Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/418

from the focus. The energy, as evidenced by its effects in each successive valley plain, further from the focus, is found abruptly reduced, the moment the intervening ridge has been passed; and when there are many such alternations, the wave of shock becomes rapidly lost and extinguished, and this altogether apart from retardations or reductions, due to discontinuity of formation, or passage through transverse stratification. This will be easily understood from Fig. 354, where $o$ is the mean focal point, $o\ v$; the

Fig. 354.

seismic vertical, $Pp''$ the surface of the earth from which rise three mountain masses in the same right line with $o$, or three parallel mountain ranges, $\mathrm {A,\ B,\ C}$. Of the whole wave $\mathrm {P\ W\ D}$, transmitted, the shaded parts are (as respects direct transmission) extinguished at the successive elevations, $a\ a'\ \mathrm {at\ A}$, $b\ b'\ \mathrm {at\ B}$, &c. The places situated upon the flanks $a\ a',b\ b'$ &c., being free-lying surfaces, are exposed to exalted earthquake action. The places between $a'\ \mathrm {and} \ p,b'\mathrm {and} \ p'$ &c., suffer less as their distances from $o$ increase; but those on the plain $c'p''$ are abruptly less acted on than those on that $b'p'$; and these, again, abruptly less than in $a'p$; while the places in these

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