This seems to be the best explanation of the facts recorded by Mallet. There is, however, a possible difficulty that should not be overlooked--namely, the apparently slight influence of the Montemurro focus on the mean direction of the shock (Fig. 9). At a few places, of course, the mean direction passes through both epicentres; at some others, as we have seen, one of the two observed directions points towards the Montemurro epicentre. It is not impossible, also, that Mallet, after the first few days' work, may occasionally have quite unconsciously selected and measured those fissures from the maze presented to him which agreed most closely with his early impressions obtained from the neighbourhood of Polla. But, for places nearer Polla than Montemurro (and these form the majority of those visited by Mallet), the probable explanation of the difficulty is that the Montemurro focus was not so deep as the Polla focus. This, as will appear more fully in the next chapter, would account for the comparatively great intensity in the immediate neighbourhood of Montemurro and for its rapid decline outwards; and it receives some support from an isolated reference by Mallet to two angles of emergence at Padula, one of 25° from the north, and the other of 8° or 10° in the perpendicular walls. ELEMENTS OF THE WAVE-MOTION. The elements of the wave-motion, as mentioned in the introductory chapter, are four in number, namely, the period, amplitude, maximum velocity, and maximum acceleration. If any two of these are known for each vibration--and the first two are now given by every accurately constructed seismograph--the others can be determined if the vibrations follow the law of simple harmonic motion.[15] _Amplitude._--To ascertain the amplitude, Mallet had to rely chiefly on the fissures made in very inelastic walls. If the parts into which such a wall are fractured are free to move, and yet, being inelastic, obliged to remain in the farthest position to which they are carried by the wave, the distance traversed by the centre of gravity of one of the displaced parts should give a "rude approximate measure" of the horizontal amplitude of the earth-wave. At Certosa, near Padula, he thus found the amplitude to be about 4 inches, at Sarconi about 4-3/4 inches, and at Tramutola about 4-1/2 inches. From somewhat similar evidence, the amplitude at Polla appears to have been about 2-1/2 or 3 inches; and, from the oscillation of a suspended clock or watch on a rough wall, about 3-1/2 inches at La Sala and 1-3/4 inches at Barielle. With the exception of Barielle, these places lie nearly on a straight line passing through Mallet's epicentre, and he gives the following table, showing an increase in amplitude with the distance from the epicentre:-- Polla. La Sala. Certosa. Tramutola. Sarconi. Distance in miles 4.0 13.4 19.0 23.8 30.8 Amplitude in inches 2-1/2 3-1/2 4 4-1/2 4-3/4