Eratosthenes in his writings also dealt with the history of geography and with physical geography. This last branch attracted a number of students about this period and later, as, for example, Agartharchides or Agatharchus of Cnidus (middle of the second century B.C.). Crates of Mallus, in the first half of the same century, expressed the view of the Stoic philosophers that the spherical earth was divided into four inhabited quarters--the Œcumene (the known world), the Antipodes, the Periœci, and the Antœci. Posidonius the Stoic (_c._ 130–50 B.C.), among his studies in various departments of knowledge, included such subjects as the ocean, volcanoes, and earthquakes; he observed the interaction of the sun and moon in their influence on tides. He recalculated the circumference of the earth, but obtained an underestimate considerably further from the truth than Eratosthenes’s overestimate; and, owing to his high scientific reputation, his error persisted in much later work. Posidonius, before settling at Rhodes, had travelled widely in Africa, Spain, and western Europe generally; and both he and Agartharchides, like other writers of this period, and Aristotle before them, recognized the importance of the human side of geography, and the influence of physical environment on the political and social _régime_. Polybius (_c._ 204–122 B.C.) of Megalopolis in Arcadia, historian, statesman, and military commander, was inspired by extensive travel to introduce the results of topographical and geographical studies throughout his history. Among travellers who rank more nearly among explorers there may be mentioned at this period Eudoxus of Cyzicus (fl. _c._ 130 B.C.), who went on a trading expedition to India, in command of a fleet which was despatched by Ptolemy Euergetes of Egypt with the specific object of exploring the Arabian Sea. Eudoxus subsequently tried without success to circumnavigate Africa, making at least two voyages along the east coast. Finally, it must be remembered that at this period the extension of the knowledge of the world was mainly due no longer to Greek, but to Roman, activities, and Roman conquests were pushed beyond the confines of accurate Greek knowledge in various directions. Much geographical material was made available by writers on Roman military expeditions. Thus Pompey was accompanied in the Caucasian region by Theophanes of Mytilene, as historian of his campaigns; Julius Cæsar wrote his own account of lands into which he carried his arms (Gaul, etc.).