This poem was once published together with Celsus De Re Medica, in 1566, by Robert Constantin, who died at an advanced age in 1605, and who added a few, but excellent notes, which have been inserted by Wernsdorf in his edition. This Constantin seems to have known that the instrument of Priscian and the _hydroscopium_ of Synesius were the same; and that they were used for determining the weight of fluids. He explains the use of them very properly; but is mistaken in supposing the cone to have been of wood, though it served to render the lower part of the instrument heavier, as the poet himself says: “cui cono interius modico pars ima gravatur.” I am almost induced to think that _interius_ implies that additional weight was given to the cone by throwing some small heavy bodies into it, through the opening above; and at present grains of leaden shot are employed for that purpose. It appears therefore that the honour of having first given a good explanation of the before-quoted passage of Synesius belongs rather to Constantin than to Fermat; but I can readily believe that the latter was not acquainted with the observations made on it by the former. Before I conclude the history of this instrument among the ancients, I shall add two remarks further. It is evidently wrong when one, with Muschenbroek and others, whose opinion I adopted before I engaged in this research, considers Hypatia as the inventress of the hydrometer. It was known at her time, and was made at Alexandria; but it seems not to have been very common, as Synesius wrote to Hypatia to procure him one, and even thought it necessary to give her a description of it. Those are mistaken likewise, who say that this instrument was called also _baryllium_. That word, as far as I have been able to learn, occurs only in Synesius, who expressly tells us that the small heavy cone alone was meant by it. In the same manner has it been understood by Constantin. In the Dictionary of Basle it is said to be _hydroscopii pars_; and in Stephen’s Dictionary it is explained by _pondusculum_, as well as in that of Ernest, where it is given as the diminutive of _baros_. It signified therefore the heavy part of the hydrometer only.