Among the many inventions attributed to Hypatia, besides the planisphere and astrolabe which she designed for the use of astronomers, are several employed in the study of natural philosophy. Probably the most useful of these is an areometer mentioned by her pupil Synesius. He calls it a hydroscope and describes it as having the form and size of a flute, and graduated in such wise that it can be used for determining the density of liquids. That Hypatia was thoroughly familiar with the science of natural philosophy, as then known, there can be no doubt. That she also contributed materially to its advancement, as well as to that of astronomy, in which she always exhibited a special interest, there is every reason to believe.[153] After the death of Hypatia, the study of natural philosophy was almost entirely neglected for more than a thousand years. The first woman in modern times to attract attention by her discussion of physical problems was the famous Marquise du Chatelet, although she was better known as a mathematician and as the translator into the French of Newton's _Principia_. In her chateau at Cirey she had a well-equipped physical cabinet in which she took special delight. But in her time, as in that of Hypatia, natural philosophy was far from being the broad experimental science which it has become through the marvelous discoveries made in heat, light, electricity and magnetism during the last hundred years, as well as through those countless brilliant investigations which have led up to our present doctrine of the correlation and conservation of the various physical forces. There was then no occasion for those delicate instruments of precision which are now found in every physical laboratory by means of which the man of science is able to investigate phenomena and determine laws that were quite unknown until a few years ago. In the time of Mme. du Chatelet, as during the century following, natural philosophy consisted rather in the mechanical and mathematical than in the physical study of nature. This is illustrated by the title of the great work on the translation of which she spent the best years of her life--Newton's immortal _Philosophiae Naturalis Principia Mathematica_.