The Eruption of Vesuvius in 1872 Part 2

5. Seismic energy _may_ become sensible at any point of the earth's surface, its efforts being, however, greater and more frequent as the great lines of elevation and of volcanic activity are approached; yet not in the inverse ratio of distance, for many of the most frequently and terribly shaken regions of the earth, as the east sh.o.r.e of the Adriatic, Syria, Asia Minor, Northern India, etc., are at great distances from active Volcanoes.

6. The surfaces of minimum or of no known disturbance are the central areas of great oceanic or of terra-oceanic basins or saucers, and the greater islands existing in shallow seas.

s.p.a.ce obliges me to pa.s.s unnoticed here many minor but not unimportant deductions. The discussions as to distribution in time and s.p.a.ce occupy seventy-two pages of this fourth and last Report, the remainder of which (thirty-one pages) embraces the description and mathematical discussion as to seismometers, to which I may refer, as comprising the most complete account of these instruments that has, I believe, been anywhere given.

The appendix to the Report comprises the entire bibliography of Earthquakes collected during those researches, and a concluding chapter on desiderata, and inquiries as to ill-understood phenomena supposed to be connected with Earthquakes.

In 1849-50, I was honoured by the request to draw up the article "Earthquake Phenomena," which has appeared in the first and subsequent editions of the "Admiralty Manual of Scientific Inquiry." Originally the subject was intended to have formed part of the article on Geology, entrusted to Mr. Darwin, who consulted me upon the subject; and upon my representing how much Earthquakes had, within a short time, become matter for the mathematician and physicist, he, with a singleness of eye to science which it is but just to place on record, took the necessary steps with the Admiralty authorities that Earthquakes should form a separate article, and advised its being placed, as it was, in my hands.

To record this will, I believe, be sufficient justification for my reference to this article, in which a good deal of information as to Seismometry is to be found.

By recurring to Mr. Hopkins's Report on Earthquake Theory, before remarked upon ("Report of British a.s.sociation, 1847"), it will be seen that the solutions of the problems which he there gives for finding the depth of focus of shock are founded upon the _velocity of propagation_ of the wave in the interior of the ma.s.s, the _apparent horizontal velocity_ and the _horizontal direction of propagation_ at any proposed point being known (p. 82).

By this it appears plainly that at that time Mr. Hopkins supposed that it was the _velocity of translation_ of the wave of shock that did the mischief, and not the _velocity of the wave particle_, or wave itself.

And, further, that the former might be obtained by reference simply to the modulus of elasticity of the rock of any given formation, as, indeed, was my own earliest view when I produced my "Dynamics of Earthquake" in 1846. From the remarks already made as to the vast difference between the actual transit velocity in more or less discontinuous rocks--such as they occur in Nature--it will be equally obvious that Mr. Hopkins's methods, as above mentioned, are impracticable, even were there no confusion between the velocity of translation of the wave and that of the wave particle or wave itself.

This applies also to the demonstration and diagram (taken from Hopkins) given by Professor Phillips ("Vesuvius," pp. 258-259).

In December, 1857, occurred the great Neapolitan Earthquake, which desolated a large portion of that kingdom; and an opportunity then arose for practically applying to the problems of finding the directions of earthquake shock at a given point through which it has pa.s.sed, and ultimately the position and depth of focus, other methods, which I had seen, from soon after the date of publication of my original Paper (1846), were easily practicable, and the details of which I had gradually matured.

Bearing in mind that, in the case of the normal vibration in any elastic solid of indefinite dimensions, the direction of motion in s.p.a.ce of the _wave particle_ coincides in the first semiphase of the wave, and at the instant of its _maximum velocity_ with the right line joining the particle and the focus or centre of disturbance, it follows that, in the case of earthquakes, the normal vibration of the wave of shock is always in a vertical plane pa.s.sing through the focus and any point on the earth's surface through which the shock pa.s.ses (a.s.suming for the present no disturbing causes after the impulse has been given), and that at such a point the movement of the wave particle in the first semiphase of the wave is in the same direction or sense as that of translation; and at the moment of maximum velocity the direction in s.p.a.ce of the motion of the wave particle is that of the right line joining the point through which the wave has pa.s.sed with the focus or centre of impulse.

If, therefore, we can determine the direction of motion of the wave particle in the first semiphase, and its maximum velocity, we can obtain, from any selected point, a line (that of emergence of the shock) _somewhere in which_, if prolonged beneath the earth, the focus must have been; and if we can obtain like results for two or more selected points, we decide the position and the depth of the focus, which must be in the intersection of the several lines of direction of the wave particle motion at each point, when prolonged downwards.

Now, as I have said, it is the _vibration of the wave itself_, _i.e._, the motion of the wave particle that does the mischief--_not_ the transit of the wave from place to place on the surface; just as in the a.n.a.logous (but _not_ similar) case of a tidal wave of translation running up an estuary and pa.s.sing a s.h.i.+p anch.o.r.ed there, it is not the transit up the channel, but the wave form itself--_i.e._, the motion of the wave particles--that lifts the s.h.i.+p, sends her a little way higher up channel, drops her to her former level, and sends her down channel again to the spot she lay in just before the arrival of the wave.

Everything, therefore, that has been permanently disturbed by an earthquake shock has been thus moved in the direction and with the maximum velocity impressed upon it by the wave particle in the first semiphase of the wave; and thus almost everything that has been so disturbed may, by the application of established dynamical principles, be made to give us more or less information as to the velocity of the wave particle (or as we, for shortness, say, the velocity of shock), the direction of its normal vibration, and the position and depth beneath the earth's surface, from which came the generating impulse. We thus arrive at these as simply and as surely as we can infer from the position taken by a billiard ball, on which certain forces are known to have acted, the forces themselves and their direction; or, from a broken beam, the pressure or the blow which fractured it.

It is obvious, then, that nearly every object disturbed, dislocated, fractured or overthrown by an earthquake shock is a sort of natural seismometer, and the best and surest of all seismometers, if we only make a judicious choice of the objects which being found after such a shock, we shall employ for our purpose. This was the principle which I proposed to the Royal Society at once to apply to the effects of the then quite recent great Neapolitan Earthquake of 1857, and which, through the liberality and aid of that body, I was enabled to employ with the result I had pretty confidently antic.i.p.ated, namely, the ascertainment of the approximate depth of the focus.

_Every_ shock-disturbed object in an earthquake-shaken country is capable of giving _some_ information as to the shock that acted upon it; but it needs a careful choice, and some mechanical ????, to select _proper_ and the best objects, so as to avoid the needless perplexity of disturbing forces _not_ proper to the shock, or other complications.

When properly chosen, these natural seismometers, or evidences fitted for observation after the shock, are of two great cla.s.ses, by which the conditions of the earthquake motion are discoverable:

1. Fractures or dislocations (chiefly in the masonry of buildings), which afford two princ.i.p.al sources and sorts of information, namely:

_a._ From the observed _directions of fractures or fissures_, by which the _wave path_, and frequently the _angle of emergence_, may be immediately inferred.

_b._ Information from the preceding, united with known conditions as to the strength of materials to resist _fracture_, by which the _velocity_ of the fracturing impulse may be calculated.

2. The overthrow or the projection, or both, of bodies large or small, simple or complex. From these we are enabled to infer:

_c._ By direct observation, the _direction in azimuth_ of the wave path.

_d._ By measurements of the horizontal and vertical distances of overthrow or of projection, to infer either the _velocity_ of projection, or _angle of emergence_.

Fractures by shock present their planes always nearly in directions transverse to the wave path. Projections or overthrow take place (unless secondarily disturbed) in the line of the wave path, or in the vertical plane pa.s.sing through it: but the direction of fall or overthrow may be either in the same direction as the wave transit (_i.e._, as the motion of the wave particle in the first semiphase), or contrary to it.

It is thus obvious that the princ.i.p.al phenomena presented by the effects of earthquake shock upon the objects usually occurring upon the surface of the inhabited parts of the earth, resolve themselves into problems of three orders, and are all amenable to mechanical treatment, viz.:

1. Problems relating to the direction and amount of velocity producing fracture or fissures.

2. Problems relating to the single or multiplied oscillations of bodies, considered as compound pendulums.

3. Problems referable to the theory of projectiles.

These three may combine in several cases, and on the part of the observer must combine with measurements, angular and linear, and with geodetic operations to be conducted in the shaken country.

The methods of application in detail are described fully, as well as their actual application and results, in my work published in 1862 (2 vols.), ent.i.tled "The First Principles of Observational Seismology, as developed in the Report to the Royal Society of London of the Expedition made by Command of the Society into the Interior of the Kingdom of Naples, to investigate the Circ.u.mstances of the Great Earthquake of December, 1857," to the many ill.u.s.trations of which the pecuniary grant, in aid, of 300 was most liberally made to the publishers (Messrs.

Chapman and Hall) by the Society.

It is not my intention here, nor would s.p.a.ce allow, of my going into the details of observation, nor of the deductions and conclusions I have recorded in those volumes. I have referred to their contents as marking the advent of a new method. I have ventured to call it a new _organon_ in the investigation of Earthquakes, and, through them, of the deep interior of our earth; and will only add that the method, on this its very first trial, proved fertile and successful. The depth of focus for this shock of December, 1857, was about seven to eight geographical miles below sea level, roughly stated. It gives me great pleasure to add that my friend, Dr. Oldham, Director-General of the Geological Survey of India, has since applied these same methods to the phenomena of the great Cachar Earthquake of the 10th January, 1869, and with success. The pressure of official duties has, he informs me, as yet prevented his fully working out his results, but they appear so far to indicate, as we should expect, a depth of focus or origin considerably greater than in the European case of 1857. Some account of Dr. Oldham's results were this year communicated to the Geological Society of London through myself, they are of great interest and importance.

Such, briefly and imperfectly sketched, is the existing state of Seismology. As a branch of exact science it is, as it were, an affair of yesterday. It is with reluctance that I have been compelled, in this review, to refer to my own work so prominently. The harvest has been and still is plenteous, but in this field of intellectual work the labourers are few. This must continue to be so as long as Geology shall continue to be viewed in public estimation (in England at least) as a fas.h.i.+onable toy, that everyone who has been to school is supposed capable of handling; and until all who profess to be geologists shall have learnt that, to make sound progress, they must first become mathematicians, physicists and chemists.

It is to the general imperfect knowledge of these sciences amongst geologists that speculative errors show such vitality, and that Geology makes such poor progress towards becoming the interpretation of the world as a machine (_Erdkunde_).

It is for the same reason that Seismology and Vulcanology make little progress; the first cannot be pursued beyond its present boundaries, nor can even its present position be understood or explained by anyone unfamiliar with the laws of wave motion, of all cla.s.ses of waves; and it would be easy to show, by quoting from various British or foreign text-books on Geology, how extremely imperfect is the grasp of some of the authors upon the subject of earthquake-wave motion, even such as they admit and endeavour to explain and apply: in fact, many geologists appear never to have framed to themselves any clear idea of what _is_ a wave of any sort, liquid or elastic. The general silence as to seismic theory of French geological writers is remarkable, to whatever cause attributable. It has been said that French philosophers show themselves little disposed to acknowledge or to follow the lead of their foreign compeers in any branch of science. If this be true, or in so far as it may be so, it is unworthy of French science, which has such boundless claims upon our homage. I am disposed to attribute the fact in this case to other circ.u.mstances; and, amongst these, to the small extent to which our language is known amongst French scientific men.

Germany has shown more desire to cultivate this branch of science.

Although, as yet, the distinct enunciation of its fundamental principles has but spa.r.s.ely found its way into her text-books, several able monographs, such as those of Schmidt and of Hottinger, prove how completely some of her philosophers have mastered and how well applied them. The men of science of Northern Italy, amongst whom so many glorious names are to be found on the roll of discovery, have shown themselves quite alive to the importance of Seismology; and I know of no more clear, exact and popular exposition of its principles and application, and of its cosmical relations, than is to be found in a small volume by Professor Gerolamo Boccardo, published at Genoa in 1869, ent.i.tled _Sismopirologia Terremoti, Vulcani e lente oscillazione del suolo, saggio di una teoria di Geographia Fisica_.

My object, so far, has been to mark the progress of ascertained theoretic notions as to Seismology. I have, therefore, pa.s.sed without notice many speculative monographs, and the treatment upon Earthquakes, whether speculative or historical, and however able, that const.i.tutes a prominent feature of nearly all systematic works on Geology.

That which may be at present viewed as achieved and certainly ascertained in theoretic Seismology is the clear conception of the nature of earthquake motion; the relations to it of great sea or other water wave commotions; the relations to it of sound waves--as to which, however, more remains to be known; and the relations of all these to secondary effects, tending in various ways to modify more or less the topographic and other conditions of the land or sea bottom. And in descriptive Seismology the present distribution of the earthquake bands or regions of greatest seismic prevalence and activity are tolerably ascertained, and their connection with volcanic lines and those of elevation rendered more evident. Viewed alone, nothing can yet be said to be absolutely ascertained as to the immediately antecedent cause or causes of the impulse. The function of Earthquake, as part of the cosmical machine, has become more clear, as the distinctive boundaries between Earthquake and permanent elevation of the earth have been made evident; and it has been seen that Earthquake, however contemporaneous occasionally with permanent elevation, is not the cause, though it may be one of the consequences of the same forces which produce elevation; and thus, that an infinite number of Earthquakes, however violent, and acting through however prolonged a time, can never act as an agent of permanent elevation, unless, indeed, on that minute scale in which surface elevation may arise from secondary effects, like that of the Ullah Bund.

Much remains to be done, and much may be expected even from the continuation, if done in a systematic and organised manner, of the statistic record of Earthquakes in connection with those other branches of cosmical statistics, Climatology, Meteorology, Terrestrial Magnetism, etc., the observation of which is already, to a certain extent, organised over a large portion of the globe.

And now let us look back for a moment to ask, How, by what mental path of discovery, have we arrived at what we have pa.s.sed in review?

The facts of Earthquakes have been before men for unknown ages "open secrets," as Nature's facts have been well called; "but eyes had they and saw not." Facts viewed through the haze of superst.i.tion, or of foregone notions of what Nature _ought_ to do, cease to be facts. When, after the great Calabrian Earthquake of 1783, the Royal Academy of Naples sent forth its commission of its learned members to examine into the effects, they had spread around them in sad profusion all that was necessary to have enabled them to arrive at a true notion of the nature of the shock, and thence a sound explanation of the varied and great secondary effects they witnessed, and of which they have left us the records in their Report, and the engravings ill.u.s.trative of it. But we look in vain for any light; the things seen, often with distortion or exaggeration, are heaped together as in the phantasmagoria of a wild and terrible dream, from which neither order nor conclusion follow.

Why was this? Why were these eminent _savants_ no more successful in explaining what they saw than the ignorant peasants they found in the Calabrian mountains?

Because physical science itself was not sufficiently advanced, no doubt; but also because they had no notion of applying such science as they had, to the very central point itself of the main problem before them, freed from all possible advent.i.tious conditions, and so, as it were, attacking it in the rear. How different might have been the result of their labours, had they begun by asking themselves, What is an earthquake? Can we not try to find out what it _is_ by observing and _measuring_ what it has done? We see the converse mode of dealing with Nature in Torricelli. "Nature abhors a vacuum," was told him, as the wisdom of his day. Possibly: but her abhorrence is limited, for I find it is _measured_ by the pressure of a column of water of thirty-four feet in height. We need not pursue the story with Pascal, up to the top of the Puy de Dome.

This lesson is instructive generally to all investigators, and particularly here; for Vulcanology, to which we are about now to turn, has occupied until almost to-day much the same position that Seismology did in those of the Neapolitan Commissioners.

Whole libraries have been written with respect to it dealing with _quality_, but _measure_ and _quant.i.ty_ remain to be applied to it.

To a very preponderant cla.s.s in the civilised world no knowledge is of much interest or value that does not point to what is called a "practical result," one measurable into utility or coin. I do not stop to remark as to the bad or as to certain good results of this tendency of mind; but I may venture to point out to all, that the exact knowledge of the nature of earthquake motion, even during the short time that it has become known, has not been barren in results absolutely practical and utilitarian. The minute investigation of the destruction of buildings, etc., and the deductions that have been made as to the relations between the form, height, materials, methods of building, combination of timber and of masonry, and many other architectural or constructive conditions, have made it certain now that earthquake-proof houses and other edifices can be constructed with facility, and at no great increase, if any at all, of cost. I can affirm that there is no physical necessity why in frequently and violently shaken countries, such as Southern Italy or the Oriental end generally of the Mediterranean, victims should hereafter continue by thousands to be sacrificed by the fall of their ill-designed and badly built houses.

Were a "Building Act" properly framed, put in force by the Italian Government in the Basilicatas and Capitanata, etc., so that new houses or existing ones, when rebuilt, should be so in accordance with certain simple rules, a not very distant time can be foreseen when Earthquakes, pa.s.sing through these rich and fertile but now frequently sorely afflicted regions, should come and go, having left but little trace of ruin or death behind. Some disasters there must always be, for we cannot make the flanks of mountains, nor the beds of torrents, etc., always secure; but the main mortality of all Earthquakes is in the houses or other inhabited buildings. Make these proof, and the wholesale slaughter is at an end.

The principles we have established have been thus practically applied in another direction. The j.a.panese Government, with the keen and rapid perception of the powers inherent in European science which characterises now that wonderful people, has commenced to illuminate its coasts by lighthouses constructed after the best European models. But j.a.pan is greatly convulsed by earthquakes, and lighthouses, as being lofty buildings, are peculiarly liable to be destroyed by them.