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 pa.s.sed 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 a.s.semblage 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, Frances...o...b..netti da Molfettu, Giuseppe Carbone da Bari, Francesco Spezzaferri da Trani, and Giovanni Busco da Casama.s.sima 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] a.s.suming 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, l.u.s.trous, acicular crystals, of the trimetric system, easily scratched, Sp. gr., 5238.
TENORITE, peroxide of copper, in thin, hexagonal plates or scales, translucent when very thin, dark steel gray, of the cubic system; hard and l.u.s.trous. Sp. gr. about 5950.--_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 cla.s.s in which the wave movements are indicated by the displacement, relative or absolute, of columns of mercury in gla.s.s 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 a.s.sociation, 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 ill.u.s.tration, Plate 8.
In Fig. 1, _E_ is a helix of bra.s.s 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 pa.s.ses 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 pa.s.sing 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 gla.s.s 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 pa.s.s from the pillar, _P_, over the ends of all the long branches, and similar bars pa.s.s 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 pa.s.ses 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 a.s.sumed 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 compa.s.s (two of them shown in section) round its inner circ.u.mference; 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 quant.i.ty 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 a.s.sociation, 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 gla.s.s tubes, set horizontally in a stand, as shown in Fig. 3. The two rings are of wood, and the gla.s.s tubes pa.s.s through holes in them; small leather washers are placed outside the outer rings; the displacement of one or more tubes is a.s.sumed 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 a.s.sistant-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 exact.i.tude 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 a.s.sured 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 _role 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 ma.s.s of the more or less liquid lava by intestine movements, as its ma.s.s 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 ("epoques Geologiques 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 circ.u.mstances not unimportant to discover.
[5] (P. 120). The _Crocella_ is a small wooden cross, erected several years ago, and which one pa.s.ses 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 a.s.signed 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 circ.u.mambient 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 const.i.tute 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 a.s.sociation 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.