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, [Greek: Peri
Kosmou], 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.