Letter 15

To the Philosopher [Hypatia]. I am in such bad luck that I need a hydroscope [an instrument for measuring the specific gravity of liquids]. Please have one cast in brass and assembled for me. The instrument is a cylindrical tube shaped like a flute and about the same size. It has notches in a vertical line that allow you to measure the weight of different liquids. A cone serves as a lid at one end, fitted tightly to the tube — the cone and the tube share a single base. This part is called the baryllium. When you place the tube in a liquid, it floats upright. You can then count the notches at your leisure and determine the specific gravity of the water.

Letter 15: A densimeter [1] To the Philosopher note [Hypatia.] I am in such evil fortune that I need a hydroscope. See that one is cast in brass for me and put together. [2] 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 baryllium. Whenever you place the tube in a liquid, it remains erect. You can then count the notches at your ease, and in this way ascertain the specific gravity of the water.