EARTHQUAKES.

New Zealand Spectator and Cook's Strait Guardian, Volume V, Issue 389, 25 April 1849, Page 4

EARTHQUAKES.

The following observations on earthquakes, extracted from a Review of Mrs. Somerville's Physical Geography in a recent number of the Quarterly Review, will be read with interest in this settlement which has j so severely suffered from them, and where they still continue occasionally to occur :-— The history of earthquakes is replete with I phenomena not less vast and terrific. We need but refer to those which in 1797 destroyCumana, Quito, and Riobamba — events minutely and vividly described by Humboldt — and to the great Lisbon earthquake of 1 755, the effects of which are still to be seen in and around that city ; and which from some sub* marine centre in the Atlantic (not, as Mrs. Somerville statei, immediately under Lisbon), spread one enormous convulsion over an area of 700,000 square miles — agitating by a single impulse the lakes of Scotland and Sweden, and the islands of the West Indian sea. Not, however, by a simultaneous shock, for the element of time comes in with the distance of undulation; and together with this another complexity of action, in the transmission of earthquake movements through the sea, arising from the different rate of progression at different depth. In the fact that the wave of the Lisbon earthquake reached Plymouth at the rate of 2*l miles per minute, and Barbadoes at 7*3 miles per minute, we have illustration of the law that the velocity of a wave is proportional to the square root of its depth, and becomes a substitute for the sounding line in fixing the mean proportional depth of different parts of this green ocean basin. Such and so striking are the connexions of the physical sciences, and th« illustrations they afford to each other? - 1

The theory of earthquake* and volcano*, while presuming a dose relation of origin of these, phenomena, is still incomplete and obscure. Subterranean heat We know to- exist undef eterjr part of the crust' of itie globV; and the various evident** lately collected from mines, wells, and springs, and carefully exanlined by Fox, Gordier, Kupjfcr, Arago; De la Rive, &c, show that the increment begins within 100 feet of the surface, and is continued afterwards in a certain ratio of progression, to the greatest depths whitih mail has ever reached* This depth has rarely exceeded two-fifths of a mile from the surface; and only in one instance attained a third of a mile below the level of the sea. The refined .methods of modern geology, based on the inclination and superposition of strata, have demonstrated in soifli* cases a succession of solid rocks seven ntilei in thickness ; but without indicating the tern* perature of the lower portion, or the point where the materials of the globe may be presumed to become liquid from heat. As a physical fact this will, perhaps, never be ascertained. Mr. Hopkins, indeed, has sought its solution from a higher source ; affirming, upon refined calculation, that the phenomena of the precession of the equinoxes cannot be what they are, unless with a certain thickness of crust, which he rates at 800 or 1000 miles, enveloping any liquid material which may occupy the centre of the earth. It is difficult to reconcile this deduction with the phenomena of volcanos and earthquakes, seeing that these involve forces and actions, which are only clearly explicable to our present knowledge under the supposition of mobility and actual change in the matter which lies at no great distance below the superficial crust. Deep rents in the earth may, indeed, give scope and issue to some of these impulsions ; but there are others, and especially those which dynamically produce the various and singular movements of the earthquake, which can hardly be comprehended under this view. Here, in truth, lies the great question of the earthquake theory ; how to explain motions or vibrations of soliJ strata, not merely perpendicular and horizontal, but undulating or even rotatory in kind ? We can reach the solution of this difficulty, we believe, but in one possible way — via. by admitting the mutual elastic action of particles, as in the propagation of light and sound ; and by extending this analogy to the interference of different lines of vibration, in explanation of the rotary and irregular motions which have so greatly perplexed the inquiry. It may be difficult for one untutored in these matters to conceive such conditions as occurring in the solid rocks of the earth ; but the simpler kinds of motion, needfully admitted to exist in this case, give proof of the possibility of those more complex ; and the whole course of modern research has tended to reveal mutual actions and conditions of change among the particles of solid bodies, altogether unsuspected or deemed impossible before. We have not space to dwell further on this interesting topic ; yet we must not quit it without briefly adverting to a series of papers by Mr. Milne, on the Earthquakes of Great Britain, published in Jameson's Philosophical Journal, from 1841 to 1843. These memoirs are much less known than they deserve to be from their minute and curious research. They record 116 earthquake shocks in England ; 31 of which were along the south coast, 30 in Wales, 14 on the borders of Yorkshire and Derbyshire ; and 139 in Scotland, of which not fewer than 85, and these the most violent, occurred in the vicinity of Comrie, in Stratherne; indicating, without the proximity of any volcanic action, some singular relation of this locality to subterranean actions going on underneath. These, memoirs establish more explicitly than heretofore many general conclusions regarding earthquakes — the fact of two shocks generally occurring in quick succession — the noise always attending them — the frequent fogs preceding the shocks — the unnatural sultriness of the air at the time, -even during winter — the sudden fall of the barometer, &c. Some of these conditions suggest' the concernment of electro-magnetic actions within the earth in the production of earthquakes \ as their antecedence, and the peculiar influences present in the atmosphere, cannot easily be explained by a regard to dynamical forces only. But the whole question remains open for future solution ; if, indeed, the elements for such solution should ever come within reach of human research.