different beds, they also should receive attention. In all cases where fossil plants are mingled with the coal or associated beds, specimens as various as can be obtained should be secured. These have a geological bearing which may often turn out of great practical importance in some given region.

It scarcely requires remark that the foregoing observations are but hints which it is hoped may be useful to those engaged in voyages of discovery and survey, or who, on more general service, may feel inclined, whenever fitting opportunities may present themselves, to devote some portion of the time not occupied by their professional duties, to the study of minerals, either for purely scientific purposes, for their useful employment, or for both combined. these opportunities do present themselves we well know, or rather if sought will be found more frequently than might be imagined. Many a walk along a coast may thus be advantageously turned to account, and an interest be excited not at first thought probable. Not only may a naval man thus add to his own stock of knowledge, but he may most materially by his exertions promote the advance of science and its applications generally, minerals being objects of great interest, whether we regard them with reference to their importance to man, and the aid many of them afford to the spread of civilization, or as connected with several sciences, even those of the highest order.

ARTICLE XIII.

FOURTH DIVISION, SECTION 3.

ON OBSERVATION OF

EARTHQUAKE PHENOMENA.

BY R. MALLET, Mem. Ins. C.E., F.R.S., M.R.I.A.

THE observation of the Facts of Earthquakes and the establishment of their theory constitute Seismology (from σεloμos, an earthquake, a movement like the shaking of a sieve), which has only become an exact science within the last twelve years. Its immediate and most important applications are to the discovery of the nature of the deep interior of our planet, and of the reactions of the interior upon the exterior, visible in volcanic action at the surface.

Whenever a blow or pressure of any sort is suddenly applied, or the passive force of a previously steady or slowly variable pressure is suddenly either increased or diminished, upon material substances, all of which, whether solid, liquid, or gaseous, are more or less elastic, then a pulse or wave of force, originated by such an impulse, is transferred, through the materials acted on, in all directions, from the origin or centre of impulse, or in such directions as the limits of the materials permit. The transfer of such an elastic wave is merely the continuous forward movement, of a change in the relative positions, a relative displacement and replacement, of the integrant molecules or particles of a determinate volume, affecting in succession the whole mass of material.

Ordinary sounds are waves of this sort in air. The shaking of the ground felt at the passage of a neighbouring

railway-train is an instance of such waves in solid ground or rock. A sound heard by a person under water, or the shock felt in a boat lying near a blast exploded under water, are examples of an elastic wave in a liquid.

The velocity with which such a wave traverses, varies in different materials, and depends principally in any given one, upon the degree of elasticity and upon the density. This transit period is constant for the same homogeneous material, and is irrespective of the amount or kind of original impulse: for example, in air its velocity is about 1140-in water about 4700-and in iron probably about 11,100 feet per second-all in round numbers. In crystallised or pseudo-crystalline bodies, such as laminated slate or other rocks, the transit period may vary in three different directions. A very great retardation of this period is produced in solids whose mass is shattered or broken, even when the fissures appear perfectly close.

Thus, if one stand upon a line of railway near the rail, and a heavy blow be delivered at a few hundred feet distant upon the iron rail, he will almost instantly hear the wave through the iron rail-directly after he will feel another wave through the ground on which he stands-and, lastly, he will again hear another wave through the air; and if there were a deep side-drain to the railway, a person immersed in the water would hear a wave of sound through it, the rate of transit of which would be different from any of the others—all these starting from the same point at the

same moment.

The size of such a wave-that is, the volume of the displaced particles of the material in motion at once, depends upon the elastic limits of the given substance, and upon the amount or power of the originating impulse. By the elastic limit in solids is meant the extent to which the particles may be relatively displaced without fracture or other permanent alteration: thus glass, although much more perfectly elastic than India rubber, has a much smaller elastic limit.

Nearly all such elastic waves as we can usually observe,

originate in impulses so comparatively small that we are only conscious of them by sounds or vibrations of various sorts, the advancing forms of whose waves are imperceptible to the eye; but when the originating impulse is very violent, and the mass of material suddenly acted on very great, as in an earthquake, the size of the wave may become so great as to produce a perceptible undulation of the surface of the ground, often visible to the eye; and by whose transit bodies upon the earth are disturbed (chiefly through their own inertia), thrown down, &c.

There is every reason to consider it established, that an earthquake is simply "the transit of a wave or waves of elastic compression in any direction, from vertically upwards to horizontally in any azimuth, through the crust and surface of the earth, from any centre of impulse or from more than one, and which may be attended with sound and tidal waves, dependent upon the impulse and upon circumstances of position as to sea and land.”

Until this was clearly grasped, the observation of earthquake phenomena, in the absence of a "guiding hypothesis," was vague and useless.

At present the objects and aim of Seismology are of the highest interest and importance to geology and terrestrial physics. It offers to us the only path to discover the real constitution and condition of the interior of our planet, and will become the key to open to us the true nature, depth of origin, and source, of volcanic heat. In these respects, one of the most primary objects of Seismometry is to arrive at a knowledge of the depth beneath the earth's surface from which earthquake shocks are delivered, i. e. the depth of the origin.

The observer must form clear conceptions of the fundamental conditions of propagation of seismic waves. Fig. 1 represents a vertical section of part of the earth in the plane of a great circle, cutting the surface at h'h and passing through the origin of impulse at A-Ap being the prime vertical to that point whose depth beneath the surface is BA. The wave starts from the origin (assuming the earth's mass homogeneous) with one normal and two

transversal vibrations; neglecting the latter for the present, the wave may be imagined transferred outwards, in all

directions in concentric spherical shells, whose volume at the same phase of the wave is constant. The interval between any two such shells therefore diminishes as r2, r being the mean radius, and the overthrowing energy of the shock in the direction of r varies inversely as the square of the distance from the origin.

The shock reaches the surface at B directly above the origin vertically, but, for all points around that, it emerges with angles getting more and more nearly horizontal as the distance measured on the surface increases. The intersecting circle of any one shell with the surface, which is that of simultaneous shock, is the coseismal line, or crest of the earth-wave; circular (like the circles on a pond into which a stone has been dropped) if in a homogeneous medium, more or less distorted if in a heterogeneous one, such as constitute the various formations of the earth, but always a closed curve. The transversal vibration is trans