Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/354

Rh about 2 geographical miles north of the main focal point, and some to another, nearly 5 geographical miles south of the same; the length or distance along the right line connecting these extremes, and passing through the main focus, being about 7 miles, and in a direction nearly 20° or 30° east of north. Many of the more southern wave-paths, at that extremity of this linear locus, are those from places such as Saponara, &c., almost parallel to the major axis of the meizoseismal area.

That the existence in part of this linear locus of foci, may be due to unavoidable errors of observation in the wave-paths, and to local changes of direction, of the wave-paths observed only at some points, there can be no doubt; but the length of this locus of foci, coupled with the facts recorded as to the sound, leave as little doubt, that the centre of effort was not confined horizontally, to a single, or a very small point of space, nor diffused horizontally, and equally, round the main focus, over a very wide one; but was limited to a surface, either in a vertical or highly-inclined plane to the horizon, which we shall see reason to believe, was a curved surface, which passed through the main focal point nearly beneath Caggiano.

Such conditions would be fulfilled, were the originating impulse due, to a great rent or fracture, suddenly produced, in the course or direction of this curved plane passing through the focal loci, and this is exactly the condition that will account for the sounds, so variously heard and described, at different points of the shaken area.

A rent or fracture produced under our conditions, must have commenced at a middle point in its length, i.e., at the main focal point, for at this point, the forces producing

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