Wearmouth and Jarrow who wrote a work On the Nature of Things, several books on the mathematical / astronomical subject of computus, the most influential entitled On the Reckoning of Time. He made original discoveries concerning the nature of the tides and his works on computus became required elements of the training of clergy, and thus greatly influenced early medieval knowledge of the natural world. Rabanus Maurus (c. 780 – 856), a Christian monk and teacher, later archbishop of Mainz, who wrote a treatise on Computus and the encyclopedic work De universo. His teaching earned him the accolade of "Praeceptor Germaniae," or "the teacher of Germany." Leo the Mathematician (c. 790 – after 869) was a Byzantine philosopher and logician associated with the Macedonian Renaissance. He wrote book-length works, poems, and many epigrams, and was also a compiler, who brought together a wide range of philosophical, medical, and astronomic texts. He composed his own medical encyclopaedia. He was archbishop of Thessalonica and later became the head of the Magnaura School of philosophy in Constantinople, where he taught Aristotelian logic. Abbas Ibn Firnas (810 – 887), a polymath and inventor in Al-Andalus, made contributions in a variety of fields and is most known for his contributions to glass-making and aviation. He developed novel ways of manufacturing and using glass. A 17th century account states he broke his back at an unsuccessful attempt at flying in 875. Pope Sylvester II (c. 946–1003), a Christian scholar, teacher, mathematician, and later pope, reintroduced the abacus and armillary sphere to Western Europe after they had been lost for centuries following the Greco-Roman era. He was also responsible in part for the spread of the Hindu–Arabic numeral system in Western Europe. Maslamah al-Majriti (died 1008), a mathematician, astronomer, and chemist in Al-Andalus, made contributions in many areas, from new techniques for surveying to updating and improving the astronomical tables of al-Khwarizmi and inventing a process for producing mercury oxide. He is most famous, though, for having helped transmit knowledge of mathematics and astronomy to Muslim Spain and Christian Western Europe. Abulcasis (936-1013), a physician and scientist in Al-Andalus, is considered to be the father of modern surgery. He wrote numerous medical texts, developed many innovative surgical instruments, and developed a variety of new surgical techniques and practices. His texts were considered the definitive works on surgery in Europe until the Renaissance. High Middle Ages Michael Psellos