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Milne, whose observatory at Shide, in the Isle of Wight, supplies, in a continuous series, autographs of earth-tremors that have travelled thither from the shores of Kamchatka or Japan, from beneath the floor of the Pacific, or the flexured declivity of the Himalayas. Their discussion, though arduous, has proved of unique interest. The vibrations of the earth supply the best means at our disposal for probing its depths; and they bring news both authentic and suggestive, though our inexperience may still partially darken its true meaning.

The various kinds of waves set up by earthquakes travel with different speeds. What is more, they evidently pursue different routes. Mr. Oldham distinguishes three types of pulsation in the seismographic records of distant shocks.* The first two are preliminary oscillations of small amplitude and short period, which appear to have been transmitted right through the globe; the third and strongest set have taken their way round its circumference, causing the surface to undulate more or less sensibly as they passed. They are surmised to be of a hybrid nature, partly elastic and partly gravitational, thus combining the properties of sound and seawaves; but their velocity, of about three kilometres a second, is much higher than that of disturbances in air or water.

The tremors, however, which anticipate their arrival attract still more curious attention because of their exploratory powers. They follow paths otherwise untrodden, unless by Dante in his voyage to the Giudecca. The two species into which they of themselves separate stand, nevertheless, apart in some important respects. The foremost group probably consist of longitudinal waves modelled on those of sound. They proceed, Dr. C. G. Knott considers, straight from point to point along the chord of the terrestrial segment interposed between the epicentre of the earthquake and the spot where its occurrence is recorded. The rapidity of their transit would else be unaccountable. They take advantage of their fellow-travellers, the large surfaceundulations, by striking out a much shorter way, and a way that shortens proportionately as the distance from the starting-post to the goal increases. For this reason, to say nothing of their tripled speed, they win the race by a continually widening interval.

The secondary group of vibrations represent, we are told, the transversal part of the disturbance. They are, in a

* Philosophical Transactions, A, vol. cxciv. p. 172.

sense, complementary to their immediate predecessors of the longitudinal type. Like them, they are propagated through the earth's interior, though not, like them, with an uniform speed. As they plunge deeper into its profundities they become accelerated owing to the augmented rigidity of the medium; and the consequent bending of their paths by refraction renders them convex towards the centre. From these deep seismic soundings something has alredy been learned regarding the structure and qualities of olossal mass of matter beneath our feet.

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Within the globe, pressure grows with eve descent, and its materials must, by the mere fac superincumbence, be rendered more homogeneous. cracks, vesicles, become obliterated; a nearly equa position is finally attained. The effacement of ineq is doubtless helped by elevation of temperature. Th with descent in mines and borings is at the rate of a one degree Fahrenheit for 64 feet; but how far it contin to prevail is uncertain. There may be a fixed thermal lim which, for aught we know, falls short of the melting-poin of rocks. Erupting lavas are, to be sure, enormously hot; but local and temporary causes may be largely concerned in producing volcanic conflagrations. It is certain, in any case, that the core beneath the crust of the earth is in a high degree rigid and elastic, and possesses, accordingly, a superior capability for the swift transmission of wavetremors. Their subterranean velocities imply, indeed, an elasticity twice (approximately) that of steel.

Dr. C. G. Knott's analysis of the statistics of earthtremors † involves an unlooked-for distinction between the elasticity and the rigidity of the bulky mass transmitting them. Should his conclusion be established, that those of longitudinal type travel uniformly at all depths-once they have penetrated the crust-while their transversal associates quicken their pace as they approach the globe's centre, it would be necessary to admit a constant ratio, throughout its innermost layers, of elasticity to density, coupled with a ratio increasing downward of rigidity to density. Let us look at the conditions. Vibrations taking place to and fro along the line of march owe their origin to the power of matter to resist and rebound against sudden compression, and they travel quicker as this faculty becomes intensified

* Dutton, 'Earthquakes,' p. 232.
+ Ibid. p. 222.

relatively to the density of the transmitting substance. The constancy, then, of their speed within the earth proves the invariability of this ratio, and makes it probable that only a moderate standard of density is reached in the deepest subterranean abysses. Waves, on the other hand, of the transverse description are set on foot by forces of recovery from distortion. Their velocity grows with rigidity, and has an inverse relation to density. So that the quickening of pace with the increasing length of the chords traversed by this kind of earth-tremors leads to the inference that the strata harden in their consistence beyond the proportion of their density as pressure is augmented. Their chemical constitution remains unknown. Pure iron, formerly assumed, by way of giving precision to ideas, as the main, if not the exclusive ingredient of the terrestrial core, does not possess the requisite qualities.* They would more probably belong to some metallic alloy. The probability is indeed strong that all the metals are present there together, although in very unequal quantities. The subject, however, is not yet nearly ripe for discussion.

Earthquakes are a sign of planetary vitality. They would seem to be characteristic of the terrestrial phase of developement. Effete globes like the moon can scarcely be subject to the stresses to which they are due; nor can they be very suitably constituted for the propagation of elastic waves. Inchoate worlds, such as Jupiter and Saturn, are still less likely to be the scenes of reverberating concussions. Their materials have not yet acquired the necessary cohesion. They are pasty, or fluid, if not partially vaporous. On the earth the seismic epoch presumably opened when, exterior solidification having commenced, the geological ages began to run. It will last so long as peaks crumble and rivers carry sediment; so long as the areal distribution of loads fluctuates, and strains evoke forces adequate for their catastrophic relief. Our globe is, by its elasticity, kept habitable. The separation of sea from dry land is thus and no otherwise maintained; the alternations of elevation and subsidence manifest the continual activity of this reserve of energy. The dimensions of the globe we inhabit depend upon the balance of pressure and expansiveness. Relaxation or enhancement of either instantly occasions a bending inward or an arching outward of the crust. Just by these sensitive reactions the planet shows itself to be alive, and seismic thrillings are the breaths it draws.

* Milne, 'Nature,' vol. lxvii. p. 539.

ART. III.—THREE PHASES OF PASTORAL

SENTIMENT.

1. Geschichte der christlichen Kunst. Von F. X. KRAUS. Vol. I. Section Pastor bonus.' 1897.

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2. Sacred and Legendary Art. By Mrs. JAMESON. 3. History of Spanish Literature. By TICKNOR. By TICKNOR. London: John Murray. 1849.

4. Poésies populaires de la Gascogne. Par BLADÉ.

5. Histoire de la Chanson populaire en France. Par J. J. TIERSOT. 1889.

6. Diana. By MONTEMAYOR; tr. B. YONGE. 1598.

7. Giovanni Segantini. By L. VILLARI. London: T. Fisher Unwin. 1901.

SENTIMENTS-sentiments emotional and imaginative,

secular and religious-have, no less than individuals, nations, and races, their periods of growth and decline, their apogees of blossom and fruit-bearing, their subsequent decadence. From phases of exigent vitality, when they clamour for manifestation and expression, they pass into limbos, more or less permanent, of non-active, non-influential, non-potent existence, they lie in abeyance, they hibernate during the winter of men's disesteem. Again, they regerminate. Why? Who can say ? Who can say? But the disused, discarded sentiment stirs afresh at the heart of the world; there comes a new impulse from within, a new call from without, a new revolution of the wheel of fashion in feeling brings with it a new evocation of semi-extinct sensation. And the dormant emotion, or its similitude, rises from the dusty tombs where, with other rags and shreds of humanity's mental equipment, it lay awaiting the day of resurrection.

Thus, on the secular plane of life, the sentiment of mediæval romanticism ran through its courses, ascendant and decadent, of empire over men's sympathies. It had its fluxes and refluxes of enthusiastic hyper-sensitiveness. Its gallant endeavours to domesticate a Quixotic ideal in our poor world of clay suffered eclipse, died maybe at the hands of the melancholy satirists of this unbelieving earth, or, more likely, expired, exhausted by the exuberance of its own efflorescence. Having died, it came forth from its grave rejuvenated, claiming its exponents in a Hugo or a Scott, its colour-devotee in a Delacroix, its sentimentalist in an Ary Scheffer. Once more it effaced its own imprint, it

slept awhile. Yet sleeping—the idea suggests itself with insistence-sentiments are capable of interludes of somnambulism, and romanticism wandered forth afresh clad in the maladive incarnations of a Burne-Jones picture, whispering through the clamorous passions of a Wagnerian Tannhäuser or the stagnant delirium of Tristan, dreaming the feverdreams sick men dream of visions which have passed away. So, too, the transit of the sentiment of antiquity, exteriorised in the diverse classicisms of Italian Renaissance or the Siècle Louis XIV., or in that phase associated with the names of David and Ingres. So, too, the transit of religious sentiment illustrates above all others both the fitfulness and the persistence of the hold abstract emotional imaginations retain over the minds of men. It has had, more than any other, perhaps, its periods of stagnation and inertness; it has had, more than any other, its tidal waves, when in revivals or reformations it has swept the souls of men upon its floodcurrents to seek new truths or new heresies, till in sterilising formulas of sectarianisms, or conventionalised pacts of man with God, emotion has worked out its especial tragedies: the declension of aspirations too vehement to endure, the base materialisation of conceptions too transcendental to admit of incorporation amongst the maxims of practical conduct or the dogmas of rational belief. Nor has this very sentiment been in later days without its dream-walking phenomena, both in printed and painted thought. The légendes dorées of the other-worlds,' above and below, are, save as tradition, effete. But the vision-seer, Blake, in the earlier decades of last century, reclothed the sentiment of religious supernaturalism with a symbolism of deeply indented actuality, overcharged with emotions of faith, hope, and aspiration; while, reversed, and turned into grim and coarse grotesques, the supernatural transmuted into the fantastic, Félicien Rops-whose genius for the first time has been made widely known to the English public *-replaced the visions of faith with his sinister nightmares, the devildreams of an imagination imbued with the religious sentiment à travers-visions which in very truth are visions of the night.

Recently M. de la Sizeranne † has traced parallel developements in the history of art; the evolution of the battle sentiment, once embodied and crystallised in national epic

International Exhibition. New Gallery, 1904.
Le Miroir de la Vie, 1902.

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