About the same period John Sacrobosco, of Holywood, a native of Halifax, in Yorkshire, who was educated at Oxford, and taught mathematics and philosophy at Paris, made an abridgment of the amalgamist of Ptolemy, and of the commentaries of the Arabs, which was long famous as an elementary book under the title of “De Sphira Mundi.” He died at Paris, in the year 1235. In the same year, Roger Bacon, an English Franciscan friar, made astonishing discoveries in science for the time he lived. He perceived the error in the Kalendar of Julius Cæsar, and proposed a plan, for the correction of it, to Pope Clement IV. in 1267. He is presumed from his writings to have known the use of optical glasses, and the composition and effects of gunpowder. He believed in planetary influence on men’s fortunes, and the transmutation of metals. On account of his vast knowledge in astronomy, mathematics, and chemistry, he was called Doctor _Mirabilis_; but, for the same reason, he was suspected of magic. Under this pretext, whilst at Paris, he was put in prison by order of the Pope’s legate; and after a long and severe confinement, he was at last, by the interest of several noble persons, liberated, returned to England, and died at Oxford in 1292, in the seventy-eighth year of his age. In the fifteenth century two events happened which changed the face of the sciences; the invention of printing, about 1440, and the taking of Constantinople by the Turks in 1453. The learned men of that city having escaped from the cruelty of the victors, fled into Italy, and again introduced the taste for classical literature; which was greatly promoted by the munificence of the Emperor Frederick III., Pope Nicholas V., and particularly of Cosmo de Medici, who justly merited the title of Father of his Country, and Patron of the Muses. The restoration of astronomy began in Germany. The first who distinguished himself, was George Purbach, born at Purbach, on the confines of Austria and Bavaria, in 1423, who was cut off in the flower of his age--only thirty-eight years old. He was succeeded by a scholar more skilful than himself, John Muller, born at Konigsberg, in 1436, who taught mathematics and astronomy with great reputation at Vienna. In February, 1471, appeared a comet, on which he published his observations. Being called to Rome by Pope Sextus IV., to assist in correcting the Kalender, he was cut off by the plague, in 1476. Bernard Waltherus, a rich citizen of Nuremberg, his friend and associate, succeeded him, who is said to have first made use of clocks in his astronomical observations, in 1484, and to have been the first of the moderns who perceived the effects of the refraction of light. Fracastorius, born at Verona, in 1483, was a celebrated astronomer, and an eminent poet and good philosopher; he made considerable discoveries in this science, and with all his abilities may be considered as the precursor of the celebrated Copernicus.