Seismology was established at Tokyo: a network of $58 stations for the registration of "quakes” of all sorts and degrees of intensity was spread over the country, and a seismic survey carried out with their aid. The animating spirit of the enterprise was Professor John Milne, whose expert services were, during twenty years, at the disposal of the Japanese Government. Amongst a crowd of able cooperators, he stands out as the veritable originator of the new seismology. Its basis was provided by his catalogue of 8,300 earthquakes, observed sorunium artem in Japan between 1885 and 1892, and finally enumerated each with a separate map showing its regional limits and central point. The distribution of seismic activity, as the upshot of seven years' sustained vigilance, became thas more thoroughly known in the Mikado's kingdom than over any other equal area of the world's surface. The performance, however, would have been impossible without instrumental appliances for automatically recording the complex tremors constituting a seismic disturbance, and the need was met by the invention of the seismograph. Essentially, this ingenious little machine was of Anglo-Japanese origin; it took form in the hands of Ewing, Milne, and Gray. Valuable, too, were the pioneering experiments of Dr. Wagener, as well as the refinements of construction subsequently introduced by Professors Sekiya and Omori of Tokyo, by Drs. Agamennone and Cancani, Vicentini and Grablowitz in Italy. In some forms of the instrument, the trace attesting the time and mode of agitation is photographic; a quivering little mirror reflects a spot of light on to a circulating roll of sensitised paper; in others, it is stylographic. A pendulum is usually employed to furnish a 'steady-point, or as the index to motion, but with endless modifications of detail. Indeed, different kinds of apparatus are required to correspond with the varied demands of intelligible record. No one seismograph, nor any number of seismographs of the same type, would avail for the inscription, in decipherable characters, of the tangled phases of a single shock. A mere by-product of the transformation of Japan, seismology, nevertheless, already claims an honoured place among the physical sciences. And the precision of its methods lends validity to the claim. The study of earthquakes was, until lately, almost wholly descriptive. It was based upon narratives of catastrophes. Robert Mallet, it is true, made resolute efforts for the attainment, by the use of clearly prescribed methods, of a define point of view; and his work IF SAL FLUBLINE, Tris inferences have lost their pristine Indy. Bu now the tale of ordinary experience is Bud y by way of supplement to more formal statements; the earth quakes "by the card'; every throb of the ground is pasiced a period and direction, and referred to its STIGECTEDBET score. The change is scarcely less than that of the amazed spectator of total solar eclipses in old times into the agent modern coserver of their spectroscopic and photographie phenomena. The pageantry of nature is out of Sade in science. The work of which we have quoted the title at the head of this article is fully abreast of recent advances in earthquake research. It appropriately belongs to the Progres"sive Science Series, and its author, Major Dutton, holds the first rank among the seismologists of the United States. He writes well and carefully, and his book supplies fairly complete and entirely authentic information on a new and difficult subject. Its pages do not allure readers with fanciful speculations, but they satisfy them with solid facts; and in our opinion the vagaries of scientific imagination are to the full as vapid and wearisome as the maundering of the most shambling poetaster who ever set out to climb the hill of Parnassus. Seismology may be regarded as the science of earthvibrations. It is hence closely allied to acoustics, which is the science of air-vibrations, and to optics, the science of ether-vibrations. For the ground under our feet, the rocky crust of our planet, is an elastic solid capable of propagating wave-motion at measurable rates, and according to determinate laws. Its manner of doing so is, nevertheless, of baffling intricacy. Air and ether transmit (sensibly) each a single type of oscillation. Sound-waves are longitudinal; they alternately compress and rarefy the medium that conveys them forward. Those of light are transversal; they are directed at right angles to the line of advance. Both kinds of undulation, however, can be generated in the earth, though they travel through it at considerably different speeds. The first intelligence of an underground shock reaches the surface by means of elastic waves of compression, analogous to the undulations of sound; waves of distortion, similar to those of light, start in their company, but arrive a little later. To this initial diversity are superMed complexities, indefinable in number and amount, due irregularities in the transmitting strata. The hetero geneity of their composition is apparent on the most casual inspection. Granite is overlaid with shales and sandstones, with limestone, slate, and conglomerate, all frequently interlarded with beds of clay, or interpenetrated with dykes of basalt or serpentine. The waves of an earthquake are not then recorded by our instruments just in their original shapes. At every breach in the continuity of the rocks they traverse, they are variously shattered and transformed. Their periods of vibration, no less than their rates of travel, undergo changes recognised as actual, while admitted to be incalculable; some, turned aside by total reflection, must be lost to observation; others, Professor Milne finds reason to suspect, reach us as echoes, which succeed and prolong the primary effects of a concussion. With musical reverberations inside the world,'* an earthquake perhaps verges to a close. No wonder, then, that 'seismograms' present to the eye mere coils and folds of enwreathed lines, baffling uninitiated attempts at decipherment. Ingenuity, however, avails to unravel the knot, and disengage its hidden meaning. With one of Professor Ewing's seismographs the three components into which terrene tremblings are resolvable obtain separate record on a revolving circular plate, one tracing-point delineating vertical displacements, a second and a third horizontal disturbances directed respectively north and south, and east and west. This designed analysis is, in a sense, counterfeited by Nature. Near the 'epicentre' of an earthquake (the point at the surface directly above the focus), no vibrational distinctions can be noted. All come 'together,' Major Dutton says, 'big and little, long and 'short, rapid quivers and slow swings.' With increasing distance from the centre, he further informs us, there is a general extension of periods and wave-lengths, and the commotion, at first short and sharp, becomes somewhat protracted and diffuse. The various forms of vibration separate as a consequence of their different velocities, and separate further, so as to arrive at wider time-intervals, the greater the distance they have had to traverse. This relation was particularly well illustrated by the Charleston earthquake of August 31, 1886, investigated with great thoroughness by our American authority. At Summerville, and other places within the epifocal tract, 'the *Report Brit. Association, 1899, p. 230. in cash came hered. No minor movements • ushered in the grazer movements' In the city of Charlesna. Twenty miles from the centre, premonitory tremors had THE X FA Noresbit thead of the main disturbance. The • vincles which dizem down walls and wrecked streets, and vìà mau have had relatively large amplitudes and kog periods, were preceded by rapid quivers of small aricioles and period' Eight seconds at the most messtored the pause between the successive wave groups. Yen wis mansabille even in the midst of a destroying *airbroke. At Sarannah, seventy-five miles off, the quick vibracices rosiderably anticipated the arrival of their femikable associates Fiber afeld, at distances from the epicentre of three to four hundred miles, nothing was felt bar "a long slow swing. No instruments were at hand by which delibate pulsatins might have been detected. Only the rough-and-ready method of human sensation was brought into requisitica. Seismograms are now obtained widely and almost continuously. But the securing of them is not an end in itself. Were they as numerously stored as the inscribed tablets in Assurbanipal's library, they would be valueless or misleading unless their meaning could be read aright. This is no easy matter; obscurity still partially prevails; yet enough has been elicited to stimulate the utmost assiduity of research. On two subjects especially these novel records must prove instructive, and both are of palmary importance to the study of geophysics. They are, first, the originating cause of earthquakes; next, the condition of our globe's interior. No one needs to be told that what constitutes an earthquake is the shaking of the ground. What occasions the shaking, however, is less readily ascertained. A subterranean event being inaccessible to direct observation, its nature can only be reasonably inferred. All the attendant circumstances have first to be considered, and already much has been done towards localising and classifying shocks. From the spreading rings on a pool it is easy to fix the point where a stone has been dropped into it, even though the splash has escaped notice; and somewhat similarly the vertical direction of an underground disturbance can, from a fair supply of reliable data, be determined with approximate accuracy. Limits, too, can be assigned for the distance below the surface at which a concussion takes place. A shallow focus gives a steep surface-gradient of decline in agitation. Its epicentral violence rapidly falls off as it spreads outward, and only a narrow area is sensibly affected. Using this criterion, Major Dutton arrives at a maximum depth of twenty miles for earthquake origins.* Professor Milne extends the limit to thirty miles; † and Mallet depressed the focus of the Riobamba cataclysm of February 4, 1797, to nearly thirty-one miles beneath the trembling soil of Ecuador. All such estimates are, indeed, admittedly insecure. They rest upon arbitrary simplifications, for the actual state of things is complex beyond what figures can convey. And there is always the possibility that, in smoothing the data of calculation, we vitiate its results. Thus, the centrum of an earthquake is, in order to facilitate clearness of conception, taken to be a point; yet it is known to be a tract of undetermined shape and extent, or even a long, oblique line of weakness in the strata. Further, the unexplored terrestrial crust, composed of an endless variety of beds dissimilar in structure, density, and elasticity, is replaced by an ideal medium, possessing the supposed average quality of the de facto transmitting vehicle, though perhaps differing from it in some important respects. Hence, only provisional values can at present be fixed for the depth of seismic foci. Those yielded by Major Dutton's method of gradients rarely exceed twelve miles, at which the source of the Charleston disaster was approximately placed, and sometimes fall short of half that amount. Earthquakes are usually divided into two classes, volcanic and tectonic. Volcanic and seismic modes of action are, in the nature of things, mutually dependent. The dependence may not be absolute, but it is close. Eruptive outbursts necessarily set the earth quivering. If they fail to obtain a vent, they set it quivering all the more violently. A classical instance is afforded by the Vesuvian earthquake of 63 A.D., by which Herculaneum and Pompeii were devastated sixteen years previously to their entombment. Mount Epomeo, in the neighbouring island of Ischia, has been similarly energetic for destruction during intervals of apparent repose. The volcano has been extinct since 1302; but twice during the nineteenth century the manacled giant in its keeping turned in his sleep, and jolted Casamicciola into ruins. The town, a paradisiacal holiday resort, was crowded with visitors on July 28, 1883. Most of them were collected, and perished, in the theatre. Altogether the victims numbered about 1,900, and the crash left one house *Earthquakes, p. 194. + Seismology, p. 198. |