Page:Great Neapolitan Earthquake of 1857.djvu/49

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and frequently the angle of emergence may be immediately inferred.

b. Information from the preceding united with known conditions as to the strength of materials to resist fracture, by which the velocity of the fracturing impulse may be calculated.

2nd. The overthrow, or the projection or both, of bodies, large or small, simple or complex. From these we are enabled to infer—

c. By direct observation the direction in azimuth of the wave-path.

d. By measurements of the horizontal and vertical distances of overthrow or of projection, to infer the velocity of projection and angle of emergence—both, or either.

Fractures or dislocations present themselves always in directions more or less transverse to the wave-path. Overthrow or projection, on the contrary, always takes place in the line of the wave-path, or in the vertical plane passing through it; but the direction of fall or of projection may be reverse (or in the contrary direction) to that of the wave transit, or it may be in the same direction with it.

At the moment of the arrival of the earth wave at any object upon the surface, whose dimensions are less than the amplitude—an obelisk or pillar or single wall for example—motion is suddenly communicated to the body; the velocity of the vibrating particles rapidly increases from zero to its maximum velocity, and returns to zero, as it completes its first semiphase or half-vibration, the direction of movement in which is in the same sense as that of the wave transit.