The trade of the Arabs, their wide conquests, and their religious pilgrimages to Mecca vastly increased their knowledge of the world. They were the best geographers of the Middle Ages. An Abbasid caliph, the son of Harun-al-Rashid, had the Greek _Geography_ of Ptolemy [25] translated into Arabic and enriched the work with illuminated maps. Arab scholars compiled encyclopedias describing foreign countries and peoples, constructed celestial spheres, and measured closely the arc of the meridian in order to calculate the size of the earth. There is some reason to believe that the mariner's compass was first introduced into Europe by the Arabs. The geographical knowledge of Christian peoples during the Middle Ages owed much, indeed, to their Moslem forerunners. EDUCATION Schools and universities flourished in Moslem lands when Christian Europe was still in the "Dark Ages." The largest institution of learning was at Cairo, where the lectures of the professors were attended by thousands of students. Famous universities also existed in Bagdad and Cordova. Moslem scholars especially delighted in the study of philosophy. Arabic translations of Aristotle's [26] writings made the ideas of that great thinker familiar to the students of western Europe, where the knowledge of Greek had all but died out. The Arabs also formed extensive libraries of many thousands of manuscripts, all carefully arranged and catalogued. Their libraries and universities, especially in Spain, were visited by many Christians, who thus became acquainted with Moslem learning and helped to introduce it into Europe. CHEMISTRY AND MEDICINE The Arabs have been considered to be the founders of modern experimental science. They were relatively skillful chemists, for they discovered a number of new compounds (such as alcohol, aqua regia, nitric acid, and corrosive sublimate) and understood the preparation of mercury and of various oxides of metals. In medicine the Arabs based their investigations on those of the Greeks, [27] but made many additional contributions to the art of healing. They studied physiology and hygiene, dissected the human body, performed difficult surgical operations, used anaesthetics, and wrote treatises on such diseases as measles and smallpox. Arab medicine and surgery were studied by the Christian peoples of Europe throughout the later period of the Middle Ages. [Illustration: INTERIOR OF THE MOSQUE OF CORDOVA The great mosque of Cordova, begun in the eighth century, was gradually enlarged during the following centuries to its present dimensions, 570 by 425 feet. The building, one of the largest in the world, has now been turned into a cathedral. The most striking feature of the interior is the forest of porphyry, jasper, and marble pillars supporting open Moorish arches. Originally there were 1200 of these pillars, but many have been destroyed.] MATHEMATICS AND ASTRONOMY