Page:Great Neapolitan Earthquake of 1857 Vol 2.djvu/402

Rh and if we retain both Tramutola and Sarconi, we get 12·107 for the mean velocity in feet per second.

The first, taking all circumstances into account, is the most probable mean, and excepting the two doubtful terms, differs from the highest determination, by only 0·810 feet per second, and from the lowest by 1·326 feet per second.

We shall refer to these differences again, under the next head. Chap. XVI.

The velocity at Sarconi, may either be a really low one, arising from the peculiar circumstances of its position (Part II. in loc.), or be defective in amount, as means did not exist for checking it, by a second determination at the same place. Reviewing all the circumstances, I am inclined to conclude that the velocity at Sarconi was really low, but that also, the determination is below the truth by about one foot per second.

The highest velocity of which any measurable evidence was found, was that of the projection of the vase from the pier of the Prior's garden gate, at the Certosa. Of the total velocity here, at least 8 feet per second, is due to the elastic oscillation of the top of the pier itself. Dr. Young has shown (Math. Prin. Nat. Phil. Art. 398) that the time of oscillation of such an elastic prism, will be that of a simple pendulum, whose length is, $$\mathrm L = \frac {0.9707\ h^4}{d,\ d\ \epsilon}$$, $$h$$ being the height of the vibrating bar, $$d$$ its thickness in the direction of vibration, and $$\epsilon$$ the elastic modulus expressed in feet; from which the velocity may be calculated when the angle or arc of vibration is known. Unluckily, in this, as in most examples that actually occur in seismic observation, the excursion of the summit of the pier is

VOL. II.Z