spheres were large and normally used for display, whereas a plane astrolabe was portable and could be used for practical measurements. The statement from Synesius's letter has sometimes been wrongly interpreted to mean that Hypatia invented the plane astrolabe herself, but the plane astrolabe was in use at least 500 years before Hypatia was born. Hypatia may have learned how to construct a plane astrolabe from her father Theon, who had written two treatises on astrolabes: one entitled Memoirs on the Little Astrolabe and another study on the armillary sphere in Ptolemy's Almagest. Theon's treatise is now lost, but it was well known to the Syrian bishop Severus Sebokht (575–667), who describes its contents in his own treatise on astrolabes. Hypatia and Theon may have also studied Ptolemy's Planisphaerium, which describes the calculations necessary in order to construct an astrolabe. Synesius's wording indicates that Hypatia did not design or construct the astrolabe herself, but merely acted as a guide and mentor during the process of constructing it. In another letter, Synesius requests Hypatia to construct him a "hydroscope", a device now known as a hydrometer, to determine the density or specific gravity of liquids. Based on this request, it has been claimed that Hypatia invented the hydrometer herself. The minute detail in which Synesius describes the instrument, however, indicates that he assumes she has never heard of the device, but trusts she will be able to replicate it based on a verbal description. Hydrometers were based on Archimedes' 3rd century BC principles, may have been invented by him, and were being described by the 2nd century AD in a poem by the Roman author Remnius. Although modern authors frequently credit Hypatia with having developed a variety of other inventions, these other attributions may all be discounted as spurious. Booth concludes, "The modern day reputation held by Hypatia as a philosopher, mathematician, astronomer, and mechanical inventor, is disproportionate to the amount of surviving evidence of her life's work. This reputation is either built on myth or hearsay as opposed to evidence. Either that or we are missing all of the evidence that would support it." Legacy Antiquity Neoplatonism and paganism both survived for centuries after Hypatia's death, and new academic lecture halls continued to be built in Alexandria after her death. Over the next 200 years, Neoplatonist philosophers such as Hierocles of Alexandria, John Philoponus, Simplicius of Cilicia, and Olympiodorus the