Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/355

290 fracture must have been most violent, and the fracture extended, or ran out thence with great rapidity, in both directions, viz., N.N.E. and S.S.W.; extending, at the same time, upwards and downwards, in height and depth, from the same point.

The noise, therefore, whether produced by the actual fracture of rocky strata, in the hard lower limestone, or in some still lower rocks, or due to the rushing of steam at high pressure, into the huge fissure as it opened along, or due, as most probable, to both united and together, could be heard nowhere upon the surface of the earth above, as an explosion, but everywhere as a prolonged sound of some sort; and the amount of prolongation, as heard by an observer at any given point, would be greater, in proportion as his station was more, in directum with the line of the rent, or with its chord, if curved, and his position further and further removed from the middle point of its length.

The sound is produced, at the point in the act of being rent, and into which, as it opens the fissure, the steam is rushing; it is therefore produced, in succession along every point of the line of fissure, from the origin or focus, to its extremities, where the rending force becomes evanescent.

Now if $$t=$$ the whole time of rending the fissure, to any given distance along it, from the origin $$x$$, and $$d=$$ the distance of the hearer upon the surface of the earth, from the same; $$v=$$ the velocity of sound, in the masses fractured and transmitting it to the ear; and omitting all consideration of intermediate transmission, through small thicknesses of the atmosphere before reaching the ear, or of sounds produced simultaneously with the longitudinal rending, by