through a medium quite homogeneous in its properties of density and elasticity, and perfectly solid and continuous throughout its entire extent. We have still to reckon with the facts that the soils and subsoils of the earth are anything but homogeneous, and that near the surface, at least the stratified rocks are by no means continuous. That the defects of homogeneity and continuity must affect the transmission of the impulses in many ways is sufficiently obvious. But that it greatly affects the variation of intensity is not so clear. That it does so in a measure is not at all doubtful, but that it affects it sufficiently to vitiate seriously the general course of reasoning and deduction based upon it, is not apparent. The facts sustain the theory sufficiently to give the theory the character of a fairly satisfactory approximation.

In the discussion thus far it has been necessary to limit it to those quakes which have a centrum or something approaching that character, and to reserve remark upon those which originate in long faults or downthrows of large blocks of ground like the Chilian, New Madrid, Assam, and Sonora earthquakes. While this indefinite, diffuse, and often irregular configuration of the place of origin renders all our inferences more complex, it does not affect the fundamental considerations upon which our reasoning proceeds. are still elastic waves originating from numberless points, more or less widely separated, sometimes simultaneous, sometimes in rapid succession, and all blending into a medley of vibrations like the sound-waves emanating from musketry and artillery in a battle. The quakes from the above causes are the most forcible and destructive, as well

These

as the most far-reaching of any that we know of. They are so because far greater amounts of energy are exercised in producing them than in the lighter quakes.

An opportunity was furnished in 1886 to compare the intensity of the Charleston quake at New Madrid with that of the New Madrid quake at Charleston. The distance separating the two places is about 575 miles. The Charleston quake at New Madrid was felt as a light tremor and oscillation on the ground, but as a long swaying motion in upper stories of buildings. It was noticed by only a few people and caused no alarm. The New Madrid quake was much more forcibly felt in Charleston, where it cracked walls, set the church bells ringing, and threw down plastering. Even in Boston, over twelve hundred miles distant, it was apparently forcible enough to attract general attention and tc cause considerable alarm.

CHAPTER XI

SPEED OF PROPAGATION OF EARTH-WAVES

Importance of this Question in Relation to the Problem of the Earth's Interior Condition-Difficulty of Ascertaining Wave Speeds-Especially at Small or Moderate Distances-Three Kinds of Waves, Each with its Own Speed-Wide Discordances of Results-Difference between True and Apparent Speed - Hopkins's View - Seebach's Method of Finding Depths of Origin from the Relation between True and Apparent Speed —August Schmidt's Proposition Concerning Effect of Variable Elasticity upon Wave Speeds and Wave Forms-Results Obtained over Short Distances-Speeds in the Charleston Quake-Speeds Obtained by Omori in Japan-Indian Quakes Discussed by Agamennone—Real Causes of the Difficulty of Securing the Proper Wave Data

N°

branch of the subject of earthquakes has received. more attention than the speed with which their vibrations are transmitted. A knowledge of this quantity has been deemed valuable in attempting the solution of some of the greatest questions of dynamical geology. For the rate of transmission is assumed to be dependent wholly upon the elasticity and density of the transmitting medium. The speed of propagation, then, ought to indicate to us something as to the elasticity and density, and, therefore, of the physical condition in general of those materials which form the interior of the earth. Nor do we seem to have at present any means of extending our inquiries experimentally into those regions. But an earthquake vibration which has

traversed a diameter or a long chord of the earth's interior may be supposed to have brought us some message from the mysterious realms it has crossed on its way, though the difficulty of translating it aright may be insuperable.

At first it might seem a very simple matter to time the passage of a shock at two points. The difference of time and the distance between the two points being known, the speed follows at once. But nature is not half so simple, and is seldom or never disposed to surrender her precious secrets so easily. Instead of timing a single, short, well-defined impulse, the observer is required to note a protracted series of them, whose beginning is more or less uncertain, whose increase is gradual up to a maximum, or even several maxima, and whose ending is through a progressive decline. The speed of transmission is always swift, several kilometres per second, and, unless the space interval between observing stations is considerable, a very small error in the observed time leads to a large error in the result. In the vast majority of earthquakes such errors are inevitable. It is only in the more powerful ones that long intervals of both distance and time are available for measurement, and these are rather uncommon.

Again, there are three distinct classes of waves whose speeds are different, being dependent upon different sets of physical constants, viz., the normal, the transverse, and the surface waves. Near the epicentre these are all more or less intermingled. As they spread out they separate, but even within two or three hundred miles of the origin the separation is so incomplete that they still overlap each other more or less, and it is not until they are two or three thousand

miles away from it that the separation is fully completed. By that time the amplitudes of the vibrations are so far reduced that only instruments of extreme sensitiveness can pick them up and properly record them. The normal and transverse waves also reach great distances by passing through the deeper portions of the earth's interior, the rays following paths which are possibly not chords, but curves due to a variation of the ratio of elasticity to density, and converge toward the earth's centre. The surface waves, true to their name, follow around the earth in arcs of great circles. By the time they reach a recording instrument four or five thousand miles away, they have travelled a greater distance from the origin than the normal and transverse

waves.

Finally, the purely elastic waves, on their way through the depths, may perhaps cross regions of varying ratio between elasticity and density, and undergo changes of speed, probably increasing at first as their paths get nearer the earth's centre, and afterwards decreasing as they get farther away from it.

These few remarks may suffice to suggest the complexity of the subject. They may also explain why recent investigators have divided that subject into two distinct fields, one of which comprises speed measurements and estimates over short distances from the epicentre or origin, and the other, those which deal with speeds through long distances of several thousand miles from the origin. Both fields of inquiry have yielded instructive results, the latter or long-distance estimates being the more valuable of the two groups. But some knowledge of the results of