The earth is shaken in various ways, and wonderful effects are produced[539]; in one place the walls of cities being thrown down, and in others swallowed up by a deep cleft[540]; sometimes great masses of earth are heaped up, and rivers forced out, sometimes even flame and hot springs[541], and at others the course of rivers is turned. A terrible noise precedes and accompanies the shock[542]; sometimes a murmuring, like the lowing of cattle, or like human voices, or the clashing of arms. This depends on the substance which receives the sound, and the shape of the caverns or crevices through which it issues; it being more shrill from a narrow opening, more hoarse from one that is curved, producing a loud reverberation from hard bodies, a sound like a boiling fluid[543] from moist substances, fluctuating in stagnant water, and roaring when forced against solid bodies. There is, therefore, often the sound without any motion. Nor is it a simple motion, but one that is tremulous and vibratory. The cleft sometimes remains, displaying what it has swallowed up; sometimes concealing it, the mouth being closed and the soil being brought over it, so that no vestige is left; the city being, as it were, devoured, and the tract of country engulfed. Maritime districts are more especially subject to shocks. Nor are mountainous tracts exempt from them; I have found, by my inquiries, that the Alps and the Apennines are frequently shaken. The shocks happen more frequently in the autumn and in the spring, as is the case also with thunder. There are seldom shocks in Gaul and in Egypt; in the latter it depends on the prevalence of summer, in the former, of winter. They also happen more frequently in the night than in the day. The greatest shocks are in the morning and the evening; but they often take place at day-break, and sometimes at noon. They also take place during eclipses of the sun and of the moon, because at that time storms are lulled. They are most frequent when great heat succeeds to showers, or showers succeed to great heat[544]. CHAP. 83. (81.)—SIGNS OF AN APPROACHING EARTHQUAKE. There is no doubt that earthquakes are felt by persons on shipboard, as they are struck by a sudden motion of the waves, without these being raised by any gust of wind. And things that are in the vessels shake as they do in houses, and give notice by their creaking; also the birds, when they settle upon the vessels, are not without their alarms. There is also a sign in the heavens; for, when a shock is near at hand, either in the daytime or a little after sunset, a cloud is stretched out in the clear sky, like a long thin line[545]. The water in wells is also more turbid than usual, and it emits a disagreeable odour[546]. CHAP. 84. (82.)—PRESERVATIVES AGAINST FUTURE EARTHQUAKES.