[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.