In the nineteenth century, researches in natural history had revealed the fact that secular vertical oscillations, vary with the locality, were taking place in the earth’s crust, and had proved that, from prehistoric times, the soil of western and southern France had been slowly sinking and the sea slowly gaining upon the land. One after another, the islands of Jersey, of Minquiers, of Chaussey, of Écrehou, of Cezembre, of Mont-Saint-Michel, had been detached from the continent by the sea; the cities of Is, Helion, Tommen, Portzmeûr, Harbour, Saint Louis, Monny, Bourgneuf, La Feillette, Paluel and Nazado had been buried beneath its waves, and the Armorican peninsula had slowly retreated before the advancing waters. The hour of this invasion by the sea had struck, from century to century, also for Herbavilla; to the west of Nantes; for Saint-Denis-Chef-de-Caux, to the north of Havre; for Saint-Etienne-de-Paluel and for Gardoine, to the north of Dol; for Tolente, to the west of Brest; more than eighty habitable cities of Holland had been submerged in the eleventh century, etc., etc. In other regions the reverse had taken place, and the sea had retired; but to the north and west of Paris this double action of the subsidence of the land and the wearing away of the shores had, in less than seven thousand years, made Paris accessible to ships of the greatest tonnage. As in the case of languages, ideas, customs and laws, so, also, the manner of reckoning time had changed. It was still reckoned by years and centuries, but the Christian era had been discarded, as also the holy days of the calendar and the eras of the Mussulman, Jewish, Chinese and African chronologies. There was now a single calendar for the entire race, composed of twelve months, divided into four equal trimesters of three months of thirty-one, thirty, and thirty days, each trimester containing exactly thirteen weeks. New Year’s Day was a fête day, and was not reckoned in with the year; every bisextile year there were two. The week had been retained. Every year commenced on the same day—Monday; and the same dates always corresponded to the same days of the week. The year began with the vernal equinox all over the world. The era, a purely astronomical division of time, began with the coincidence of the December solstice with perihelion, and was renewed every 25,765 years. This rational method had succeeded the fantastic divisions of time formerly in use.