Page:Great Neapolitan Earthquake of 1857.djvu/450

360 held by its impression in the soft soil precisely in the spot in which it had fallen.

The mortar joint at $$c$$, Fig. 6, had obviously possessed scarcely any bond whatever, and the only resistance to fall by the shock was, therefore, the stability of the superimposed two blocks upon the base or joint $$c$$. The form of the surface of separation, showed that the blocks had overturned precisely in a vertical plane, passing through the axis of the longer block as found prostrate, which corroborated the assurance of the Syndic and others present, that they had not been moved by any one since their fall. The direction of the axis of the fallen block, $$\mathrm{B}$$, was exactly 15° W. of N., and such was the wave-path that overthrew it, the movement having been from N. to S.

We may view the velocity that overthrew the shaft, separately from that which projected it from its base to the horizontal distance given, and compounding these two horizontal velocities, and resolving in the direction of the wave-path, obtain the total velocity impressed upon it, and hence that of the wave itself.

And first we obtain the horizontal velocity for overturning only from the equation Here $$a = \text{50 inches}, b = \text{17 inches}, \theta = 19^\circ$$; and solving for $$V$$ we have