In addition to her works on astronomy and mathematics, Hypatia is credited with several inventions of importance, some of which are still in daily use. Among these are an apparatus for distilling water, another for measuring the level of water, and a third an instrument for determining the specific gravity of liquids--what we should now call an areometer. Besides these apparatus, she was likewise the inventor of an astrolabe and a planisphere. One of her most distinguished pupils was the eminent Neo-platonist philosopher, Synesius, who became the Bishop of Ptolemais in the Pentapolis of Libya. His letters constitute our chief source of information respecting this remarkable woman. Seven of them are addressed to her, and in four others he makes mention of her. In one of them he writes: "We have seen and we have heard her who presides at the sacred mysteries of philosophy." In another he apostrophizes her as "My benefactress, my teacher,--_magistra_--my sister, my mother." In science Hypatia was among the women of antiquity what Sappho was in poetry and what Aspasia was in philosophy and eloquence--the chiefest glory of her sex. In profundity of knowledge and variety of attainments she had few peers among her contemporaries, and she is entitled to a conspicuous place among such luminaries of science as Ptolemy, Euclid, Apollonius, Diophantus and Hipparchus.[112] It is a matter of regret to the admirers of this favored daughter of the Muses that she is absent from Raphael's _School of Athens_; but, had her achievements been as well known and appreciated in his day as they are now, we can readily believe that the incomparable artist would have found a place for her in this masterpiece with the matchless form and features of his beloved Fornarina. After the death of Hypatia the science of mathematics remained stationary for many long centuries. Outside of certain Moors in Spain, the only mathematicians of note in Europe, until the Renaissance, were Gerbert, afterward Pope Silvester II, and Leonardo da Pisa. The first woman to attract special attention for her knowledge of mathematics was Heloise, the noted pupil of Abelard. According to Franciscus Ambrosius, who edited the works of Abelard and Heloise in 1616, the famous prioress of The Paraclete was a prodigy of learning, for besides having a knowledge of Latin, Greek and Hebrew, which was something extremely rare in her time, she was also well versed in philosophy, theology and mathematics, and inferior in these branches only to Abelard himself, who was probably the most eminent scholar of his age.[113]