One of the most distinguished scholars of the tenth century was Bishop Gerbert (ca. 940-1003), later Pope Sylvester II, of whose learning we possess reliable evidence.[82] His astronomical knowledge so astonished his contemporaries that he was thought to be a necromancer and was accused of being in league with the evil one.[83] He was a diligent student of the literature of antiquity, which had survived to his day, especially surpassing all others, it is reported, in his acquaintance with the learning of pagan Rome. In the instruction which he gave in astronomical science he made use of various instruments, to the end that his pupils might the better understand the subject, among which instruments were celestial globes and armillary spheres. These were a source of much wonderment to his contemporaries. It is said that one of these instruments was so skilfully constructed that even the untrained by its use, having one constellation pointed out, would be able to locate all others “with the aid of a globe and without the aid of a teacher.”[84] In a letter to the monastic teacher Constantius, with whom Gerbert stood in the friendliest relations for many years at Rheims, he refers to the construction of a celestial globe, and in a more detailed manner he makes mention of this when writing to Remigius of Trier. In four of his letters to this last named prelate, Gerbert touches upon his purpose to construct a globe, but on account of the added duties which were his, occasioned by the death of Archbishop Adalbero, he seems not to have been able to complete his work. He expresses himself, in the third one of these letters, as hopeful that a favorable time might yet come for him to take up the plan, but the increasing opposition of his enemies left him no leisure for scientific labors of this character, and it does not appear that he turned his attention again to globe making.[85] The thirteenth century furnishes us with the names of two distinguished princes who were especially active in advancing scientific studies of their times. One of these was the Hohenstaufen Frederick II, concerning whom we are informed that he directed a learned Arabian, who sojourned at his court, to construct for him a celestial globe of gold on which the stars were to be represented by pearls.[86] We are further told that as an outcome of his friendly relations with the rulers of the East, the Sultan of Egypt sent to him an astronomical tent of wonderful construction. In this the sun and the moon were represented and by means of a skilfully constructed mechanism they were made to rise and set, marking out the hours of day and night.[87]