It is equally erroneous when one says, with Muschenbroek and others, that those who among the Romans made it their employment to examine the quality of water with the hydrometer, were called _baryllistæ_ or _barynilæ_. These words do not occur in the works of the ancient Latin authors, nor in any of the completest dictionaries. We read only the following passage in some editions of the Commentary of Servius upon Virgil: “Scrutatores et repertores aquarum (aquilices dicuntur) barinulas dixerunt[416].” If these words were really written by Servius, who lived in the fifth century, he either confounded the water-searchers, _aquilices_, those who sought for springs, with those who examined the nature of water when found, as the hydrometer was of no service to the former in their business; or both employments must at that time have been followed by the same people, and these must have acquired their name from a part only of one instrument they used, which is not at all probable. I think we may with certainty believe that the hydrometer was not known to Seneca, Pliny, or Galen, who died about the end of the second century. Were not this the case, it would certainly have been mentioned by the first, where he speaks so minutely of the specific gravity of hard and fluid bodies[417]; by the second, where he says that the weight of water was ascertained by a common balance[418]; and by the last, where he gives directions how to discover its lightness. Galen adds, that in his time a method had been invented of determining the quality of salt-lye by placing an egg in it, and observing whether it floated[419]. Have we not reason to think that on this occasion the hydrometer must have occurred to him had it been then used? But however well-known it may have been in the fifth century, it seems that it was afterwards entirely forgotten, and that towards the end of the sixteenth it was again for the first time revived or invented anew. To George Agricola it was scarcely known; for where he speaks of the weight of different kinds of water, and particularly of that of salt springs[420], he does not mention it. Constantin, however, who lived at the same time, must have been acquainted with it, else he could not have explained the before-mentioned passages of Synesius and Priscian.