What early Christianity lost by looking askance at classical theory the Arabs, after the establishment of Muhammadan power in the seventh century, in great measure gained, for they were free of scruple as regarded the earlier learning, and eager for knowledge. Early in the ninth century the Caliph Al-Mamun of Bagdad gave a strong impulse to geographical and kindred studies. He caused translations to be made of Ptolemy’s astronomical and geographical works, and of those of other ancient authorities, among whom was Marinus of Tyre. Muhammad ben Musa, librarian of Bagdad, compiled a _Description of the World_, or gazetteer of place-names with their positions, on the Ptolemaic model. Degrees were measured in Syria and Mesopotamia. Among Arab descriptive works the first which survives is that of Suleiman, a merchant, who made voyages to India and China in the middle of the ninth century. In the first half of the following century Masudi travelled widely--to India and Ceylon, probably to China, to Madagascar, to the Caspian, and in Syria and Egypt. His work, the _Meadows of Gold and Mines of Precious Stones_, is an example of the application of the results of travel and personal observation to history; he was commonly compared with Pliny. The Muhammadan demand for geographical works at this period appears to have been great, from the fact that Abu Zaid’s work, written about 921, was revised thirty years later by Istakhri, who travelled all through the Muhammadan lands, in his _Book of Climates_ (or Zones); and this was again revised and extended by Ibn Haukal in 977 in his _Book of Roads and Kingdoms_. A more important figure is Idrisi (_c._ 1099–1154), an Arab of Spanish birth, who was probably educated at the great centre of learning, Cordova. He travelled in North Africa and Asia Minor, and, settling in Sicily, made a celestial sphere and a map of the world in silver for King Roger II. Idrisi conceived a substitute for a projection by dividing the inhabited world into seven climates or zones between the equatorial line and the arctic region; each of these was divided into eleven equal parts by perpendicular lines. The squares thus formed were used (to the disregard of natural or political divisions) in a description of the earth carried out by Idrisi for the king, which is noteworthy as having been put together, at least in part, from the reports of an organized system of observers, who were despatched at Roger’s order to various countries. Arabian cartography, however, so far as is known, was primitive. Yet the Arab astronomers made some close meridianal determinations--an error of only three degrees between Toledo and Bagdad, for example, and a calculation of the major axis of the Mediterranean which was very near the truth.