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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.

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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; by the second, where he says that the weight of water was ascertained by a common balance; 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.

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completely and accurately as possible. It contains much useful information, as well as many fragments of works now lost; and on this account cannot well be entirely dispensed with.

* Quæst. nat. iii. 25. P. 726.

↑ Hist. nat. xxxi. 3. sect. 23. p. 552: Quidam statera judicant de salubritate, frustrante diligentia, quando perrarum est, ut levion sit aliqua. Athen. ii. p. 46. Plutarchi Quæst. nat. 7.

De simplic. med. facultatibus, iv. 20. p. 61. ed. Gesneri.

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, 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.

I am inclined to think that the first account of the hydrometer being again brought into use, must be found in the oldest German books on saltworks. It is, at any rate, certain that from these the modern philosophers became first acquainted with it. One of the earliest who has described it is the Jesuit Cabeus, who wrote about the year

Quin et modum jam invenerunt, moderatam ad saliendum conficiendi salsuginem, si ovum in ea videatur natare. Nam ubi etiamnum sidit, ac nondum super salsuginis superficiem innatat, aquosa magis est et dulcis; graviter vero salsa, ubi tanta est salis copia indita, ut amplius liquari qui postea adjicitur nequeat. Αλλα και μετρον ηδη τι πεποίηνται του την ακμην εύκρατον ὑπάρχειν εις τας ταρίχειας, ει φαίνοιτο κατ' αυτην επιπλεον ωον. This passage occurs in the Greek edition of Basle, part ii. p. 52. 49.

* De natura eorum quæ effluunt ex terra, lib. ii. p. 124.

1644; but he confesses that he acquired his information from a German treatise by Tholden, whom Kircher† calls a German artist. He was, however, not properly an artist. He was a native of Hesse; a good chemist for his time; and resided about the year 1600 or 1614 as overseer of the salt-works at Frankenhausen in Thuringia. His treatise, which Cabeus had in his possession, was entitled Tholden's Haligraphia, printed at Leipsic in 1603. Another edition, printed at the same place in 1613, is mentioned by Draudius; but at present I have not been able to find it; and can say only from Cabeus and Leupold, that Tholden's hydrometer had a weight suspended to it; and that he speaks of the instrument not as a new but a well-known invention, and on that ac count has described it only imperfectly.

Kircher, whose works were generally read, seems to have principally contributed towards making it publicly known; and Schott, Sturm § and others, in their account of it, refer to his writings.

* Philosophia experimentalis, sive Commentaria in Aristotelis Meteorolog. lib. ii. textus 26. quæst. 2, tom. ii. p. 158, b. Inveni hoc instrumentum positum a quodam Jo. Tholden, in libello Germanice scripto de sale; sed aut auctor ille non intellexit causam et formam instrumenti, aut certe occultare voluit, non vulgare, nec publicum facere.

+ Mundus subterraneus, vol. i. p. 254; and also Physiologia Kircheriana, Amstelod. 1680. fol. tom. i. p. 29.

Cursus mathemat. p. 455. icon. 20. f. 469.

§ Collegii experimentalis pars secunda. Norimberge 1715. 4to.

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The artists at Nuremberg, who worked in glass, and who constructed a great many hydrometers which were every where sold, assisted in this likewise. One, above all, made by Michael Sigismund Hack, was highly valued about the beginning of the last century, as we are told by J. Henry Muller,* professor at Altorf. Of this artist, often mentioned by Sturm and other philosophers, an account has been given by Doppelmayr. He died in 1724.

Many improvements, or perhaps only alterations, have been made in this instrument in latter times by a variety of artists. The task of collecting these completely, in chronological order, with explanations, I shall leave to others; and only mention a few of them. One of the first who endeavoured to adapt the hydrometer for determining the specific gravity and purity of metals was Monconys. Almost about the same period Cornelius Meyer and Mr. Boyle seem to have conceived the idea of facilitating the weighing of solid bodies by a weighing-scale added to the instrument. The former affirms that this improvement was invented by him so early as the year

* Dissertat. de hydrometro. Altorfi 1723, 4to. p. 9.

† Page 275.

In the third part, p. 3. of the letters printed with his Travels, which addition seems to have been made in the year 1664. I quote the edition printed at Lyons 1665 and 1666 three volumes in quarto, Journal des voyages de Monconys.

1668; * whereas Boyle did not make his known till 1675. Besides these the following also are worthy of notice: Feuille, ‡ Fahrenheit, Clarke, § and Leutmann, whose improvements have been described by Wolf, ¶ Leupold, ** Gesner, †t Weigel, and others.

LIGHTING OF STREETS.

THE lighting of streets while it greatly contributes to ornament our principal cities, adds considerably also to the convenience and security of the inhabitants. But, of whatever benefit it may be,

* Nuovi ritrovamenti divisi in due parte. Roma 1696, fol. I shall take this opportunity of observing that a good account of Meyer and of his works, which are scarce, may be found in Scheibels Mathematische bücherkunde, ii. p. 443.

Hydrostatica medica, and in the Philosoph. transact. 1675. No. cxv. p. 329, where an engraving is given of all the parts, P. 340.

Journal des observations physiques et mathematiques. Paris 1714, 4to. i. P. 16.

Philosoph. transact. No. ccclxxxiv. p. 140; and Numb. ccccxiii.

p. 277.

Commentarii Acad. Petropolit. v. p. 274.

In his Versuchen. Halle 1737, 8vo. i. P. 556.

** Pars ii. Theatri statici universalis, sive Theatrum hydrostaticum.

++ In his Dissertation mentioned in the first note to this article. ‡‡ C. E. Weigel, Programma de historiæ barylliorum rudimentis. Gryphiæ 1785, 4to.

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