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 calllapilli, 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 containfragments 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.
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. (SeePlate 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 offire 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.
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. Myapparatus, 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 insulatingmastic, having a little plate or cylindrical cavity of gilded brass at the top (Figs. 2 and 3), with two armsd 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. Atn, 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 electrifythe 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 theimpulsiveone, the index returns, and, after only two oscillations, comes to rest at what I shall call thedefinitearc.
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.1/2α
α(β - α) / β = tang.1/2α
In which β is the impulsive arc and α the definite arc, showing that α comes out nearly equal to1/2β. 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 anydispersionsand of what nature; for if the definite arc is not close to the limit of the impulsive arc, it is a sign ofdispersionshaving 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 Iknow 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 comparedwith 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 fis a hempen cord, which is used to raise and lower the conductor.
iis 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 afixed 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 thestudy 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 mouthsabove 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 thecondensation 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 thethunderbolts 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]
We may conclude from what I have stated:
Ia. The Cone of Vesuvius, in 1870, from a Photograph taken near the Observatory.
IIa. Profile of Vesuvius, taken from a Photograph of the Observatory in the month of September, 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.
Va. Profile of Vesuvius after the Eruption of the 26th April, 1872, from a Photograph taken near the Observatory.
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."
[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.
[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.
[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.
[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.
[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.
[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.
[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.
[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,Eis 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 (byT) 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,CandD; these, during the circulation of a current,attract their armatures, which are connected with levers. The action ofC'slever 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 ofD, 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 ofD, 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 fourU-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 fromR, 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,CandD. The description of oneUtube,n, will apply to all the others;nis 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 theUwill cause the mercury to oscillate in the tubes, and more sensibly in that with the smaller diameter; when it rises up inthe latter, so as to touch the platinum point, the connection betweenPandRis made and the circuit completed, starting the action of the electro-magnetsCandD, 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 theU, 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 displacementof 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, bya 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 anypractical value,or any certainty beyond that which a certain amount ofmere experienceas to therôle commonly playedby 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; Vesuviusmightdouble 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 smallfusillademay 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.Themainmischief 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.
[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,Eis 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 (byT) 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,CandD; these, during the circulation of a current,attract their armatures, which are connected with levers. The action ofC'slever 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 ofD, 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 ofD, 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 fourU-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 fromR, 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,CandD. The description of oneUtube,n, will apply to all the others;nis 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 theUwill cause the mercury to oscillate in the tubes, and more sensibly in that with the smaller diameter; when it rises up inthe latter, so as to touch the platinum point, the connection betweenPandRis made and the circuit completed, starting the action of the electro-magnetsCandD, 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 theU, 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 displacementof 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, bya 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 anypractical value,or any certainty beyond that which a certain amount ofmere experienceas to therôle commonly playedby 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; Vesuviusmightdouble 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 smallfusillademay 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.
Themainmischief 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.