerated the products which are constantly collected in
fumaroles, although they are not all found at the same time or place, in
order to show that the sublimations follow a certain law in their
appearance. _Tenorite_, for instance, was formerly considered an
accidental product of certain eruptions, and I have always found it; but
if you visit the fumarole when the acids have had time to transform it,
you will no longer see it. I found the crystallized chloride of lead, or
"cotunuite," as it is called, for the first time in the lavas of 1855,
and thought it a singular circumstance; but from that time I recognised
it in all the lavas, though not always so beautiful and abundant; and
even when not found as a distinct substance, I observed it in
combination with chloride of copper. In the lavas of the 26th April
_cotunuite_ and _tenorite_[E] were not very abundant, because the
chloride of iron disturbed the greater number of the sublimations. I
found sal ammoniac very abundantly on the fumaroles of the lavas that
invaded the cultivated ground. Although chloride of ammonia, contrary to
opinion, was not wanting in the sublimations of the fumaroles of the
lavas deposited on other lavas, yet it was neither abundant nor
crystallized, but combined in small quantities with other substances. It
appeared in great abundance in all the fumaroles of lavas which covered
cultivated or woody ground. At first it was scarce enough, and mixed
with chloride of sodium; but when the rains came the sea-salt was washed
away, and sal ammoniac formed beautiful crystals, nearly free from
adventitious matters, as was the case with the fumaroles of the last
lava. Afterwards, when chloride of iron was produced, ferro-chloride of
ammonia was found. Crystals of sal ammoniac were sometimes found of a
beautiful amber yellow. This colour was, in the opinion of my colleague,
Professor Scacchi, produced by such small traces of chloride of iron
that neither Professor Guiscardi nor I, nor indeed any other chemists to
whom I submitted specimens for examination, could detect any. What I can
affirm with certainty is, that these limpid crystals of a yellow colour
were almost always attached to an amorphous substance, soluble in water,
composed of various chlorides, in which iron was often detected.

From these remarks, it is evident that in the tranquil lavas the
sublimations appear with a certain order of succession, and in the
violent lavas, and those which flow most copiously, they are more
complicated, and render both chemical analysis and spectroscopic
researches more difficult. Notwithstanding, I observed traces of lithium
and thallium, which I had previously perceived in some sublimations of
1871. I purpose submitting many sublimations which I have collected to
more complete spectroscopic investigation, although I am persuaded that
the discovery of traces of certain bodies in the sublimations or in the
lavas is a matter of small importance to the science of volcanoes. I
must say, however, that calcium was discovered on this occasion in great
abundance, not only by the spectroscope, but also by chemical analysis.
Sulphate of lime has often been found in larger or smaller proportions,
but this was the first time I had observed chloride of calcium both
close to the craters, and also in the sublimations of the fumaroles upon
the lavas. The white stalactites which I collected beneath the great
mass or bomb above described were almost exclusively composed of
chloride of calcium, and only a few green drops manifested, with the
usual re-agents, the presence of iron.

I did not fail to look often at the spectrum of the flowing lavas
covered with the smoke which issued from them, but I always had a
continuous spectrum. The spectroscope employed was Hoffmann's
construction, with direct vision; but I think it would be better on
other occasions to use a spectroscope combined with a telescope, like
those used by astronomers.

But avoiding minute particulars of these sublimates, let us see what is
the general direction and the order of their appearance. Sublimations
are generally oxides, chlorides and sulphates, sometimes sulphides.
Among the oxides, we must enumerate in the first place "tenorite" and
_feroligiste_ or micaceous peroxide of iron. The first is almost always
found at the commencement of activity in the fumaroles, simultaneously
with the sublimation of chloride of sodium; the second--which is,
perhaps, never wanting in eruptive cones that are often found lined with
it inside--is seldom generated in the fumaroles of the lava, and
therefore it is not easy to define the moment of its appearance.
Sometimes one collects micaceous peroxide of iron on the lava, but it is
often transported there from the mouths of eruption, as happened on this
occasion.

Trustworthy writers are of opinion that all the oxides are derived from
the decomposition of the chlorides, but I think I have clearly
demonstrated that, with regard to copper and lead, the opposite
statement may be affirmed; for the oxides are changed into chlorides,
and hydrochloric acid liberated. Oxide of copper forms sublimates at the
beginning, at the same time as the sea-salt; and if the fumarole be
anhydrous or, as Deville would say, _dry_, this oxide does not change
into either a chloride or a sulphate; but if the fumarole gives watery
vapour, after a little hydrochloric acid is formed, which changes the
oxide into a chloride, and if whilst this is going on oxide of lead be
developed, it is changed into the chloride of lead, so frequently found
in combination with chloride of copper. Then the sublimations change
from white to red or yellow, and specimens when carried away gradually
turn light blue, but when heated on platinum over a spirit lamp they
resume their yellow tint. Sometimes the yellow colour remains longer,
and in time changes to green; this also happens on the fumarole itself,
the green commencing at the zones furthest removed from the centre,
where the temperature is highest. When these sublimations are greenish,
they become far less soluble than at first. The yellow, so common at a
certain period on the fumaroles of the tranquil lavas, never attracted
attention before I first examined it, doubtless, because it was
considered chloride of iron, and yet in small eruptions this is only
found close to the discharging mouths, and never in the sublimations of
the fumaroles of the lava; but, on the other hand, it is the most
copious and common product on the lavas of the great eruptions. This
probably also accounts for the fact that lead, which is so obvious in
the fumaroles of the lavas, had never previously been observed. In 1855,
I noticed the crystallized chloride of lead in a fumarole in the Fossa
della Vetrana, and this induced me always to look for it on the
fumaroles of the later lavas; and I ascertained that, if it did not
always appear as a distinct mineral, it was easily discovered in
combination with other chlorides. The specimens which I have collected
are not the most beautiful, but the presence of lead in the sublimations
is not less common.

Micaceous peroxide of iron, when found on the lava, has been mostly
conveyed from the eruptive mouths, as I have already stated, and perhaps
never so abundantly and evidently as on this occasion. The lava of the
26th of April carried along a large quantity of round masses or bombs,
varying in size, among which were found antecedent lava more or less
covered with micaceous iron, either collected in the cavities of the
lava, or incorporated with its mass. Sometimes the micaceous iron
appears like little veins in the paste of new lava enveloping the
exterior of these rounded masses, an exterior compact and lithoidal, and
not resembling scoriæ. Among these spherical masses I found one of
enormous size, four to five metres in diameter, which, having broken up
where the exterior envelope was thinnest, I found filled with a great
mass of lapilli and fragments of other lavas covered with micaceous
iron. This bomb still preserves (June 5th) an elevated temperature
within, and emits smoke and hydrochloric acid, which, meeting the
micaceous iron discovered by breaking the envelope with blows of a
hammer, transforms it superficially into chloride of iron, showing most
clearly how, on some occasions at least, chloride of iron is formed from
the oxide which precedes it. That those lapilli and the pieces of lava
were solid when enveloped in the paste of the new lava, we infer from
seeing the impressions on the inside of the said envelope. The chloride
of calcium, which I found in this spherical mass almost pure, caused me
to suspect that the sulphate of lime which is so often found on Vesuvius
is a transformation of the chloride produced by the contact of
sulphurous acid, which easily becomes transformed into sulphuric acid.
The hydrochloric acid which escapes from a fumarole coming into contact
with the scoriæ near its mouth, produces chloride of iron, which is,
therefore, not always obtained by sublimation, although, when the
temperature is very high, chloride of iron is conveyed from the interior
of the lava, and sublimes on the exterior and colder parts; for
instance, the chloride of iron which issues from the eruptive cones is
sometimes found sublimed on the rocks of Monte di Somma. When chloride
of iron has been produced by sublimation, we may collect it inside a
glass bell placed over the fumarole, or upon a piece of brick; but when
it is produced by the action of hydrochloric acid on the scoriæ, it will
only be found on the scoriæ themselves.

If, therefore, the origin of micaceous peroxide of iron were due to the
decomposition of the sesqui-chloride of iron requiring a more elevated
temperature for its decomposition, it would follow that its genesis
would be easier near the discharging mouths, and more difficult on the
lavas, but there the fact was verified: for example, in the great bomb
on the fumarole, where we observed micaceous iron transformed into
chloride of iron. We may therefore consider it _proved_ that some
chlorides--for instance, chloride of sodium--issue from the lava itself,
either being there pre-existent, or being formed there; and that others
are derived from the oxides which precede them, as undoubtedly is the
case with chloride of copper; hence, the theory that derives the oxides
always from the chlorides cannot be considered true. Granting that this
theory might be applicable to the origin of micaceous iron, we should
still want to know how it is found with the paste of the new lava
itself, which forms the exterior coating of the bombs above described.

Many of these rounded masses, which have been rolled along by the lava,
contain scoriæ partly decomposed by the long action of the acids found
on the fumaroles of the craters. They disintegrate easily, and have a
more or less yellowish tint. In the greater number of cases the interior
of these masses is formed of leucitic lava, with cavities lined with
micaceous iron. In short, their contents appeared to me quite similar to
the material of the cone of 1871 and 1872, which in all probability was
engulfed in the large crevasse or fissure that opened below it; and the
fragments having thus fallen down into the lava, were enveloped by it
and carried out by it after having been more or less rounded. The
external envelope of these spheres is not at all scoriaceous, but
compact and lithoidal, and sometimes composed of concentric folds or
plaits.

As to the gaseous emanations of fumaroles, watery vapour with few
exceptions comes first; this conveys the material which first appears in
the sublimations, viz., sea-salt, and for the most part oxide of copper.
If the fumarole continue active, it passes from the neutral period to
the acid period, and first hydrochloric acid is produced, which, in
small lava streams, never conveys chloride of iron, and rarely attacks
the scoriæ to form that salt, but expends its force in changing the
sublimations already there. For this reason chloride of iron, though
completely absent in the lavas of 1871, was abundantly found in those of
the 26th April, 1872. Sulphurous acid follows hydrochloric at a later
period, and sulphuretted hydrogen occasionally succeeds.

Having examined the gases of fumaroles by means of a graduated tube, and
the pyrogallate of potash, I always found that it contained less oxygen
than the surrounding atmosphere.

For several years I wished to see whether the fumaroles of the lavas had
a period of evolution of carbonic acid, as sometimes happens with
fumaroles near the craters, but I have always obtained negative
results. I often found that the atmosphere on the lavas contained an
excess of carbonic acid, but as these lavas had burnt many trees, and it
was probable that carbonic acid springs had formed under the lava, I
never considered it safe to form any conclusion on the subject.



IV.

BOMBS, LAPILLI AND ASHES.


The bombs ejected from the craters are like those carried down by the
lavas, but of smaller size, and they seldomer contain a nucleus similar
to those found in the latter. With the bombs properly so called, many
pieces of incandescent lava were thrown up, and in their fall went
beyond the base of the cone. A quantity of small scoriæ varying in size
accompanied these projectiles, and those fragments, which we call
_lapilli_, fell at a greater distance. With the lapilli, and sometimes
without them, the smoke carried a very minute dust or sand, which is
generally called ashes. These ashes, when washed with water, lose
soluble constituents which they have collected in the smoke--such as
chloride of sodium and other chlorides and often free acids. The
insoluble part originates in the detritus of lava, and with the
microscope we can detect abundant fragments of those crystals which most
frequently occur in the lava of the same eruption.

The lavas of 1871, which were eminently leucitic, and almost entirely
deprived of pyroxene, resembled the ashes, which appeared to be
fragments of crystals of leucite, more or less enveloped in the paste of
the lava, so that having triturated the scoriæ of the lava, and looked
at the powder through the microscope, it was apparently quite the same
as the ashes.

But at the beginning of the eruption of the 26th April, a white sand
fell in the Atria del Cavallo, close to the Crocella[5], which on the
dark scoriæ of 1871 looked like snow. Its fall had a limit so well
defined that one passed without any gradation from white to black.
Having collected some of this sand that very morning, I put it up in
white paper, for at that moment it was impossible for me to examine it.
Taking it out some days after, I found it had become reddish, and having
put it under the microscope, I observed that it was exclusively formed
of little pebbles more or less round, of a transparent vitreous matter,
partly covered with a red substance. Fragments of green crystals
occurred in this sand, upon which no red was perceptible. I consulted
our eminent crystallographer, Arcangelo Scacchi, whether these little
pebbles were leucite, as I suspected, and whether the green particles
were pyroxene: he confirmed my suspicion, and remarked that the red
colour was superficial only. We then washed a little of the sand in hot
water, and saw the pebbles become whitish; but having heated some on
platinum, we observed that they first turned black and then became
perfectly white, proving that the red was a deposit of organic matter.
To see these leucites, rounded like small pebbles transported by a
torrent, deprived of the soluble chlorides which generally accompany
Vesuvian ashes, is a matter worthy of attention. Whilst heating this
sand upon platinum, decrepitation was audible, which indicated the
cracking of some of the little pebbles. It is evident, therefore, that
crystals of leucite raised to a certain temperature may break, and thus
we can understand how almost all Vesuvian ashes contain fragments of
the said crystals enveloped in the paste of the lava. It is evident that
the soluble part of the ashes is obtained from the smoke through which
it passes. On this occasion the smoke from the craters did not
apparently contain much acids, for no bad smell was perceptible, and the
water in which I washed the ashes scarcely reddened litmus paper. Even
chloride of iron, which was so abundant in the lavas, was scarcely
perceptible in the smoke, which almost exclusively deposited sea-salt on
the surrounding rocks; I say sea-salt advisedly, and not chloride of
sodium, to show that I include all that sea-salt contains. The slight
disturbance it manifested with chloride of barium, and the small
precipitate with oxalate of ammonia, reveal sulphate of lime, without
excluding the possibility of the chloride.

But how can these ashes do so much injury to the vegetation of the
ground they cover, especially at the first fall of rain? I think that
the damage is due partly to the sea-salt, and partly to the acids
contained either in the ashes or in the rain-water itself. Upon watering
the tender tops of some plants with a saturated solution of the salt
from Vesuvius itself, I noticed that they withered away after a few
hours. But very often the rain alone which traverses the smoke of
Vesuvius, or is produced by condensation from it, gives manifest acid
reactions, and destroys the grass and the tops of the trees. The
peasants believe that the rain is warm or of boiling water, from
observing that the tender parts of the plants are, by its deposit, all
burnt up. Vegetation is now recovering, but without flowers, and
consequently without fruit.



V.

THE CRATERS AND THEIR FUMAROLES.


The greater part of the lava issued from the base of the great fissure
in the cone which I have described; and although two other lava streams
descended from the top of the mountain, neither proceeded from the
crater, but from apertures near it. The great crater, divided in two as
already described, opened wide on the morning of the 26th April,
destroying the brim of the antecedent crater, and remaking it in another
shape with ejected matter, except on the south-west side, where the brim
was split. (See Plate 5.)

From this double crater, copious smoke, bombs and incandescent scoriæ,
with ashes and lapilli, issued with violence, and from the depths below
came dreadful detonations and bellowings, producing great terror. And
yet the lava poured out into the Atria del Cavallo without any noise,
and not even a column of smoke marked its origin of issue--namely, from
the fissure.

When the eruption was over, the sight of the vertical walls of these
deep craters, of almost horizontal strata of scoriæ and lithoidal
masses, with a fracture fresh, and as if they had never undergone the
action of fire or of acid vapours, without recent scoriæ and without
fumaroles, was to me a marvellous spectacle. The fumaroles were almost
all on the brims of the craters, with emanations of hydrochloric and
sulphurous acid. In a few that were more removed from the brim,
sulphuretted hydrogen was perceptible. In the sublimations, chloride of
iron was most abundant, in combination with other chlorides, for
example, of sodium, magnesium and calcium. This last chloride was
frequent even among the sublimations of the fumaroles of the lavas, and
it was the first time it was ever remarked, but I do not think it was
the first time that it was ever produced: being in combination with
chloride of iron, and very deliquescent, it did not attract attention
from anyone. In a hollow fragment of scoriæ I observed a yellowish
substance, which looked like sulphur in a viscid state, and which boiled
at a temperature of 120°, and evolved hydrochloric acid. Having
collected this substance and poured it into a glass phial, it quickly
coagulated into an amorphous mass of the same colour; but before I
reached the Observatory, I found that it had become liquid by
deliquescence. It consisted of a mixture of the aforesaid chlorides,
according to an analysis made by Professor Silvestro Zinno and myself.
In some fumaroles, where I perceived the smell of sulphuretted hydrogen,
I found sublimed sulphur under the scoriæ.

At the source of the lava stream that flowed towards the Camaldoli, on
the seaward flank of Vesuvius, I observed large fumaroles of steam only,
pure aqueous vapour.

There was no trace of carbonic acid in these fumaroles, but that fact
does not imply that there was none at a later period, for, since the
first investigations of Deville, it is known that carbonic acid is found
under certain conditions on the very summit of Vesuvius.



VI.

THE ELECTRICITY OF THE SMOKE AND ASHES.


Our ancestors could judge that a great amount of electricity was
occasionally evolved in the smoke, from their observation of the
lightning flashes that darted through the Vesuvian pine tree; but they
had no proper instruments for ascertaining whether this evolution of
electricity was constant or accidental, or what laws regulated its
manifestations. My _apparatus, with movable conductor_, by which
comparative observations of electric meteorology can be made, and the
errors arising from dispersion corrected, supplied me with an easy
method of studying the electricity evolved during eruptions.

I must begin by describing the bifilar electrometer, in order to explain
the apparatus which I have named as above, "_Apparechio a conduttore
mobile_."

_A A_ (Plate VIa, Fig. 1) is a glass cylinder, the lower edge of which
is ground, well varnished with gum lac, and let into a wooden base, B,
furnished with three levelling screws. Through a sufficiently wide glass
tube, _a a_, runs a copper rod covered with insulating mastic, having a
little plate or cylindrical cavity of gilded brass at the top (Figs. 2
and 3), with two arms _d d_, _d' d_. In the plate a disc of aluminium,
_m_, is suspended by means of two silk fibres, and to the disc a very
fine aluminium wire is attached, _f f'_, bent a little at the ends, as
are the arms, _d d_, _d' d_. The disc has about three millimetres less
diameter than the plate. The diameter of the plate may vary within
certain limits, but I have found it convenient to make it eighteen
millimetres. The glass tube, _a a_ (Fig. 1), should descend below the
base as much as it rises above it, that is three to four centimetres.
The length of the index is about one decimetre.

The upper ends of the two silk fibres, by which the disc and index are
suspended, are attached to the top of the glass tube, _C_, by a
contrivance which permits a change in the distance between the two
points of suspension, and a screw, _p_, is provided to raise and lower
the disc with the index. At _n_, at the lower part of the tube, _C_,
there is a kind of torsion micrometer, arranged so as to bring the index
to the zero of the scale engraved on the graduated ring, _B_, which is
formed of a strip of good paper pasted on the rim of a glass disc. The
index must be placed at the zero of the scale, and must be some distance
from the ends of the arms of the plate with which it is parallel. The
plate is about three millimetres deep.

Having levelled the instrument, so as to render the disc concentric with
the plate, and placed the index at zero, it is obvious that if an
electric charge through the wire, _h_, reach the plate with the arms, it
will electrify the disc and index: the disc will have the opposite
electricity, and the extremities of the index will take the same
electricity as the arms, and consequently the index will describe an arc
more or less great. The motion of the index is sufficiently slow to
allow the eye conveniently to follow it. Having traversed the first arc,
which I call the _impulsive_ one, the index returns, and, after only two
oscillations, comes to rest at what I shall call the _definite_ arc.

When the electric charges are of very brief duration, the impulsive arcs
are within certain limits proportional to the tensions, and the ratio
between the impulsive and definite arcs is expressed by the following
equation:

     α(β - α) / β = tang. (1/2) α

In which β is the impulsive arc and α the definite arc, showing that α
comes out nearly equal to 1/2 β. In dry weather all goes perfectly
within the limits of proportion, and I can tell whether, during the time
in which the index traversed the impulsive arc, there were any
_dispersions_ and of what nature; for if the definite arc is not close
to the limit of the impulsive arc, it is a sign of _dispersions_ having
taken place during the motions of the index. Every degree less in the
definite arc denotes two degrees of loss for the impulsive arc; but as
the index employs double the time traversing the definite as it does the
impulsive arc, we may consider the loss of one equal to the loss of the
other.

In excessively damp weather the index gives no definite arc, and it is
necessary to resort to artificial heat in order to dry the insulators.
The most simple means I know of is to hold the instrument over some
hollow vessel, which, for the time, is converted into a stove by the
introduction of a spirit lamp.

From Gauss's formula for the bifilar system of instruments of this
class, we learn that the maximum sensitiveness of such instruments is
given when the length of the suspending fibres is greatest, and the
distance between them is smallest, with the weight of the movable or
rotating member a minimum; and these elements being the same, the
sensitiveness of the instruments is invariable.

To some electrometers, in order to avoid errors of parallax, a small
telescope, with a micrometer wire, has been added; but, with a little
practice, we can read accurately without this refinement. In order to
obtain comparative measurements, it is necessary to select some given
unit of tension. I have observed that by making a galvanic pile of
copper, zinc and distilled water, and insulating it well, each pole has
a tension which remains the same for many days, if the conditions of
temperature and the moisture of the surrounding atmosphere are not very
different. With thirty pairs of this pile, each element having
twenty-five square centimetres of surface, I have on the electrometer a
definite arc of 15°, with the temperature of the atmosphere at 20° C.,
and with the difference of 4° to 5° C. between the thermometers of the
psychrometer of August's construction. The first observation was made
twenty-four hours after mounting the pile. For unit of tension I took
that which corresponded to a single pair, that is, the thirtieth part of
the total tension. Other electrometers may be compared with one already
properly adjusted, without always having recourse to the pile.

This done, let us see the arrangement of all the apparatus:

_H H_ (Plate VIIa, Fig. 1) is the ceiling of a well-situated lofty
room, with an opening, _o o_, at the upper part.

_M M_, a bracket or table fastened against the wall, about a metre
distant from the ceiling, _H H_.

_N N_, a wooden platform for the observer.

_A_, the bifilar electrometer.

_B_, Bohnenberger's electroscope.

_a a_, a movable conductor formed of a brass rod 15 to 18 millimetres in
diameter, insulated below by means of a glass rod, well varnished with
gum lac, having a suspending pulley, _c_, and a wooden guide-rod
underneath it, _l_, within the guiding tube, _k_. At the upper part of
this conductor, _a a_, there is a sliding roof, _b_, which can be
adjusted so as to prevent rain entering at the opening, _o o_. The
conductor terminates in a disc made of a sheet of thin brass, _d_, 24
centimetres in diameter. Upon this disc, or even in place of it, we may
use metallic points.

As a support to the conductor at the upper part, I have made use of a
triangular ring, _x_, drawn at its full size in Fig. 2. The conductor
passes between three springs, and the triangular ring is held in place
by three silk cords, _m m m_. Their material should not be mixed with
any cotton, and it may be advisable to saturate them with an alcoholic
solution of gum lac.

_f f f_ is a hempen cord, which is used to raise and lower the
conductor.

_i_ is a copper wire covered with silk, by means of which the triangular
ring, _x_, and through that and its springs the conductor communicates
with either the electrometer or the electroscope.

Quickly raising the conductor by pulling the cord, _f_, the index of the
electrometer will describe a more or less large impulsive arc, and,
after two oscillations, will stop at the definite arc. Having thus
measured the electric tension of the air, and having lowered the
conductor, I next place the wire, _i_, in communication with the
electroscope, _B_, and by again raising the conductor, I ascertain
whether the electricity be positive or negative. It is scarcely
necessary to say that the conductor, when raised, gives electricity of
the same nature as that prevailing at the moment in the atmosphere; and
when lowered, manifests the opposite. In some conjunctures we must keep
the conductor raised and in communication with the electroscope, in
order to observe certain phenomena which I shall presently describe:
this method I call observation with a _fixed conductor_.

I have also constructed a similar but portable apparatus for use on
eruptive cones, when required.

Having given this description of the apparatus, it remains for me to
relate the results obtained, especially on the occasion of the last
eruption of Vesuvius.

The Observatory is distant, in a direct line from the central crater of
Vesuvius, 2,380 metres, so that, when the smoke is copious, it is
properly situated for the study of electricity, particularly when the
wind inclines the pine-tree cloud in the direction of the Observatory,
as frequently happened on the last occasion.

With smoke alone, without ashes, we obtained strong tensions of positive
electricity; with ashes only, which sometimes fell while the smoke
turned in the other direction, we had strong negative electricity; when
the smoke inclined towards the Observatory, accompanied with ashes and
lapilli, we had sometimes one kind of electricity, and sometimes the
other, just as the smoke or the ashes predominated; and often with a
"fixed conductor" we obtained negative electricity, and with a "movable
conductor" positive electricity. In Naples, too, at the Meteorological
Observatory attached to the University, my colleague, Professor Eugenio
Semmola, observed negative electricity of strong tension whilst ashes
were falling there in abundance. The tensions on this occasion were so
strong as to equal those obtained at changes of weather or during storms
(_temporali_), and, being beyond measure with a delicate electrometer,
we marked them with the symbol ∞: the same phenomena were observed when
lightnings flashed.

When there is but little smoke, it is necessary to approach the eruptive
mouths with a portable apparatus, in order to observe those phenomena
which, in great eruptions, may be studied from the Observatory itself.

The conditions under which (_folgori_) lightning flashes are seen from
the cloud of smoke are, that it is conveying great abundance of ashes.
In 1861, there were small flashes even from the line of eccentric mouths
above Torre del Greco, although the smoke was not very great; and when
these ceased to discharge, and the central crater became somewhat
active, with a moderate amount of smoke but a great deal of ashes, small
and frequent lightning flashes were observed in the twilight darting
through the smoke, which was dark in colour. In 1850 the eruption was
more vigorous, the smoke more abundant, and the ashes scarce, but the
flashes were very rare. In 1855, 1858, and 1868, with a scanty supply of
ashes and at intervals, no flashes were observed, and the electricity
remained constantly positive. But having regard to the facts of
antecedent eruptions, one sees that the flashes are always derived, from
the midst of smoke accompanied with ashes and lapilli, which separate
like rain from the rolling volumes of smoke, in the midst of which they
were ejected.

But how can we account for the positive electricity of the smoke, and
the negative electricity of the falling ashes? Without denying the
probability that a part of the positive electricity depends upon the
elevation of the smoke, as in the case of every other conductor we raise
aloft, or with a jet of water sent from a vessel by compressed air, I
think that the greater part of the electricity proceeds from the rapid
condensation of vapours, which are changed from the gaseous condition
into dense clouds; for even when the smoke issues tranquilly and does
not rise, because carried away horizontally by the wind, it gives signs
of positive electricity. From all my studies of atmospheric electricity,
and from some experiments made specially, it follows that the
condensation of vapours is the origin of this development of positive
electricity.

The negative electricity of the falling ashes certainly arises from the
fact itself of their fall; for if we place a metallic vessel full of
ashes upon an elevated and well-situated terrace, while the atmospheric
electricity is positive, and cause the ashes from the vessel to fall
gradually into an insulated metallic cup, communicating with
Bohnenberger's electroscope placed at three or four metres distance from
the vessel, the electroscope will manifest negative electricity. If the
upper vessel be insulated, and the ashes permitted to fall upon the
ground, we shall obtain, from the vessel, positive electricity. The
intensity of these electric manifestations depends (other things being
equal) upon that predominant at the moment in the air; so that if the
experiment be made while negative electricity prevails, the falling
ashes will manifest positive electricity, the upper vessel then showing
negative electricity. Now, as the ashes separate from the positively
electrified smoke in order to approach the ground, which is negatively
electrified, it follows that they must manifest negative electricity
upon touching the ground, leaving the positive electricity in the smoke
above. For this reason, the electric tension of the smoke is increased
by the descent of the ashes and lapilli, so that discharges between the
upper and lower part of the pine-tree cloud, or the surface of the
crater, are rendered possible. Hence it follows that the flashes of
lightning of Vesuvius play through the smoke, and with difficulty strike
bodies upon the earth; and from this circumstance our ancestors believed
the thunderbolts of Vesuvius to be harmless. However, if the smoke were
very great, and driven by the force of the wind to some distance from
the crater, with an abundant fall of ashes, it would be possible to have
lightning flashes proceed from the smoke to the earth. I possess some
documents which relate that, in 1631, thunderbolts fell upon the Church
of Santa Maria del Arco, and other places on the coast of Sorrento.

After upwards of twenty years' study and observation of meteoric
electricity, I am enabled to prove that atmospheric electricity is never
manifested without rain, hail or snow, and that manifestations of light
are always accompanied by thunder--manifestations of light (_lampi_),
thunder and rain being most closely connected. We may have rain without
manifestations of light, but never the latter without rain or hail. I
cannot here repeat what I have demonstrated in other memoirs; I can only
say that the lightnings of Vesuvius, erroneously believed to be not
accompanied by thunder, are really not accompanied by rain, but are
induced by the descent of ashes and lapilli.[6]



GENERAL CONCLUSIONS.


We may conclude from what I have stated:

1. That by the assiduous study of the central crater, and the
indications afforded by the "Apparatus of Variations" and the
"Electro-Magnetic Seismograph," we can obtain precursory signals of
eruptions; and that the other premonitory signs pointed out by our
ancestors, such as the drying up of wells, either only happen
occasionally or are mere coincidences, such as those of the coincidence
of a dry or a rainy season, the prevalence of certain winds, etc.[F]

2. That the fumaroles of the lavas are communications between the
external surface of the lava, hardened and more or less cooled, and the
interior lava still pasty, or at least incandescent.

3. That from the lava, while flowing, there is no escape of acid
vapours, neither from the fumaroles at the first period of their
existence, but these, if they last long enough, arrive at an acid
period.

4. That hydrochloric is the first acid that appears, combined afterwards
with sulphurous acid, and, still later, with sulphuretted hydrogen.

5. That vigorous lava streams may have eruptive fumaroles. (See
Translator's Note 2 to p. 94.)

6. That the sublimations follow a certain order in their appearance. In
the neutral period we get sea-salt mixed with some metallic oxides, the
first of which is oxide of copper. But in the great lavas, chloride of
iron appears simultaneously with the acid period. Hydrochloric acid
transforms the oxides into chlorides, which, in their turn, change into
sulphurets or sulphates on the appearance of sulphurous acid.

7. That the acids, by attacking the scoriæ, create new chlorides and
sulphates, which are thus not products merely of sublimation.

8. That micaceous peroxide of iron--so common and abundant near the
eruptive mouths--is very scarce and rare on the lavas, unless conveyed
there from the craters.

9. That chloride of iron--so manifest on the fumaroles of the great
lavas--is only found in small eruptions close to the discharging mouths.

10. That the frequency of chloride of iron in the lavas of great
eruptions masks the order of transformation of the other products.

11. The fumaroles at the summit of Vesuvius present even greater
gradations, for they often emit carbonic acid or pure watery vapour.

12. Lead, which I first discovered in the fumaroles of the lavas of
1855, is a constant product of fumaroles which have a certain duration.
It is often obtained as a distinct and crystallized chloride, and often
is found in combination with other products.

13. Oxide of copper is also a constant and primary (_primitivo_) product
of fumaroles. The chloride and sulphate of copper are formed from the
oxide, directly contrary to general belief.

14. I do not think that the chloride of calcium, which I found on this
occasion in almost all the deliquescent sublimations, is a product
peculiar to this eruption only, in which alone, however, I found it. I
was, therefore, induced to look for it in other sublimates, in which I
might possibly have overlooked it, as, without doubt, my predecessors
have done, owing to the deliquescence of the chloride of iron with which
it was constantly combined. I think that this chloride, in accordance
with the general law, is transformed into a sulphate--a transformation
which readily occurs on Vesuvius.

15. Copious and well-crystallized sal ammoniac is only found on the
fumaroles of those lavas which have covered cultivated or wooded ground.

16. The scarcity of oxygen in the gases of fumaroles may possibly arise
from the formation of the oxides which precede the chlorides.

17. Lavas give a continuous spectrum, although covered with smoke, when
looked at with Hoffmann's spectroscope with direct vision.[G]

18. The smoke gives positive electricity, and the falling ashes negative
electricity.



REFERENCE TO THE PLATES.


PLATE

Ia. The Cone of Vesuvius, in 1870, from a Photograph taken
    near the Observatory.

    _a._ The Atria del Cavallo.
  _b b._ Fossa della Vetrana.
    _c._ Punta del Crocella.
    _d._ Lava of 1858 and 1867.
    _e._ Police Barrack near the Observatory.
    _f._ Part of Monte Somma.

IIa. Profile of Vesuvius, taken from a Photograph of the
     Observatory in the month of September, 1871.

    1. The Cone, on the 13th January, 1871.
 2, 2. Lava of 1871.

IIIa. Profile of Vesuvius on the 16th April, 1872, about ten
      days before the last Conflagration.

IVa. Vesuvius, on the 26th April, 1872, from a Photograph taken
     in the neighbourhood of Naples.

    1. The Observatory.
    2. Fossa della Vetrana.
    3. Eruption of Smoke and Ashes, with Stones, from the
         surface of the Lava.
    4. The Novelle, St. Sebastiano, and Massa.
    5. Lava which took the direction of Resina.
    6. Lava which, from the Crater, took the direction of the
         Camaldoli.
    7. The Grain Stores, near Naples.
    8. Resina.
    9. Torre del Greco.
   10. The Camaldoli.

Va. Profile of Vesuvius after the Eruption of the 26th April,
    1872, from a Photograph taken near the Observatory.

   1, 1. The Fissures of the 26th of April.
   2, 3. Small Hill thrown up on the morning of the 26th of
         April, from below which issued the great current of
         Lava.
4, 4, 4. The Mouths out of which the Lava issued.
   5, 5. The larger Lava Stream, which passed near the Observatory
         by the Fossa della Vetrana.
   6, 6. The other Lava Stream, which, after dividing from
         the last, took the direction of Resina.
   7, 7. The Lava which ran down towards the Camaldoli.
  8 & 9. The two Craters on the summit of the Cone.

VIa. The Bifilar Electrometer of Signor Palmieri.
     (_Details._)

VIIa. The assemblage of the Electroscopic Apparatus of Signor
      Palmieri, as arranged at the Vesuvian Observatory.

VIII. Professor Palmieri's Seismographic Apparatus.


[A] This small cone, as it appeared on the 1st April, is described and
drawn in a Memoir of Professor von Rath, of the University of Bonn, on
"Vesuvius on the 1st and 17th of April, 1871."

[B] Eight young medical students perished beneath the lava, with others
unknown by name. They were all youths of good promise; their names will
be recorded on the marble monument to be erected near the Observatory.
They are: Girolamo Pausini, Antonio and Maurizio Fraggiacomo, Francesco
Binetti da Molfettu, Giuseppe Carbone da Bari, Francesco Spezzaferri da
Trani, and Giovanni Busco da Casamassima and Vitangelo Poli.

[C] If this enormous height of projection really means, that above the
brim of the crater, it involves an initial velocity of projection of
above 600 feet (British) per second.

Observations of the height of ascent of volcanic blocks are always
difficult and deceptive, and never free from error.--_Translator._

[D] Assuming these flashes to have emanated from somewhere within the
cloudy volume of steam and dust called "the head of the pine-tree," this
interval would indicate that the mean height of this cloudy volume
itself was not more than about four thousand feet above the top of the
cone; and, if so, that is not very far from the limit in height of
projection of the dust and lapilli.--_Translator._

[E] COTUNUITE, chloride of lead, in white, lustrous, acicular crystals,
of the trimetric system, easily scratched, Sp. gr., 5·238.

TENORITE, peroxide of copper, in thin, hexagonal plates or scales,
translucent when very thin, dark steel gray, of the cubic system; hard
and lustrous. Sp. gr. about 5·950.--_Translator._

[F] Earthquakes, though in distant regions, usually precede eruptions.
The Earthquake of Melfi preceded the great Eruption of Etna in 1852; the
Earthquake of Basilicata of December, 1857, terminated with the Eruption
of 1858, which filled the Fossa Grande with lava; the Earthquakes of
Calabria of 1867 and 1870 were the precursors of the Vesuvian
conflagrations of 1868, 1871, 1872. A Volcano, also, in the Island of
Java had a great eruption in the month of April, some days before the
last conflagration of Vesuvius, as I learnt from a letter addressed to
Signor Herzel, Swiss Consul at Palermo, communicated to me[7] by the
astronomer, Signor Cacciatore.--_Palmieri._

[G] I have made a large collection of sublimates, which I purpose
examining with the spectroscope, and I shall be able to place some at
the disposal of experimentalists who may desire to pursue investigations
of this kind.



NOTES

BY THE TRANSLATOR.


[1] (P. 82, text). Professor Palmieri has not given any description
in this Memoir of his seismograph--the instruments described being those
only which have relation to atmospheric electricity. The following brief
account of his seismograph will, therefore, form a not unsuitable
complement to his Memoir. The instrument, in general terms, is of that
class in which the wave movements are indicated by the displacement,
relative or absolute, of columns of mercury in glass tubes. It is a
self-recording instrument, composed of two distinct portions--one for
record of horizontal, or rather of what are called undulatory shocks;
the other for vertical shocks. In point of general principle, therefore,
it is very similar to that proposed by me ("Transactions, Royal Irish
Academy," in 1846), and in certain respects appears to me less
advantageous than the latter. Some account of the Palmieri instrument,
together with some critical remarks as to its action, may be found in my
"Fourth Report on Earthquakes" ("Reports, British Association, 1858,"
pp. 75-81). The following description of the instrument is derived from
"The Engineer," of 7th June, 1872, and the publishers have to thank the
proprietors of that journal for permission to use the illustration,
Plate 8.

In Fig. 1, _E_ is a helix of brass wire (gauge about one millimetre);
the helix consists of fourteen or fifteen turns, and has a diameter of
from twenty to twenty-five millimetres; it hangs from a fine metal
spring, and can be raised or lowered by a thumb screw. From the lower
end of the helix hangs a copper cone with a platinum point; the latter
is kept close to the surface of mercury in the iron basin, _f_, which
rests on an insulating column of wood or marble, _G_. The distance of
the point from the surface of the mercury remains constant, as the metal
pillar, _T_, is of such a length that its expansion or contraction by
change of temperature compensates that of the helix; the latter is in
connection (by _T_) with one pole of a Daniell's battery of two cells,
and the basin, _f_, is connected with the other pole. Any vertical
movement, however slight, makes the platinum point dip into the mercury,
and thus completes the circuit. In this circuit are included two
electro-magnets, _C_ and _D_; these, during the circulation of a
current, attract their armatures, which are connected with levers. The
action of _C's_ lever is to stop the clock, _A_, which thus records, to
a half-second, the time of the occurrence of the shock, at the same
instant that the clock strikes an alarm bell, which attracts the
attention of an observer. The lever, attached to the armature of _D_, at
the first instant of the current frees the pendulum of the clock, _B_,
which was before kept from swinging, in a position out of the vertical;
the clock then acts as a time-piece, and its motion unrolls a band of
paper, _k k k_, at a rate of three metres an hour. At the same time the
armature of _D_, while attracted, presses a pencil point against the
band of paper which passes over the roller, _m_, marking on it, while
the earthquake lasts, a series of points or strokes which occupy a
length of paper corresponding to its duration, and which record the work
of the shock. After it is over the paper continues to unroll from the
drum, _i_, and passing round the clock, rolls on to the drum, _l_. If a
fresh shock occur the pencil indicates it, as before, on the paper, and
the length of blank paper between the two sets of marks is a measure of
the interval of time between the shocks. By way of additional check,
several helices, _h h h_, are hung from a stand, with small permanent
magnets suspended from their ends; below and close to these latter are
small basins, holding iron filings; into these the points of the magnets
dip, when their helices oscillate vertically, and some filings remain
sticking to the magnets as a record of the shock. One of the magnets has
a shoulder on it which moves an index hand along a graduated arc, as
shown in Fig. 2, thus again registering the amount of the vertical
movement. Such are the arrangements intended for the record of the
undulatory or horizontal elements of the wave of shock.

The following are the arrangements proposed for recording the horizontal
motions: On the stand, to the right of the clock, _A_, are set four
U-shaped glass tubes, open at their ends. One of each pair of vertical
branches must have a diameter at least double that of the other. These
pairs, with their supporting columns, are shown in plan, where one pair
lies N. and S., another E. and W., a third N.E. and S.W., and the other
N.W. and S.E. It will be observed that metallic bars pass from the
pillar, _P_, over the ends of all the long branches, and similar bars
pass from _R_, over the ends of the short branches; the pillars
themselves, as in the case of the other instruments, are each connected
with one pole of a Daniell's battery, the connections including the
electro-magnets, _C_ and _D_. The description of one U tube, _n_, will
apply to all the others; _n_ is partly filled with mercury, and an iron
or platinum wire, _o_, suspended from the bar above the short branch,
dips into the mercury therein, while another platinum wire hung from the
bar over the mouth of the longer branch, has its end very close to the
surface of the mercury in that branch. Any shock which is not
perpendicular in direction to the plane of the branches of the U will
cause the mercury to oscillate in the tubes, and more sensibly in that
with the smaller diameter; when it rises up in the latter, so as to
touch the platinum point, the connection between _P_ and _R_ is made and
the circuit completed, starting the action of the electro-magnets _C_
and _D_, which record the shock, as already described. By having the
planes of the tubes set in the different azimuths, already mentioned,
one or more of the pairs is sure to be acted upon, and by observing in
which the oscillation takes place the direction of the shock is supposed
to be ascertained. Besides this, each long branch of the U, viz., that
of smaller diameter, has a small ivory pulley, _q_, fixed above it, over
which passes a single fibre of silk, with an iron float at one end,
resting on the surface of the mercury; at the other end of the fibre
hangs a counterpoise; fixed to the pulley is a fine index hand, capable
of moving along a graduated arc. When the shock takes place the mercury,
rising in the long branch, raises the float on its surface, the silk
fibre at the same time makes the pulley revolve with its index hand,
which afterwards remains stationary, as the counterpoise prevents the
float from sinking again with the mercury. The reading on the graduated
arc is thus a measure of the movements produced in the instrument by the
horizontal element of the shock, and is supposed to measure that shock.
It is assumed that in all these instruments shocks, however small, can
be recorded with certainty by adjusting the distance between the
platinum points and the mercury.

The arrangement of Daniell's battery used for the seismograph is shown
in Fig. 4, where, for convenience of cleaning, the copper element is
made of wire (about No. 8 Birmingham wire gauge) coiled flat without the
spirals touching. Crystals of sulphate of copper are placed at the
bottom of the outer cell, into which water is poured; and the inner
cell, into which the zinc plate goes, is filled with siliceous sand.

In addition to the above some instruments of a rougher description are
employed as checks. Thus, at the foot of the pillar, _G_, there is a
wooden trough with eight holes, facing as many equidistant points of the
compass (two of them shown in section) round its inner circumference;
mercury is poured into the basin until its level is nearly up to the
lips of the holes. The effect of a shock is to throw some of the mercury
into one or more of these holes, and the greater the oscillation the
more mercury is thrown into the cells through the holes. The screws
shown outside are for drawing off the mercury from the cells, when its
quantity can be measured. The direction of the shock is shown by seeing
which cells are filled with mercury. This is the old Cacciatore
seismometer which has been long employed in Italy. (See 4 "Report of
British Association, 1858," p. 73), and Daubeny's "Volcanoes," Appendix.
The following is another contrivance. From the arm of the pillar, _G_, a
fine metal wire hangs, with a metal ball at its end, which, by its
oscillation, thrusts out one or more light glass tubes, set horizontally
in a stand, as shown in Fig. 3. The two rings are of wood, and the glass
tubes pass through holes in them; small leather washers are placed
outside the outer rings; the displacement of one or more tubes is
assumed to measure the horizontal element of the shock. By means of this
apparatus the time of the first shock is recorded, as well as the
interval between the shocks, and the duration of each; their direction,
whether vertical or horizontal, is given, as also the maximum of
intensity. Professor Palmieri has the instruments examined three times a
day, and an assistant-observer is always at hand to attend to the bell,
and put back the apparatus to its normal position for fresh observation.

It has been stated that this instrument is sensible to most of the
shocks which occur in the Mediterranean basin.

It is not my intention here to offer any criticism as to the
construction or performances of this instrument, the rather as I must
confess I do not quite share the high opinion of its inventor as to the
certainty or exactitude of its indications.

There can be no question as to the extreme importance to science of the
establishment and continued use of a seismographic instrument of
unexceptionable construction at the Observatory upon Vesuvius; and it
would be a valuable gift to science, were the Italian Government to
enable Signor Palmieri to establish such an one. Its great value and the
very first problem to set the instrument to solve should be, by _a rigid
determination of the direction of propagation of the wave of shock_, of
those slight or stronger pulsations which precede or accompany the
Vesuvian like all other eruptions, on arriving at the Observatory, _to
fix the depth, and the position vertically beneath the cone, whence
these pulses are derived_. This would be, in fact, to fix the depth and
position beneath the mountain at which the volcanic focus is situated
for the time, or, at least, where the volcanic activity is at the time
greatest. And the assured knowledge, even within moderate limits of
accuracy, of this depth, and even for this single mountain, would be an
immense accession to our positive knowledge, and a really new stage
gained for future advances. At present, we know but little as to the
actual depth below our globe's surface at which volcanic activity
occurs, or to which it is limited, either upwards or downwards. I have,
myself, established some data upon the flanks of Etna, not yet
published, which may enable me to afford some information on the subject
hereafter. Meanwhile, Professor Palmieri possesses unrivalled
opportunities for such observations; and I trust health, life and means
may be afforded him, to become the first who shall have made this great
addition to our positive knowledge of Vulcanology.

So far, popularly at least, the alleged chief uses and value of these
seismographic instruments, at the Observatory of Vesuvius, have been
made to depend upon their being presumed to afford means for foretelling
eruptions, or affording precursory warnings of their probable progress
and destructive course.

I feel compelled to express my own total disbelief in the possibility of
any such predictions in the present state of science, by the help of any
instruments whatsoever, of such a nature as to be of any _practical
value_, or any certainty beyond that which a certain amount of _mere
experience_ as to the _rôle commonly played_ by Vesuvius or other
Volcanoes in pretty habitual activity affords to the observer for a
lengthened period. And even this affords scarcely any guide as to what
may happen next. Monte Nuovo was thrown up in a night; Vesuvius _might_
double its volume in a night, or might sink into a hollow like that of
the Val del Bove in a not much longer time. A small _fusillade_ may go
on for months, and yet, without an hour's notice, by any premonitory
sign, may waken up to a roar and darken the air with ashes and lapilli
such as those which overwhelmed Pompeii. One eruption may blow forth
little but dust and ashes (so called), another may pour out rivers of
lava and little else.

The _main_ mischief of all eruptions is effected in two ways: by the
deposit of dust and ashes, lapilli, etc., to the injury or destruction
of fertile land, and by the streams of lava which overwhelm it, as well
as buildings, etc. But what information of any value can seismographic
observation afford as to the course that either of these may take in any
eruption? The volume of pulverulent material that may be ejected cannot
be foreseen; its distribution depends mainly upon its nature and upon
the direction and force of the wind at the time; or again, how shall
these warn us as to the course that the lava, if it appear, shall take,
when we cannot possibly foretell when, how, or by what mouth it may
issue. Even in this late eruption of 1872, with Palmieri stoutly at his
post upon the mountain, and the Observatory instruments in full
activity, they gave no forewarning of the sudden and unexpected belch
forth from the base of the cone, of that tremendous gush of liquid lava
which in a few minutes cut off from life the unhappy visitors whose
deaths he has recorded.

[2] (P. 94). It can scarcely be supposed that these small
eruptive-looking belchings forth from the lava stream, _en route_, are
truly of an eruptive nature at all, _i.e._, in any way connected with
forces seated deeply beneath the bed of the lava stream, or in any way
connected with the volcanic ducts of the cone or beneath it. They are
most probably merely the bursting upwards of large bubbles; that is, of
cavities formed in the mass of the more or less liquid lava by intestine
movements, as its mass winds and rolls along, and by the aggregation of
smaller cavities--all being filled with steam and gases--together with
dust and volatile products which are ejected when the cavity opens up,
and its contents escape at the upper surface of the lava stream in
virtue of the continuation of the twistings and convolutions due to the
stream motion itself, and to the unbalanced hydrostatic pressures acting
upon the parietes of the bubble. Very large single bubbles of like
character rise in the fluid lava within craters in vigorous action, and
often so regularly that their recurrence causes a sort of rhythmical
rush and roar in the column of steam, etc., issuing above the mouth.
This was evident in the discharges issuing in 1857 from the highly
instructive minor _bocca_, then existing, examined by me, and referred
to ("Report, Naples, Earthquakes," etc. Vol. II., pp. 313, 314), as
presenting at the time great facilities for determining pyrometrically
the temperature of the lava within, and of the dry superheated steam
issuing with a rhythmic roar from it. M. Le Coq ("Époques Géologiques
d'Auvergne," Tome IV.) has recorded some examples of the formation and
opening-out of large bubble-like cavities in lava already ejected.
Perhaps that able and laborious vulcanologist, whose death a few months
ago science still deplores, attributes too much importance as well as
magnitude to them, when attributing the formation of what he has
denominated "craters of explosion," to the mechanism of the rise and
bursting of such bubbles upon a gigantic scale. Such blowings forth,
sudden or prolonged, from particular spots of lava streams, _en route_,
undoubtedly may also have their origin in damp places, or water or
air-filled cavities in or beneath the bed over which the lava rolls,
which, getting gradually heated, generate steam, or air or gases under
tension by expansion, etc., which thus at length blow through the liquid
or pasty lava flowing above, and which in bursting through delivers much
dust also, and so simulates a little eruptive crater. Examples of this,
upon a great and convincing scale, can be pointed to in the Val di
Calanna and elsewhere on Etna.

[3] (P. 96). There are strong grounds for the gravest doubts that there
exists any real connection of a physical character between Volcanic
Eruptions, and Earthquakes more or less _approximately_ coincident only,
in time of occurrence; the respective sites being widely apart, and the
less the probability as the intervening distance is greater. The
discussions of the large number of records that are to be found of such
coincidences--mostly but partial, and in but _very_ few instances
complete coincidences--by Perrey, von Hoff, and others, as well as by
myself, do not tend to sustain the view that such imperfect
contemporaneity is based upon any causative connection. The seismic
region of Greece appears to have no _direct_ connection with that of
Southern Italy: the band of connection, if any, seems to lie between
Northern Italy, across the Northern Adriatic, by Ragusa, and thence
spreading into Asia Minor.

[4] (P. 97). The abundance of coleoptera and of various other forms of
insect life about lava beds, both recent and old, is a very singular
fact, and one worthy of the careful observation of entomologists. In the
autumn of 1864, at mid-day, when sitting sketching upon the lava about
the middle of the Val del Bove (Etna), I found it almost impossible to
work, or even to remain for an instant still, in consequence of the
continual cloud of insects, large and small, that struck against me in
flight, endangered the eyes, and swarmed upon my clothes. It is quite
possible that this local superabundance of insect life may arise merely
from the general dryness and warmth of such places, and the plentiful
_nidus_ that the innumerable cavities in lava afford for the eggs and
earlier stages of insect life; still, this apparition of one form of
life may also be connected with other circumstances not unimportant to
discover.

[5] (P. 120). The _Crocella_ is a small wooden cross, erected several
years ago, and which one passes to the right hand at the upper end of
the path along the ridge of tufa and volcanic conglomerate upon which
the Observatory stands, in ascending thence to the Atria del Cavallo.

[6] (P. 134). That the causes assigned by Professor Palmieri for the
potent developments of electricity (positive or negative) which
characterise the ascent of the issuing columns of (chiefly if not
always) _dry_ steam, with a relatively small volume of various gases,
and throwing up, in their blast, volumes of small solid particles in
ashes and lapilli, etc., and the subsequent fall as a mineral or stony
hail-shower of the latter, through the partially condensing vapours and
the circumambient air, are the main causes of electrical development
evidencing itself in lightning flashes, is no doubt true. We must not,
however, lose sight of the many other and very effective agencies at
work here to produce electric excitement. The actual _bocca_ of the
volcanic vent whence the steam roars off constitute the cone a veritable
hydro-electric machine. Mechanical energy in various forms is
transformed into electric energy. Chemical action is going on both in
the solid and in the vapourous and gaseous emanations as they rush into
and remain in the air or descend from it, and chemical action is
transformed in part into electric energy. Percussion between ascending
and descending particles and fragments, fractures and breaking up of
more or less of these, thus and by sudden changes of temperature in
cooling, are likewise operative. In addition, great and violent
movements in the atmosphere itself result from the large local
accessions of temperature by the heated volume driven up into it, and
which in turn give rise to electric disturbance of the same character as
those produced in wind storms and whirlwinds, brought about by the
natural causes which every day effect disturbances in our atmosphere all
over the globe.

[7] (P. 135). The views stated in note 3 (to page 96) may here again be
referred to as in point. How is it possible, in the present state of
science at least, to establish any physical connection between an
eruption in Java and one of Vesuvius, "with half the world between,"
when not even having the solitary connecting link of complete
contemporaneity, and which, if it existed, yet might be nothing but
accidental? A list of shocks upon record, which have occurred more or
less nearly simultaneously at distant parts of the world, may be found
in my fourth Report, ("Facts of Earthquakes," "British Association
Reports, 1858") and the reasons are there given for rejecting the notion
of any direct physical connection between the origins of the respective
shocks.

Shocks, emanating from the close neighbourhood of volcanic vents, or
simultaneity of eruption, in vents not far distant from each other,
stand upon a different footing.



Transcriber's Notes.

Preserved the unusual, but consistent, spelling of "develope."

Preserved the unusual, and inconsistent, references to the Plates.
Sometimes Arabic numerals are used, but usually Roman numerals. Most
have "A" attached to the name (which was changed to "a" to avoid
confusion in the Roman numerals), but not VIII, and sometimes not in the
references to them.

Equations were converted to linear text, adding spacing and parentheses
as necessary.

Changed "fumarolles" to "fumaroles" on page 3: "fumaroles and
salfatares."

Changed "Lyall" to "Lyell" in footnote originally on page 9: "Daubeny,
Lyell, Phillips and others."

Changed "throught" to "through" on page 35: "passing through it."

Removed duplicated word "the" on page 40: "the great Calabrian
Earthquake."

Page 89 refers to the "Fossa del Vetrano;" elsewhere there are
references to "Fossa della Vetrana," which may be what was intended.
However, I did not change this.

Changed "hydrochloride" to "hydrochloric" on page 109: "the hydrochloric
acid which it discharged."

Changed "disk" to "disc" on page 126: "the disc and index."

Changed "azismuths" to "azimuths" on page 143: "the different azimuths."

Changed "silicious" to "siliceous" on page 143: "filled with siliceous
sand."





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