Before beginning to read Japanese history it is necessary to know something of the chronology followed in its pages. There have been in Japan four systems for counting the passage of time. The first is by the reigns of the Emperors. That is to say, the first year of a sovereign's reign--reckoning from the New Year's day following his accession--became the 1 of the series, and the years were thenceforth numbered consecutively until his death or abdication. This method might be sufficiently accurate if the exact duration of each reign were known as well as the exact sequence of the reigns. But no such precision could be expected in the case of unwritten history, transmitted orally from generation to generation. Thus, while Japanese annalists, by accepting the aggregate duration of all the reigns known to them, arrive at the conclusion that the first Emperor, Jimmu, ascended the throne in the year 660 B.C., it is found on analysis that their figures assign to the first seventeen sovereigns an average age of 109 years. The second system was by means of periods deriving their name (nengo) from some remarkable incident. Thus, the discovery of copper in Japan was commemorated by calling the year Wado (Japanese copper), and the era so called lasted seven years. Such a plan was even more liable to error than the device of reckoning by reigns, and a specially confusing feature was that the first year of the period dated retrospectively from the previous New Year's day, so that events were often recorded as having occurred in the final year of one period and in the opening year of another. This system was originally imported from China in the year A.D. 645, and is at present in use, the year 1910 being the forty-third of the Meiji (Enlightenment and Peace) period.