Part 2

And some indication of the general truth of the fact was derivable from comparing the rude previous approximations to the transit rate of some great Earthquakes. In the case of that of Lisbon, estimated by Mitchell at 1,760 feet per second. It was still desirable to extend similar experiments to the harder classes of stratified and of contorted rocks. This I was enabled to carry into effect, at the great Quarries at Holyhead (whence the slate and quartz rocks have been obtained for the construction of the Asylum Harbour there), taking advantage of the impulses generated at that period by the great mines of powder exploded in these rocks.

The results have been published in the "Philosophical Transactions for 1861 and 1862 (Appendix)." They show that the mean lowest rate of wave transit in those rocks, through measured ranges of from 5,038 to 6,582 feet, was 1,089 feet per second; and the mean highest, 1,352 feet per second; and the general mean 1,320 feet per second.

By a separate train of experiments on the compressibility of solid cubes of these rocks, I obtained the mean modulus of elasticity of the material when perfectly continuous and unshattered, with this remarkable result--that in these rocks, as they exist at Holyhead, _nearly seven-eighths of the full velocity of wave transmission due to the material, if solid and continuous, is lost by reason of the heterogeneity and discontinuity_ of the rocky masses as they are found piled together in Nature.

I also proved that the wave-transit period of the unshattered material of these rocks was greatest in a direction _transverse_ to the bedding, and least in line parallel with that; but the effect of this in the rocky mass itself may be _more_ than counterbalanced by the discontinuity and imperfect contact of the adjacent beds.

These results indicate, therefore, that the superficial rate of translation of the solitary sea-wave of earthquakes may, when over very deep water, equal or even exceed the transit rate (in some cases) of the elastic wave of shock itself.

These results have since received general confirmation by the careful determinations of the transit rates of actual earthquake waves, in the rocks of the Rhine Country and in Hungary, by Nöggerath and Schmidt respectively, and by those made since by myself in those of Southern Italy, to which I shall again refer. In an elastic wave propagated from a centre of impulse in an infinitely extended volume of a perfect gas, normal vibrations are alone propagated--as is the case with sound in air.

In the case of like movements propagated in elastic and perfectly homogeneous and isotropic solids, the wave possesses both normal and transversal vibrations, and is, in so far, analogous to the case of light. Mr. Hopkins, in his Report above referred to, has based certain speculations upon the assumed necessary co-existence of both orders of vibration in actual earthquake shocks in the materials of which our earthy crust is actually composed.

The existence of transversal vibration in those materials has not been yet proved experimentally, though there is sufficient ground to preclude our denying their probable existence.

That if they do exist they play but a very subordinate part in the observable phenomena of actual Earthquake is highly probable. This is the view, supported not only by observations of the effects of such shocks in Nature, but by the theoretic consideration of the effects of discontinuity of formations in planes or beds more or less transverse to the wave path (or line joining the centre of impulse with the mean centre of wave disturbance at any point of its transit). If we suppose, for illustration sake, such an elastic wave transmitted perpendicularly through a mass of glass plates, each indefinitely thin, and all in absolute contact with each other, but without adhesion or friction, more or less of the transversal vibration of the wave would be cut off and lost at each transit from plate to plate, as the elastic compression can, by the conditions, be transmitted only normally or by direct push perpendicularly from plate to plate. This must take place in Nature, and to a very great extent, and the consideration, with others, enabled me generally to apply the normal wave motion of shock alone to my investigation as to the depth of the centre of impulse of the great Neapolitan Earthquake of 1857, an account of which was published in 1862, and to be presently further referred to.

Hitherto the multitudinous facts, or supposed facts, recorded in numberless accounts of Earthquakes had remained almost wholly unclassified, and so far as they had been discussed--in a very partial manner, as incidental portions of geological treatises--with little attempt to sift the fabulous from the real, or to connect the phenomena admitted by reference to any general mechanical or physical causes. In 1850 my first "Report upon the Facts of Earthquakes," called for by the British Association in 1847, was read and published in the Reports of that body for that year. In this, for the first time, the many recorded phenomena of Earthquakes are classified, and the important division of the phenomena into primary and secondary effects of the shock was established. Several facts or phenomena, previously held as marvellous or inexplicable, were either, on sufficient grounds, rejected, or were, for the first time, shown susceptible of explanation. Amongst the more noticeable results were the pointing out that fissures and fractures of rock or of incoherent formations were but secondary effects, and, in the latter, were, in fact, generally of the nature of inceptive landslips. This last was not accepted, I believe, by geologists at the time; but the correctness of the views then propounded as to earth fissures--the nature of the spouting from them of water or mud--the appearances taken for smoke issuing from them, etc.--have since been fully confirmed, first, by my own observations upon the effects of the Great Neapolitan Earthquake of 1857, and more lately by those of Dr. Oldham upon the Earthquake of Cachar (India), where he was enabled to observe fissures of immense magnitude, the nature of the production of which he has well described and explained in the "Proceedings, Geological Society, London, 1872."

The relations between meteorological phenomena proper and Earthquakes have always been a subject of popular belief and superstition.

This was here carefully discussed, and with the result of disproving any connection, or, if any, but of an indirect nature. I also, to some extent, towards the end of this Report, discussed the question of the possible nature of the _impulse itself_ which originates the shock; I showed that it must be of the nature of a blow, and ventured to offer _conjecturally_ five possible causes of the impulse:

1. Sudden fractures of rock, resulting from the steady and slow increase of elevatory pressure.

2. Sudden evolution (under special conditions) of steam.

3. Sudden condensation of steam, also under special conditions.

4. Sudden dislocations in the rocky crust of the earth, through pressure acting in any direction.

5. Occasionally through the recoil due to explosive effects at volcanic foci (p. 79-80).

The first and last of these I am, through subsequent light, disposed now to withdraw or greatly to modify.

The first, the supposed "_snap and jar_, occasioned by the sudden and violent rupture of solid rock masses," to which Mr. Scrope, in his very admirable work on Volcanoes, is disposed to refer the impulse of earthquake shocks (Scrope, 2nd edit., p. 294), I believe may be proved on acknowledged physical principles--when applied to the known elasticities and extensibilities of rocks, and keeping in view the small thicknesses fractured _at the same instant_--to be capable of only the most insignificant impulsive effects; and if we also take into consideration that strata, if so fractured, are necessarily not _free_, but surrounded by others above and below, any such impulsive effect emanating from fracture may be held as non-existent or impossible. In the statement of his views which follows, and in objecting to my second and third possible causes (p. 295-296, headed "Objections to Mallet's Theory"), Mr. Scrope appears to me to have fallen into the error of assuming that the nature of the _impulse_, or the cause producing it, forms any part of "my theory of earthquake movement," or in anywise affects it. I carefully guarded against this in the original Paper ("Transactions, Royal Irish Academy," Vol. XXI., p. 60, and again, p. 97), when I stated "it is quite immaterial to the truth of my theory of earthquake motion what view be adopted, or what mechanism be assigned, to account for the original impulse."

As regards the fifth conjecture suggested by me, I am now, with better knowledge and larger observation of volcanic phenomena, not prepared to admit any single explosion at volcanic vents of a magnitude sufficient to produce by its recoil an earthquake wave of any importance, or extending to any great distance in the earth's crust. The rock of 200 tons weight, said to have been projected nine miles from the crater of Cotopaxi, which I quoted from Humboldt as an example,[E] I believe to be as purely mythical as the rock (_bloc rejetté_) of perhaps one-sixth of that weight which, previous to the late eruption, lay in the middle of the Atria dell Cavallo, and which it was roundly affirmed had been _blown_ out of the crater, but which in reality had at some time rolled down from near the top of the cone, after having been dislodged from some part of the upper lip of the crater walls, where, as its wonderful hardness and texture and its enamel-like surface showed, it had been roasted for years probably.

Nor do I believe in the _sudden_ blowing away of one-half the crater and cone of Vesuvius, or of any other volcano, at one effort, however affirmed.

Nothing more than conjecture as to the nature of the impulse producing great or small Earthquakes can, I believe, as yet be produced. That there is some one master mechanism productive of most of the impulses of great shocks is highly probable, but that more causes than one may produce these impulses, and that the causes operative in small and long repeated shocks, like those of Visp-Comrie and East Haddam, differ much from those producing great Earthquakes, is almost certain.

We shall be better prepared to assign all of these when we have admitted a true theory of volcanic action, and so are better able to see the intimate relations in mechanism between seismic and volcanic actions.

It is not difficult meanwhile to assign the very probable mechanism of those comparatively petty repercussions which are experienced in close proximity to volcanic vents when in eruption, and which, though certainly seismic in their nature, and powerful enough, as upon the flanks of Etna, to crack and fissure well-built church-towers, can scarcely be termed Earthquakes.

In my First Report I stated that almost nothing was known then of the distribution of recorded Earthquakes in time or in space over our globe's surface, and I proposed the formation and discussion of a complete catalogue of all recorded Earthquakes, with this in view.

This was approved by the Council of the British Association and at once undertaken by me, with the zealous and efficient co-operation of my eldest son, Dr. J. W. Mallet. Nearly the whole of the Second British Association Report, of 1851, is occupied with the account of the experiments as to the transit rate of artificially made shocks in sand and granite, as already referred to.

The Third Report, of 1852-1854, contains the whole of this, "The Earthquake Catalogue of the British Association" (of which, through the liberality of that body, more than one hundred copies were distributed freely), in which are given, in columnar form, the following particulars, from the earliest known dates to the end of 1842:

1. The date and time of day, as nearly as recorded.

2. The locality or place of occurrence.

3. The direction, duration, and number of shocks so far recorded.

4. Phenomena connected with the sea--great sea-waves, tides, etc.

5. Phenomena connected with the land--meteorological phenomena preceding and succeeding. Secondary phenomena--all minor or remarkable phenomena recorded.

6. The authority for the record.

Though most materially assisted by the previous labours and partial catalogues of Von Hoff, Cotte, Hoffman, Merrian, and, above all, of Perrey, the preparation of this catalogue--which demanded visits to the chief libraries of Europe, and the collating of some thousands of authors in various languages and of all time--was a work of great and sustained labour, which, except for my dear son's help, I should never have found time and power to complete. Professor Perrey, formerly of the Faculté des Sciences of Dijon, now _en retrait_, who has devoted a long and useful life to assiduous labours in connection with Seismology, was our great ally; and his catalogues are so large and complete for most known parts of the world after 1842, that we were able to arrest our own catalogue at that date, and take M. Perrey's as their continuation up to 1850.

The whole British Association Catalogue thus embraces the long historic period of from 1606 B.C. of vulgar chronology, when the first known Earthquake is recorded, to A.D. 1850; and the base of induction which it presents as to the facts recorded extends to between 6,000 and 7,000 separate Earthquakes. My Fourth Report ("Reports, British Association, 1858,") is occupied principally with the discussion of this great catalogue, and with that of several special catalogues produced by other authors with limited areas or objects.

The discussion of M. Perrey's local catalogues with those of others, in reference to a supposed prevalent apparent horizontal direction of shock in certain regions--as to distribution, as to season, months, time of day or night, relation to state of tide--the bearings of the views of Zantedeschi and others as to the probable existence of a terrane tide--the supposed relations of the occurrence of Earthquakes upon the age of the moon, as deduced by Perrey, viz.: that 1st, Earthquakes occur most frequently at the syzygies; 2nd, that their frequency increases at the perigee and diminishes at the apogee; 3rd, that they are more frequent when the moon is on the meridian than when she is 90° away from it--and the views of several authorities as to the distribution of Earthquakes in time and in space--occupy the first 46 pages of this Report.

It then proceeds to discuss the distribution in time and in space as deduced from the full base of the great catalogue.

The results as to time are reduced to curves, and those as to space (or distribution over our globe's surface) to the great seismic map (Mercator's projection), upon which and in accordance with certain principles and conventional laws, which admit of the indication of both intensity and frequency, all recorded Earthquakes have been so laid down as to present a real indication of the distribution of seismic energy for the whole historic period and all over the world.

The original of this map, which also shows the Volcano (size, about 7 feet by 5 feet), remains for reference in the custody of the Royal Society. A reduced copy was published with the Report, and to a still more reduced scale has been reproduced in other places. It is impossible here to do more than refer to a few of the more salient points.

As regards distribution in time, durational seismic energy may be considered as probably constant during historic time, though it is probably a decaying energy viewed in reference to much longer periods. It does not appear of the nature of a distinctly periodic force.

1. Whilst the minimum paroxysmal interval may be a year or two, the average interval is from five to ten years of comparative repose.

2. The shorter intervals are in connection with periods of fewer Earthquakes, not always with those of least intensity, but usually so.

3. The alternations of paroxysm and of repose appear to follow no absolute law deducible from these causes.

4. Two marked periods of extreme paroxysm are observable in each century (for the last three centuries), one greater than the other--that of greatest number and intensity occurring about the middle of each century, and the other towards the end of each.

As respects season, there appear distinct indications of a maximum about the winter solstice, and equally so of a minimum rather before the autumnal equinox. It is not improbable that there is a remote relation between Earthquakes and the annual march of barometric pressure.

We may expect, at present, one great Earthquake about every eight months, and were we possessed of a sufficient report from all parts of our globe, we should probably find scarcely a day pass without a very sensible Earthquake occurring somewhere, whilst, as regards still smaller tremors, it might almost be said that our globe, as a whole, is scarcely ever free from them.

As respects the distribution of seismic energy in space of our earth's surface, it is that of bands of variable and of great breadth, with sensible seismic influence extending to from 5° to 15° transversely, which very generally follow:

1. The lines of elevated tracts which mark and divide the great oceanic or terra-oceanic basins (or _saucers_, as I have called them, from their shallowness in relation to surface, in this discussion) of the earth's surface.

2. And in so far as these are frequently the lines of mountain chains, and these latter those of volcanic vents, so the seismic bands are found to follow these likewise. Isolated Volcanoes are found in these bands also.

3. While sensible seismic influence is generally limited to the average width of the band, paroxysmal efforts are occasionally propagated to great distances transversely beyond that.

4. The sensible width of the band depends upon the energy developed at each point of the length, and upon the accidental geologic and topographic conditions along the same.

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 shore 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.

Space obliges me to pass unnoticed here many minor but not unimportant deductions. The discussions as to distribution in time and space 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 Association, 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 mass, 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 passed, 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 space 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 passing through the focus and any point on the earth's surface through which the shock passes (assuming 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 space of the motion of the wave particle is that of the right line joining the point through which the wave has passed with the focus or centre of impulse.