Again, in an unfinished tunnel, about three-quarters of a mile long, between the harbour of Genoa and the eastern railway-station, the vibrations were very slightly felt. Even in the tunnels traversed by the coast railway from Genoa to Nice--that is, in those situated within the meizoseismal area--the shock was either very weak or not felt at all, and not one of the tunnels suffered the slightest injury. To men at work inside a long tunnel, the conditions for observing earthquakes are somewhat imperfect, but these facts, nevertheless, bring out very clearly the inferior intensity of the shock at some depth below the surface. AFTER-SHOCKS. While the unfelt earth-waves of the great earthquake were still wending their way over the zone that surrounds the disturbed area, the central regions were again shaken, at 6.29 A.M., by a shock strong enough to produce fresh ruins in the stricken towns along the coast. Nearly two and a half hours of quiet followed, broken only by a few subterranean rumblings in the central part of the meizoseismal area. Then, at 8.51 A.M., occurred another shock, short and sharp, and inferior in strength only to the principal earthquake. Both of these after-shocks were felt in Western Switzerland; indeed, they were perceptible nearly as far as the great shock; the second, however, a little farther than the first, for it alone was noticed at such places as Vicenza, Forlì, and Florence. The shock at 6.29 was usually described as long and its vibrations as undulatory only; that at 8.51 as rather subsultory than undulatory and of very brief duration. The latter, however, was followed after an interval of a few seconds by another shock so weak that it generally passed unobserved. Both shocks were preceded by a rumbling sound. During the next two days, tremors and earth-sounds were frequent in the Riviera; once an hour, on an average, the greater part of the meizoseismal area was shaken by vibrations more or less slight. But, between one shock and another, at Diano Marina and Alassio, and even as far as Nice, it only required attention from a careful observer to perceive an almost continual throbbing of the ground. Only one of these shocks, that of February 24th, at 2.10 A.M., was strong enough to cause slight damage to buildings. It disturbed an area, not exceeded by any of the later shocks, the boundary of which, shown by the dotted line A in Fig. 33, extends to the north and east as far as Piacenza and Spezia, while to the west it includes Cannes. The centre of the curve so drawn lies on land, but, as the shock was not felt in Corsica, there is no evidence as to the southerly extension of the disturbed area; and it is probable, as Professor Mercalli suggests, that the shock originated in the eastern or Oneglia focus of the great earthquake.