The Eruption of Vesuvius in 1872

by Luigi Palmieri

NOTES
BY THE TRANSLATOR.

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

In Fig. 1, E is a helix of brass wire (gauge about one millimetre); the helix consists of fourteen or fifteen turns, and has a diameter of from twenty to twenty-five millimetres; it hangs from a fine metal spring, and can be raised or lowered by a thumb screw. From the lower end of the helix hangs a copper cone with a platinum point; the latter is kept close to the surface of mercury in the iron basin, f, which rests on an insulating column of wood or marble, G. The distance of the point from the surface of the mercury remains constant, as the metal pillar, T, is of such a length that its expansion or contraction by change of temperature compensates that of the helix; the latter is in connection (by T) with one pole of a Daniell's battery of two cells, and the basin, f, is connected with the other pole. Any vertical movement, however slight, makes the platinum point dip into the mercury, and thus completes the circuit. In this circuit are included two electro-magnets, C and D; these, during the circulation of a current, attract their armatures, which are connected with levers. The action of C's lever is to stop the clock, A, which thus records, to a half-second, the time of the occurrence of the shock, at the same instant that the clock strikes an alarm bell, which attracts the attention of an observer. The lever, attached to the armature of D, at the first instant of the current frees the pendulum of the clock, B, which was before kept from swinging, in a position out of the vertical; the clock then acts as a time-piece, and its motion unrolls a band of paper, k k k, at a rate of three metres an hour. At the same time the armature of D, while attracted, presses a pencil point against the band of paper which passes over the roller, m, marking on it, while the earthquake lasts, a series of points or strokes which occupy a length of paper corresponding to its duration, and which record the work of the shock. After it is over the paper continues to unroll from the drum, i, and passing round the clock, rolls on to the drum, l. If a fresh shock occur the pencil indicates it, as before, on the paper, and the length of blank paper between the two sets of marks is a measure of the interval of time between the shocks. By way of additional check, several helices, h h h, are hung from a stand, with small permanent magnets suspended from their ends; below and close to these latter are small basins, holding iron filings; into these the points of the magnets dip, when their helices oscillate vertically, and some filings remain sticking to the magnets as a record of the shock. One of the magnets has a shoulder on it which moves an index hand along a graduated arc, as shown in Fig. 2, thus again registering the amount of the vertical movement. Such are the arrangements intended for the record of the undulatory or horizontal elements of the wave of shock.

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

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

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

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

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

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