width of the seismic band, paroxysmal efforts are occasionally propagated to great superficial distances beyond it. 6th. The sensible width of the seismic band depends upon the energy developed, and upon the accidental geologic and topographic conditions at each point along its entire length. 7th. Seismic energy may become sensible at any point of the earth's surface, its efforts being, however, greater and more frequent as the great volcanic lines of activity are approached. 8th. The surfaces of minimum or of no known disturbance, are the central areas of great oceanic or terr-oceanic basins or saucers, and the greater islands existing in shallow seas. The fact that certain low-lying river-basins, such as the Mississippi and the Ganges, are the seats of earthquake disturbance, does not conflict with the last proposition. In these cases, the impulse is propagated into the plain from the band of the bounding ridges; and when these are very large in relation to the basin, the breadth of the seismic band may overlap its whole surface,-as for example in the basin of the Ganges, where the seismic banks of the Himalaya and Vindhya mountains cover the whole plain of Northern India. We have thus extracted all the information that our Catalogue, or indeed any further cataloguing of earthquakes, seems capable of giving us; future research must take a more distinctly physical character. I therefore proceed to some observations upon instrumental seismometry and the construction of seismometers, upon which our future progress must much depend. Twelve years ago, at the period of the author's paper (Trans. R. I. Acad. vol. xxi. 1846) "On the Dynamics of Earthquakes," the construction of seismometric instruments appeared a comparatively easy matter; there did not seem to be much difficulty in producing even a self-registering instrument that should give every element of the earth-wave at the surface, whose normal velocity of propagation was then assumed to be extremely great, to approximate to that theoretically due to the elasticity of solid rocky media, and not to vary very materially in direction of propagation during its transit from the origin, to any distant point of the earth's surface. It is only at a very recent period that experiments and observations as to the actual phænomena, the velocity and direction of shock, &c. have begun to show the real difficulties of the subject; and as these are apparently not very generally recognized, I propose pointing some of them out here, prior to indicating the limits within which for the present, it appears to me, we must be content to restrict our seismometric aims and instruments, and describing what form of instrument, and in what localities placed, would appear, with our existing knowledge, the best to give us some information-approximate only, and incomplete without doubt, but yet such as can be made a safe basis for a future higher step with more refined and comprehensive instruments. I shall avoid as much as possible (as out of place in this Report) any mathematical treatment of the subject. The antecedent history of seismometers is in brief as follows: All the instruments hitherto devised or set up may be divided into two great classes:-1, observational, those whose motions must be observed and recorded after each shock; 2, self-registering, which record their own past movements however repeated, and admit of their observation at any subsequent period within certain limits. Each of these classes is again divided into two sorts:-a. instruments dependent upon the movements by displacement of liquids; b. those dependent upon the partial displacements of solids. Of the first class, there have been 1 (a). That of Cacciatore of Palermo, long in use in Sicily. It consists of a wooden circular dish about 10 in. diameter, placed horizontally and filled with mercury to the brim-level of eight notches that face the cardinal points and the bisecting rhumbs between, and are cut down through the lip of the dish, equally in width and depth all round. Beneath each such notch a small cup is placed, to receive such mercury as may be thrown out of each notch by an oscillatory displacement of the main mass of mercury, due to a general oscillation of the whole system. Either the volume or the weight of mercury found in each cup is supposed to ineasure the value of the displacement, and hence of the shock in its direction in azimuth. 2 (a). The wooden or other bowl of molasses, or other such viscid liquid, suggested for use by Mr. Babbage. 3 (a). A cylindric tub with chalked or whitewashed sides, and partially filled with some heavy and permanently coloured liquid of deep tint. (Mallet, Admiralty Manual, sect. vii. p. 218.) 4 (a). Tubes partially filled with mercury, -shaped, with the horizontal and open limbs directed to the cardinal points, for the horizontal component of shock; and U-shaped for the vertical component, both sets being provided with marking indices, to show previous displacement of the mercury. (Mallet, Admiralty Manual, sect. vii. p. 214.) 5 (b). The oldest, probably, of seismometers, long set up in Italy and southern Europe. A pendulum, free to move in any direction, carries below the bob a stile partly immersed in a stratum of dry fine sand spread to uniform thickness over the concave surface of a circular dish placed beneath, marked to the cardinal points, whose centre is beneath the point of suspension of the pendulum when at rest, and whose concavity is that of a spherical segment of a radius equal to the length of the pendulum and stile, plus rather more than the depth of the stratum of sand. It was supposed that the stile would mark a right line when seen in a plane vertical to the sand-bed, and in the direction of the shock. 6 (6). The inverted pendulum, held vertical when at rest by its forming part of a spring at the base (like the watchmakers' noddy), armed with a chalk tracer or pencil above the bob, marking a line or lines upon the concave lower surface of a dish in form like that of the preceding. This was understood to be one of the instruments adopted by the observers of the repeated shocks of Comrie, &c., and the invention, in its improved form, of Prof. J. Forbes. (Phil. Trans. Edin. vol. xv. part 1; Trans. Brit. Ass. 1841-42.) 7 (6). The inverted spring and ratchet pendulum seismometer, proposed in 1854 by Robert F. Budge, Esq. of Valparaiso, in a letter (12th March 1854) to Mr. Patterson of Belfast, and obligingly forwarded by him to the author. Four cylindrical or square rods of spring steel, each carrying a spherical bob (an iron shot) at top, are fixed vertically. Each is provided with a ratchet, finely cut upon the rod, and a pall, the planes of motion of the four palls passing through the cardinal points, so that each spring pendulum is free to make one semioscillation only in its own direction, or that of its ratchet and pall, and be arrested there by the latter until its position of displacement be observed and it be released. Thus, in the figure (2), p W is the spring pendulum (which, it may be remarked, would be better a flat ribbon of spring steel, Fig. 2. W m the broad dimension being transverse to the are of vibration, than The main object proposed by the author 8 (b). The pendulum seismometer of Santi. Two pendula suspended close to the faces of two walls, ranging in vertical planes traversing through the cardinal points, are free to oscillate in those planes only. N Each is provided with a chalk tracer, which marks the arc of oscillation N. and S. or E. and W., or vice versa as to either, upon the prepared face of the wall. This has been long in use in Italy. The length of the horizontal chord of the are traced is assumed to be equal to the horizontal component of shock in the direction marked, and intermediate movements are to be obtained from comparison of the lengths of both cardinal chords by the known laws of compounded motions. 9 (b). A vertical inverted spring pendulum, formed of an elastic rod (wood or cane), with bobs of iron shot, is fixed within a hoop, with certain extemporaneous means of marking its oscillations in any plane, or more than one, for horizontal component. Such pendula, fixed horizontally in a wall, or in two N. and S. and E. and W. walls, may be used for vertical element, or a shot hung from a spiral spring of wire (Mallet, Admiralty Manual, sect. vii. p. 217, 218.); these were intended for extemporaneous use. The spiral spring arrangement has had several different proposers, some anterior to the above. Such are the principal instruments of the first class, used or proposed, in addition to which may be noticed the balanced circular dish, or wheelformed seismometer, suggested, I believe, by Professor J. Forbes and Col. James, R.E.,-a disk of cast-iron or other metal with a heavy rim, upon a central point of suspension slightly above the centre of gravity, and provided with a central tracing-stile, either above or below. The sensibility and power of horizontal recovery or stability of this instrument are nearly identical with those of the common balance. It is liable to all the objections that apply to pendula, whose properties in oscillation it still partakes of; and it is difficult to see any one special advantage offered by it. Of the second class, or self-registering seismometers, the number is much more limited. 1 (a). The first completely self-registering seismometer proposed, the author believes to have been that invented by himself, an account of which was read to the Royal Irish Academy in June 1846 (Trans. R. I. A., xxi. p. 107). It consists essentially of five fluid pendula,-glass tubes, partially filled with mercury, four for horizontal, and one for vertical elements of the shock. The displacement of the mercurial columns breaks contact, in an otherwise closed galvanic circuit, which, acting upon some simple contrivances, cause a pencil to trace a line upon ruled paper, whose length is proportionate to the time that contact remains broken, or to the amplitude and altitude of the earth-wave. The ruled paper, placed upon a cylinder, is maintained in motion by a clock; the position of the commencement of the pencil line traced on the moving paper, therefore, gives the moment in time, of the arrival of the wave, or initial instant of shock. The displacement of the mercurial columns is dependent upon inertia, and on the relative mass of mercury in the adjacent limbs of each bent tube. 2 (a). Professor Palmieri, of Naples, has, some time since, constructed an instrument, in point of general principle, very similar to the preceding, and which has been at work, as he informs me, with satisfactory results, at the Royal Meteorological Observatory upon Vesuvius, and for a considerable period. His instrument consists of two distinct systems, one for vertical, the other for horizontal, or rather undulatory movements. The former consists of a clock, constantly going, and registering date and time. A galvanic circuit, which includes an electro-magnet, remains always unclosed, except at the instant of the arrival of a vertical movement of the whole instrument, when one pole of copper or platinum wire, held suspended from a heavy bob at the lower end of a spiral spring--as in 9 (6), last sentenceclose over the surface of a mercurial cup (the other pole), drops by inertia, and making good the contact, establishes the electro-magnet's action, and by it stops the clock and rings a bell. The range of vertical movement is, I believe, deduced from the direct motion of this contact-maker. The system for horizontal (?) or undulatory movements consists of a similar clock and galvanic arrangement, and of four U-shaped glass tubes, open at both ends, and containing equal vertical columns of mercury, The vertical planes of two of these U-tubes are N. and S. and E. and W.; those of the other two in intermediate rhumbs. Close above, but not in contact with, the mercurial surface in one limb of each tube, is held suspended a platinum pole, the mercury itself being the other pole of the open circuit. Upon the surface of the mercury in the opposite limb a small float rests, connected by a silk cord over a pulley in a vertical plane, with a little counterpoise, slightly heavier than the float. If, now, such a movement be given to any one or more of these U-tubes as shall kant it over or throw it out of plumb, and so alter the relative levels of the opposite surfaces of mercury in the two limbs of the tube, the U-tube that shall incline towards the limb that contains the platinum galvanic pole will then make contact, and at the moment of doing so will stop the clock and ring a bell as before. The amount of displacement as to level of the two surfaces of mercury in the opposite limbs will be made observable by the distance to which the small float shall be found elevated above the surface of the mercury in the opposite limb. A description of this instrument has been given, but without a figure, in De la Rive's Treatise on Electricity and its Applications,' English edition, vol. iii. p. 508*. 3 (b). The last self-registering instrument to be noticed is that of Herr Kreil of Vienna, of which an account appeared in 1855. This ingenious and simple instrument can hardly be made intelligible more briefly than in the author's own words, which I translate (with the addition of a word or two) from the 'Sitzungsberichte der Kais. Akad. d. Wissensch.' Band. xv. p. 111, Heft for March 1855 : Fig. 3. d a C "A good seismometer is a desideratum still to be devoutly wished for. It should not only show the commencement of the stronger, but also of the weaker shocks, as well as their duration, direction, and strength, a task which is too great for a self-registering apparatus. Therefore every idea towards the improvement of such instruments must be welcome; and on this account I venture to bring forward the following design (fig. 3). Let de be a rod of wood or metal suspended at a, which at d is fastened to the elastic spring c, like the pendulum of a clock, and therefore can swing in the plane of this spring in a vertical direction. Letabbe a second spring upon the first vertical one, which permits the bar of the pendulum, de, to swing in the plane of the spring c, i.e. at right angles to the former vertical plane. The bar de and the weight fastened to it can therefore swing in every direction, without its being permitted to turn on its own axis of vertical length, and as if there were but a thread or thin wire at b. The cylinder fghi contains clockwork, which obliges it to turn round upon the bar of the pendulum f e 9 2 13 451 0 P (as its perpendicular axis fixed with reference to rotation) once in 24 hours. It is covered with paper or other material, which can be marked on without great pressure. It contains on the lower edge the numbers of the hours, which can move behind an index m, fastened to the plate kl, which is fixed to the axis of the pendulum. Upon a neighbouring pin, op, is an elastic and thin arm of brass, on, which carries a pencil at n, which, by means of a screw (spring?), can be pressed against the cylinder and removed from it. It is in firm contact with this, and marks upon it an uninterrupted line so long as the pendulum remains at rest; if, however, this begins to swing, in consequence of the whole system being shaken, this line will be broken, and strokes produced which will have a horizontal direction if the pendulum swings in the plane of no, but will be perpendicular and crossways if swinging in the plane perpendicular to no. The force and length of * Since this report was commenced, I have myself had the advantage of seeing this instrument, and conversing with its distinguished inventor, as to its principles and construction. Prof. Palmieri informed me that it had been arrested by the celebrated shock of 16th December 1857, and had given indications that he deemed satisfactory. [R. M., May 1858.] |