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159. We shall begin this Chapter with a statement of the leading features of seismic records and of the most important steps that have been taken in their interpretation.

As long ago as 1830 it was proved by Poisson that a homogeneous isotropic elastic solid body of unlimited extent can transmit two kinds of waves with different velocities, and that, at a great distance from the source of disturbance, the motion transmitted by the quicker wave is longitudinal, that is to say the displacement is parallel to the direction of propagation, and the motion transmitted by the slower wave is transverse, that is to say the displacement is at right angles to the direction of propagation. It was afterwards proved by Stokes that the quicker wave is a wave of irrotational dilatation, and the slower wave is a wave of equivoluminal distortion characterized by differential rotation of the elements of the body, the velocities of the two waves being √{λ + 2μ/ρ} and √(μ/ρ), where ρ denotes the density, μ the rigidity, and forumla the modulus of compression. These two velocities will be denoted by a and b. Stokes proved also that, if any disturbance takes place within a limited volume of the body, waves spread out from the disturbed region in the following way:—At a distant point no movement occurs until sufficient time has elapsed for the travelling of the disturbance with velocity a from the nearest point of the initially disturbed region, and again

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