The period produced some influential English scholars. Roger Bacon, a philosopher and Franciscan friar, produced works on natural philosophy, astronomy and alchemy; his work set out the theoretical basis for future experimentation in the natural sciences. William of Ockham helped to fuse Latin, Greek and Islamic writing into a general theory of logic; "Ockham's Razor" was one of his oft-cited conclusions. English scholars since the time of Bede had believed the world was probably round, but Johannes de Sacrobosco estimated the circumference of the earth in the 13th century. Despite the limitations of medieval medicine, Gilbertus Anglicus published the "Compendium Medicinae", one of the longest medical works ever written in Latin. Prominent historical and science texts began to be translated into English for the first time in the second half of the 14th century, including the "Polychronicon" and "The Travels of Sir John Mandeville". The universities of Oxford and Cambridge were established during the 11th and 12th centuries, drawing on the model of the University of Paris.