The greatest aid in fixing Egyptian chronology is the “Sothic Cycle.” At an early date the Egyptians adopted a calendar which made up a year of 365 days. Their year originally began when the rapid rising of the Nile coincided with the rising of the star Sirius, called by them Sothis. These events coincided on July 19th. As their calendar made no allowance for leap year, in four years their new year began a day too soon, in eight years two days too soon, and so on. In 1,460 years (_i. e._, 365 × 4) their New Year’s Day would make a complete circuit of the year. These periods of 1,460 years are called Sothic Cycles. Censorinus, in Chapters XVIII and XXI of his _De Die Natali_, written in 238 A. D., tells us that a new Sothic cycle began at some time between 140 and 144 A. D. If a new cycle began in 140 A. D., the previous one began in 1320 B. C.; the one before that, in 2780 B. C.; and the one before that,--if they had their calendar so early,--in 4240 B. C. Reisner holds that the Egyptians adopted their calendar in 2780 B. C., but Meyer and Breasted hold that it is unthinkable that they should have been without a calendar until that time, as by that date the civilization of the pyramid builders was at its height; they accordingly maintain that the Egyptian calendar was adopted in 4240 B. C. An illustration will show how the Sothic cycle helps in determining dates. A priest in the 120th year of the twelfth dynasty wrote a letter to his subordinates, to inform them that the rising of Sothis would occur on the fifteenth day of the eighth month. As there were thirty days in each month, the year diverged at this time 225 days. This date, then, was just 900 years after the beginning of the cycle in 2780 B. C.; _i. e._, the letter was written in the year 1880 B. C. It proves that the twelfth dynasty began in 2000 B. C., and fixes for us all the dates of that dynasty. The calendar in the so-called _Papyrus Ebers_ shows that in the tenth year of Amenophis I, of the eighteenth dynasty, the divergence had increased to 308 days. This must have been 1,232 years after the beginning of the cycle, which was the year 1548 B. C. Data gained from these sources are supplemented by what is called dead reckoning; _i. e._, by adding together all the specific dates of the length of reigns which are given in the inscriptions, and testing them by collateral references. Meyer and Breasted have worked out the chronology from these data in this way. Meyer places the accession of Mena at 3200 B. C., while Breasted places it at 3400 B. C. This difference is slight when compared with the differences in the chronologies of the older Egyptologists. =6. Outline of History.=[3]--The history of Egypt, as it concerns our subject, extends over a period of five thousand years. It falls into twelve periods: