These same places[547], however, afford protection, and this is also the case where there is a number of caverns, for they give vent to the confined vapour, a circumstance which has been remarked in certain towns, which have been less shaken where they have been excavated by many sewers. And, in the same town, those parts that are excavated[548] are safer than the other parts, as is understood to be the case at Naples in Italy, the part of it which is solid being more liable to injury. Arched buildings are also the most safe, also the angles of walls, the shocks counteracting each other; walls made of brick also suffer less from the shocks[549]. There is also a great difference in the nature of the motions[550], where various motions are experienced. It is the safest when it vibrates and causes a creaking in the building, and where it swells and rises upwards, and settles with an alternate motion. It is also harmless when the buildings coming together butt against each other in opposite directions, for the motions counteract each other. A movement like the rolling of waves is dangerous, or when the motion is impelled in one direction. The tremors cease when the vapour bursts out[551]; but if they do not soon cease, they continue for forty days; generally, indeed, for a longer time: some have lasted even for one or two years. CHAP. 85. (83.)—PRODIGIES OF THE EARTH WHICH HAVE OCCURRED ONCE ONLY. A great prodigy of the earth, which never happened more than once, I have found mentioned in the books of the Etruscan ceremonies, as having taken place in the district of Mutina, during the consulship of Lucius Martius and Sextus Julius[552]. Two mountains rushed together, falling upon each other with a very loud crash, and then receding; while in the daytime flame and smoke issued from them; a great crowd of Roman knights, and families of people, and travellers on the Æmilian way, being spectators of it. All the farm-houses were thrown down by the shock, and a great number of animals that were in them were killed; it was in the year before the Social war; and I am in doubt whether this event or the civil commotions were more fatal to the territory of Italy. The prodigy which happened in our own age was no less wonderful; in the last year of the emperor Nero[553], as I have related in my history of his times[554], when certain fields and olive grounds in the district of Marrucinum, belonging to Vectius Marcellus, a Roman knight, the steward of Nero, changed places with each other[555], although the public highway was interposed. CHAP. 86. (84.)—WONDERFUL CIRCUMSTANCES ATTENDING EARTHQUAKES.