These same places[1], 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[2] 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[3]. There is also a great difference in the nature of the motions[4], 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[5]; 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.