The best historical Atlas is the copious German work of VON SPRUNER. FREEMAN'S _Historical Geography of Europe_ is a work of great value. DROVSEN'S _Allg. Hist. Atlas._ Smaller atlases are those of PUTZGER, Rhode, Appleton's _Hist. Atlas_, the _International_, and the _Collegiate_. Smaller still, Keith Johnston's Crown Atlases and Half-Crown Atlases. On Mediæval History, Labberton's Atlas; also, Koeppen: in Ancient Geography, SMITH'S work, KIEPERT'S, Long's. On Physical Geography, GUYOT'S text-books; Vaughan's _Connection between History and Physical Geography_, in _Contemp. Review_, vol. v.; Hall's _Methods of Studying History_, etc., p. 201 _seq._, _Encycl. Brit._, Art. _Geography_. CHRONOLOGY.--An exact method of establishing dates was slowly reached. The invention of eras was indispensable to this end. The earliest definite time for the dating of events was established at Babylon,--the era of Nabonassar, 747 B.C. The Greeks, from about 300 B.C., dated events from the first recorded victory at the Olympic games, 776 B.C. These games occurred every fourth year. Each Olympiad was thus a period of four years. The Romans, though not until some centuries after the founding of Rome, dated from that event; i.e., from 753 B.C. The Mohammedan era begins at the Hegira, or flight of Mohammed from Mecca, 622 A.D. The method of dating from the birth of Jesus was introduced by Dionysius Exiguus, a Roman abbot, about the middle of the sixth century. This epoch was placed by him about four years too late. This requires us to fix the date of the birth of Christ at 4 B.C. The day was the simplest and earliest division of time. The week has been in use for this purpose in the East from time immemorial. It was not introduced among the Romans until after the spread of Christianity in the Empire. The month was the earlier unit for periods of greater length. To make the lunar and the solar years correspond, and to determine the exact length of the solar year, was a work of difficulty, and was only gradually effected. _Julius_ _Cæsar_ reformed the calendar in 46 B.C., the date of the Julian era. This made the year eleven minutes too long. _Pope Gregory XIII_. corrected the reckoning, in 1582, by ordering Oct. 5th to be called the 15th, and instituted the "Gregorian calendar." The change, or the "New Style," was subsequently adopted by Great Britain (in 1752), and by the other Protestant nations. The difference for the present century between the Old and the New Style is twelve days: during the last century it was eleven. The Julian civil year began with Jan. 1. It was not until the eighteenth century that this became the uniform date for the commencement of the legal year among the Latin Christian nations.