Page:Great Neapolitan Earthquake of 1857.djvu/393

Rh further west from the centre line of the "camine," and $$F$$ in the same direction nearer to the same line. The centre of gravity of the whole lintel block, in the same way was posited, at a point a little to the westward, of the vertical plane normal to the face of the wall, and passing through it as it had stood erect. The inner face (or cove) of the lintel, was uppermost as it lay on the ground, proving that it had simply been thrown down, without any secondary disturbing force.

Now this overthrow might have been produced either by shock from the south to the north acting upon the mass by its inertia, or by a projecting force, of a shock from north to south, carrying the wall along with it, forcing forward the chimney breast, and then at the return stroke, (or second half of the wave,) projecting it towards the south. Each assumption would lead to different results; but as there exists abundant and quite independent evidence that the general direction of wave movement at Polla was one from north to south, and not the contrary, we must conclude that this "camine," was projected, and thrown by the forward stroke of the wave. The rotation of the side jambs, and the position of final deposit of the lintel, prove that the direction was somewhat, from the east of north to the west of south, and geometrical considerations based on careful measurements on the spot, prove the wave-path to have been 165° W. of north which produced the overthrow.

Referring to Eq. IV., Part I., we can adopt the fall of the pieces of this "camine," as a means to determine, both the angle of emergence, of the wave-path here, and the velocity of the wave particle at its maximum.

The lintel $$G$$ moving through a very small arc at the