Page:Great Neapolitan Earthquake of 1857.djvu/57

point $f$ in the second semiphase by a force measured by $ef$.

In fact, no velocity of earth wave occurring in nature, even with the emergent angle $e=0$, i.e., horizontally, could overturn a block proportioned as in Fig. 6. If resting on a bed of earth or stone, it might slide and plough along upon it, and knowing certain coefficients, the length and dimensions of the channel or course cut by it would enable the wave velocity to be arrived at. In the case of the other block (Fig. 7), however, it would be overturned by a wave emergent in the direction $a$ to $b$ in either semiphase of the wave, the forces of overthrow in each being proportionate to $dc$ and $ef$. So in more regular solids, the column shaft (Fig. 8) may be overturned by a sufficient velocity of wave, in either semiphase, emergent at any angle between $he$, passing through the centre of gravity and the horizontal wave-

path passing through the same, the overthrowing force, with a given emergence and direction of wave-path, $a$ to $b$, being proportionate to $cd$ in the first semiphase and to $ef$ in the second.

But the pedestal or "cippus" (Fig. 9) can only be overturned in the second semiphase of the wave, however great its velocity, if emergent in the direction $a$ to $b$, nor