Part 3

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 analogous (but _not_ similar) case of a tidal wave of translation running up an estuary and passing a ship anchored 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 ship, 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 anticipated, 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 [Greek: nous], 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 classes, by which the conditions of the earthquake motion are discoverable:

1. Fractures or dislocations (chiefly in the masonry of buildings), which afford two principal 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 passing 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 principal 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.), entitled "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 Circumstances of the Great Earthquake of December, 1857," to the many illustrations 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 space 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 fashionable 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 classes 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 circumstances; 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 sparsely found its way into her text-books, several able monographs, such as those of Schmidt and of Höttinger, 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, entitled _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, passed without notice many speculative monographs, and the treatment upon Earthquakes, whether speculative or historical, and however able, that constitutes 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 passed 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 superstition, 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 illustrative 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 adventitious 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 Dôme.

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 _quantity_ remain to be applied to it.

To a very preponderant class 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, passing 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 Japanese 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 Japan is greatly convulsed by earthquakes, and lighthouses, as being lofty buildings, are peculiarly liable to be destroyed by them.

The engineer of the Japanese Government for these lights, Mr. Thomas Stevenson, C.E. (one of the engineers to the Commissioners of Northern Lights), was instructed to have regard, in the design of those lighthouses, to their exposure to shock. I was consulted by Mr. Stevenson as to the general principles to be observed; and those edifices have been constructed so that they are presumedly proof against the most violent shocks likely to visit Japan; not, perhaps, upon the best possible plan, but upon such as is truly based upon the principles I have developed. Mr. Stevenson has published some account of their construction.

The earthquake regions of South America might with incalculable benefit apply those ideas; and, indeed, they have been, to some extent, already applied by my friend, Mr. William Lloyd, Member of the Institution of Civil Engineers, to the New Custom Houses constructed from his designs at Valparaiso.

As one of these utilitarian views, and an important one, it will occur to many to ask--Can the moment of the occurrence or the degree of intensity of earthquake shock be predicted, or is it probable that at a future day we may be able to predict them? At present, any prediction, either of the one or the other, is impossible; and those few who have professed themselves in possession of sufficient grounds for such prediction are deceivers or deceived. Nor is it likely that, for very many years to come, if ever, science shall have advanced so as to render any such prediction possible; but it is neither impossible nor improbable that the time shall arrive when, within certain, perhaps wide, limits as to space, previous time, and instant of occurrence, such forewarnings may be obtainable.

Earthquakes, like storms and tempests, and nearly all changes of weather, are not periodic phenomena, nor yet absolutely uncertain or, so to say, accidental as to recurrence.

They are quasi-periodic, that is to say, some of their conditions as to causation rest upon a really periodic basis, as, for example, the recurrence of storms upon the periodic march of the earth, and sun and moon, etc., and the recurrence of Earthquakes upon the secular cooling of our earth; but the conditions in both are so numerous and complicated with particulars, that we cannot fully analyse them--hence, cannot reduce the phenomena to law, and so cannot predict recurrence. Yet storms and tempests--which were, along with pestilences and Earthquakes, amongst the natural phenomena which Bishop Butler deemed in his own day impossible of human prediction--have already, through the persistent and systematised efforts of meteorological observers, become to a certain extent foreseeable; and medical science assures us that it has rendered that, though to a much less degree of probability, true of pestilences.

We may, therefore, give the utilitarian some hope, that if he will help us along--who value our accessions of knowledge primarily upon a different standard to his--in our talk of discovery, our posterity, in a century or two hence, may not improbably possess the advantage of being able, in some degree, to predict their Earthquakes. I fear the inducement will go but a small way with the utilitarian generation, whose bent tends much towards asking, "What has posterity ever done for them?"

But though we cannot as yet predict the time when an Earthquake may take place in any locality, we can, on mixed statistic and dynamic grounds, in many cases state the limits of probable violence of the next that may recur. For example, the three shafts of marble columns of the Temple of Serapis, at Pozzuoli, each of about 41-1/2 feet in height, and 4 feet 10 inches in diameter at the base, remain standing alone, since they were uncovered, in the year 1750.

Now, as we can calculate exactly what velocity of earthquake-wave motion would be required to overset these, we are certain that, during the last one hundred and twenty-two years, the site of the Temple, and we may say Naples and the Phlegræan fields generally, have never experienced a shock as great as the very moderate one that would overset these columns. A shock whose wave particle had a horizontal velocity of only about 3-1/2 feet (British) per second would overturn these columns; which is only about one-fourth the velocity (within the meizoseismic area) of the great shock of 1857, that produced wide-spread destruction in the Basilicatas, and not enough to throw down any reasonably well-built house of moderate height.

Naples, so far as Earthquake is concerned, whether coming from the throes of Vesuvius or elsewhere, has a pretty good chance of safety. She may possibly (though not probably) be some day smothered in ashes; but is in little danger of being shaken to the earth. During this time there have been taking place, larger eruptions of Vesuvius and earthquake shocks from other centres, together probably about the same number of times as the numbers of those years, when those columns have been more or less shaken.

We may therefore affirm that the probability (on the basis of this experience _only_) is, say 120 to 1, that the next shock, whether derived from Vesuvius, or elsewhere, that may shake Pozzuoli, will be one less in power than would be needed to overturn the shafts of the Temple of Serapis there.

* * * * *

Let us now turn to the second branch of our subject--viz., Vulcanology--upon which, as yet, we have secured less firm standing ground than we have seen we possess in Seismology, for which reason we took that first into consideration.

It is the part of Vulcanology to co-ordinate and explain all the phenomena of past or present times visible on our globe which are evidences of the existence and action, whether local or general, of temperatures within our globe greatly in excess of those of the surface, and which reach the fusing points of various mineral compounds as found arriving, heated or fused, at the surface.