Did she invent the Hydroscope (Hydrometer)?
Let’s see now how the literature pans out regarding her alleged invention of the hydroscope…
In letter no. 15 of Fitzgerald’s translation of Synesius’ letters, it appears that Synesius required Hypatia to make a hydroscope (which we now call a hydrometer or densimeter for measuring the weight of water). His purpose is not stated, but the tone with which he writes suggests that hydroscope was something she might have known about and that the minimal specifications were all she needed to fulfill his order.
I am in such evil fortune that I need a hydroscope. See that one is cast in brass for me and put together. The instrument in question is a cylindrical tube, which has the shape of a flute and is about the same size. It has notches in a perpendicular line, by means of which we are able to test the weight of the waters. A cone forms a lid at one of the extremities, closely fitted to the tube. The cone and the tube have one base only. This is called the beryllium. Whenever you place the tube in water, it remains erect. You can then count the notches at your ease, and in this way ascertain the weight of the water. (page 99)