Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/410

Rh The results are concordant as nearly as could be expected, from the arcs of oscillation having been only repeated for my observation by the tact of the two observers, and some weeks after the earthquake, and they show the small difference between the wave velocities in the first and second semiphase.

If a succession of $$n$$waves act upon the pendulum, the time of each being small, as compared with that of oscillation, and the arc of oscillation be due to the conjoint action of the whole, then the difference of velocity between the first and second semiphases will be $$\frac {\mathrm V}{n}$$. Such was the case at Naples; seven or eight distinct waves, at equal and short intervals, constituted the first movement, according to Guiscardi, and the other most reliable observers. The mean value of $$\mathrm V$$ given by the two chandeliers is $$= 1.295$$ feet per second, and the difference of velocity therefore $$= \frac {1.295} {n}$$, or $$= \frac {1.295}{7}$$, or $$\frac {1.295}{8}=0.185$$ feet per second for the first, and $$=0.162$$ feet per second for the second, number of pulses.

This result gives a difference of velocity, in the first and second semiphases, at most, of less than th of a foot per second; so that the determinations we have made of wave velocity, are not materially affected, whether they be derived from projections or overthrows, &c. produced by the first semiphase, or by the second; while on the other hand this small difference, in forward and backward velocity, is quite sufficient to account for the observed difference, in fissuring effect upon masonry, of the two semiphases, and so confirms the explanation given of the fact, on other grounds, in Part. I.