If we take 4 inches for the amplitude of the largest variation, and 12 feet per second for the maximum velocity, and assume the motion to have been of a simple harmonic character, the period of a complete vibration would be less than one-fifth of a second.[17] Now, we know from seismographic records that this is roughly the period of the small tremors that form the commencement of an earthquake-shock, while the period of the largest vibrations may amount to as much as one or two seconds. We may therefore conclude either that the assumption of simple harmonic motion is incorrect, or that the maximum velocity is too great, or more probably perhaps that the amplitude is too small.[18]
SOUND-PHENOMENA.
Mallet was one of the first seismologists to realise the significance of the earthquake-sound; and he attended closely to the subject, though finding the sound even more elusive of precise observation than the shock.
The chief result obtained by him was the comparative smallness of the area over which the sound was heard. He estimates it at little more than 3,300 square miles, or about one-twelfth of that over which the shock was felt. It extends north and south from Melfi to Lagonegro, and east and west from Monte Peloso to Duchessa and Senerchia. The sound was thus confined to the region in which the shock attained its most destructive character.
Towards the north and south ends of the sound-area all observers described the sound as a low, grating, heavy, sighing rush, lasting from twenty to sixty seconds, some adding that it was also of a rumbling nature. Near the centre and the east and west boundaries, the sound was distinctly more rumbling; it was shorter in duration, and began and ended more abruptly.
The earthquake, Mallet remarks, "began everywhere with tremors; the sounds generally arrived at the same time; the apparent direction of movement of the tremulous oscillations appeared rapidly to change, and still more rapidly to increase in amplitude; then the great shove of the destructive shock arrived, in some places rather before, in some a little after, the moment of loudest sound, and it died away suddenly (i.e., with extreme rapidity) into tremors again, but differing in direction from that of the great shock itself."[19]
The earthquake-sound will be described more fully in the chapter dealing with the Hereford earthquake of 1896, in which it will be found that the phenomena recorded by Mallet are equally characteristic of the slighter shocks felt in this country.
VELOCITY OF THE EARTH-WAVES.
In 1857 little was known about the velocity of earthquake-waves. Experiments had been made by Mallet himself in 1849 in the neighbourhood of Dublin. These gave 825 feet per second for the velocity in dense wet sand, 1,306 feet per second in discontinuous granite, and 1,665 feet per second in more solid granite.[20] The only earthquake for which the velocity had been calculated was the Rhenish earthquake of 1846, the value ascertained by Schmidt being 1,376 French feet, or 1,466 English feet, per second.
The accurate public measurement of time, which, as Mallet remarks, is one of the surest indications of advancing civilisation, was, however, unknown in the kingdom of Naples; and his attempt was therefore fettered by the rarity of precise estimates of the time of occurrence. Throughout the whole disturbed area only six good records could be obtained, and three of these (at Vietri di Potenza, Atella, and Naples) were derived from stopped clocks, witnesses of rather doubtful value. At Montefermo and Barielle the time was at once read from a watch, and at Melfi from an accurate pocket chronometer. The times given vary from 9h. 59m. 16s. P.M. (Naples mean time) at Vietri di Potenza to 10h. 7m. 44s. at Naples. Allowing for the supposed change of direction by refraction at the Monte St. Angelo range on the way to Naples, Mallet finds the mean surface velocity to be 787 feet per second. Omitting the Naples record, and taking account of the calculated depth of the focus, the mean velocity becomes 804 feet per second.