Chronology is the science of computing and adjusting the periods of time. It ascertains when events occurred, and assigns to each its correct date. Thus we learn from it that the world was created 4004 years before Christ, and that the flood took place 1656 years after the creation; and so of all other known and ascertained events, each one is placed in connection with its proper period or year. Of the transactions between the Creation and the Flood, we know nothing except from Scripture, and of many of those which occurred after the flood, and before the time of Christ, we know nothing with certainty, except from the same source; but about 800 or 900 years before our Saviour’s time, a succession of profane historians arose, from whom, especially those of Greece and Rome, numerous facts in Chronology have been obtained. Various Eras, Epochs, or methods of Chronology, have been adopted by different nations. The Greeks reckoned time by Olympiads of four years each, commencing from the year 776 before Christ. In marking a date by this method, the year and Olympiad were both given; for example, the year 1845 is the first of 656th Olympiad. The Romans reckoned time from the founding of Rome, 753 years before Christ. Dates reckoned from this Era are designated by the initials A. U. C. (ab urbe condita; that is, from the building of the city). The year 1845 is the 2598th year of the Roman Era. The Christian Era, now in use amongst all Christian nations, was first introduced in the sixth century, but was not very generally adopted for some centuries after. This begins 4004 years after the creation of the world, and four years after the birth of our Saviour. Dates reckoned backwards are usually marked B.C., or before Christ, but those reckoned forward are distinguished by the prefix A.D., signifying Anno Domini, or in the year of our Lord. The Mahomedans reckon time from the Hegira, or flight of Mahomet from Mecca to Medina, in the year 622 after Christ; but they use the lunar year of 354 days; so that thirty-two of our years make thirty-three of theirs. The year 1845 is the 1260th year of the Hegira. Many other epochs or eras have been used in different countries, and at different periods. The Jews, Egyptians, Tyrians, Babylonians, Persians, and other ancient nations, have each had their eras. The Hindoos and Chinese of the present day have modes of reckoning time which differ from each other, as well as from our method. In the United States, public documents, proclamations, &c. have often, besides the date in common use, the year of the national independence attached to them. This is computed from July 4th, 1776, and hence may be reckoned a national era or chronological period.