When positive evidences fail, the only method of research is by synchronism, or fixing the date of an event by its connection with some other event or person of known date. Fortu nately the records afford considerable help in this, if less than could be wished; they mention international battles, captures, treaties, appeals, threats, correspondence, etc., where one date must apply to both. Noted examples of this are the biblical records as to relations of Palestine with Assyria, Egypt, Tyre, etc.: sometimes con fusing events and dates, occasionally confound ing persons, and needing correction from other sources, but still of extreme value. Another specimen is the Assyrian "synchronous history' of the relations between Assyria and Babylonia, a tablet of about 800 n.c. When we come down to classic times, these synchronisms are the very basis of historical work on those periods. There is also a synchronism of arts, products ex changed between countries, systems of writing, etc., and even to some extent of institutions; which, though demanding expert knowledge and careful judgment, is often of the first value. On this basis much of the Homeric and pre-Homeric history of Greece is taking shape, and even the origins of China can perhaps be related to Babylonia.
The• chronological sense of different races presents extreme diversities; but it is .fairly ac curate to say that none of the older ones had any appreciable amount except the Babylonians, and that in them and their pupils, the Assyrians, it was early and remarkably developed. Their dated records (in the primitive fashion above) go back to the third millennium B.C. at least; the Assyrians' eponym canon antedates .by many centuries any similar attempt elsewhere; and the Babylonians were much the first to adopt an epoch, On the other hand, their cofounders of the earliest civilization, the Egyptians, were utterly destitute of it; and the dates in their history back of the time.of close contact with Babylonia, where synchronism can be utilized, are almost pure guesswork 2000 11.C. the divergences of estimated Egyptian dates among the most authoritative scholars vary from 500 to 1,500 years, and even down to 1500 ac. there is often 100 or 200 years' difference; and only the vaguest inferential proof is avail able. The Hindus, the Chinese, the Jews, the Greeks, were nearly as devoid of chronological instinct. The case of the great Greek historians has been mentioned; and the vague Jewish *forty for a generation or an Indefinite long time is familiar and not conducive to accurate chronology.
The obscurity due to lack of dates in general was of course enhanced by the fact that each state counted from its own events or persons, so that in place of one problem, the historical antiquary has scores. The chief method — for many ages the only one — by which this Babel of chronologies was finally reduced to uni formity over large areas was,political absorption and the disuse of provincial systems. Even
when fixed epochs came to be adopted, though the continuity of history was assured, the inter national confusion was not lessened nor com parative chronology made easier, as each selected a different epoch; and the same remedy only was operative here. Nor, indeed, for some time did even the individual system become the accurate instrument of research it is now; for the succession was not at first of years, but of annual magistrates, as with the Assyrians, Athenians and Romans sometimes more vaguely of kings and priests. Moreover, with one dubious exception the date of the epoch was not that of its adoption, but centuries back ; and the dates affixed by the new systems to former events were often wildly fanciful, as was that of the very event which furnished the starting point. The first epoch ever used, so far as known, was the accession of Nabonassar of Babylonia 747 B.C. ; it is generally believed to have been used from the beginning, but this is less probable now than formerly. The Roman era was the founda tion of Rome, somewhere about 750 ac. accord ing to Roman writers ; it is not known when it was first adopted, but probably not before the 2d century s.c. The Greek reckoning was from the alleged foundation of the Olympic games in 776 B.C., quite as apocryphal as the other; it was first used as a basis of calculation by Timaeus of Sicily in the 3.1 century s.c. It furnishes a scientific one when the year of the Olympiad is mentioned, which is not always, so that a mar gin of three years is left. Note the curious fact that all three epochs begin within a quarter cen tury or so of. other, though the last two are merely guesswork. It seems incredible that the year, apparently the most simple and obvious of natural units, was first suggested and used in• 194 ac. by Eratosthenes, the great Greek ed itor, mathematician, geographer and chronog rapher. Of course it had been employed for thousands of years as a unit within other units, such as reigns, but not alone in sequence from a fixed epoch. The Olympiad, however, for rea sons given below, held its own till the middle of the 5th century. Even after the official adop tion of, the Christian era,— which, like its prede cessors, antedated its use by centuries,— cen turies elapsed still before it was employed to the exclusion of other systems. Many other methods of dating, local or ecclesiastical, flour ished besides it, and made nearly as much labor necessary in modern as in ancient times to synchronize dates. An acquaintance with these is indispensable to the study of the chronicles, charters and other legal and Church documents of the Middle Ages.