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.

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 baryllista or barynila. 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." 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; by the second, where he says that the weight of water was ascertained by a common balances; 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+. 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', he does not mention it. Constantin, however, who lived at the same time, must have been acquainted with it,

1 On Georg. i. 109. These words are quoted by Emmenessius, the editor of the Variorum edition of Virgil, but in the edition of Servius, Venetiis, 1562, fol., they are not to be found. The Commentary of Servius may at present be no longer indispensable for explaining Virgil; but it deserves to be printed once more as 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. 2 Quæst. Nat. iii. 25, p. 726.

3 Hist. Nat. xxxi. 3, sect. 23, p. 552.-Athen. ii. p. 46.-Plutarchi Quæst. Nat. 7.

4 De Simplic. Med. Facultatibus, iv. 20.

5 De Natura eorum quæ effluunt ex Terra, lib. ii. p. 124.

else he could not have explained the before-mentioned pas sages 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 salt-works. 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 16441; 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 account has described it only imperfectly.

Kircher, whose works were generally read, seems to have principally contributed towards making it publicly known; and Schott, Sturm1 and others, in their account of it, refer to his writings. The artists at Nuremberg, who worked in glass, and who constructed a great many hydrometers which were everywhere 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 Doppelmayer. He died in 1724.

Many improvements, or perhaps only alterations, have been made in this instrument in later times by a variety of artists. The task of collecting these completely in chronological

1 Philosophia Experimentalis, sive Commentaria in Aristotelis Meteorolog. lib. ii. textus 26, quæst. 2, tom. ii. p. 158, b.

2 Mundus Subterraneus, vol. i. p. 254.

3 Cursus Mathemat. p. 455, icon. 20.

4 Collegii Experiment. pars ii. Norimb. 1715, 4to.

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 Mayer and 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 as early as the year 16681; whereas Boyle did not make his known till 16752. Besides these the following also are worthy of notice: Feuille3, Fahrenheit, Clark 4, and Leutmann3, whose improvements have been described by Wolf, Leupold7, Gesner, Weigel and others.

[The principal hydrometer now in use is that of Sykes, this is adopted in estimating excise duties on liquids. That of Baumé is principally employed abroad. Those of Beck or Cartier are but rarely used. These instruments differ merely in their graduation. Sykes's plan of increasing the extent of the indications without enlarging the instrument is ingenious. It is effected by means of a number of weights which may be appended as collars to the stem of the instru

ment.

A useful method of ascertaining specific gravities for commercial purposes, consists in using a series of glass beads, previously adjusted and numbered. When thrown into any liquid, the heavier ones sink and the lighter float on the surface; but the one which has the same density as the liquid will remain indifferent, or perhaps slightly below the surface. The specific gravity is then found by the number with which it is marked.]

'Nuovi Ritrovamenti. Roma, 1696, fol.

2 Philosoph. Transact. 1675: where an engraving is given of all the parts. 3 Journal des Observations Physiques et Math. Par. 1714, 4to. 4 Philosoph. Transact. No. 384, p. 140; and No. 413, p. 277.

5 Comment. Acad. Petrop. v. p. 274.

6 In his Versuchen. Halle, 1737, 8vo, i. p. 556.

7 Theatrum Hydrostaticum.

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, it is generally considered as a modern invention. M. St. Evremond says, "The invention of lighting the streets of Paris during the night, by a multitude of lamps, deserves that the most distant nations should go to see what neither the Greeks nor the Romans ever thought of for the police of their republics." This opinion appears to be wellfounded; for I have never yet met with any information which proves that the streets of Rome were lighted. Some passages, indeed, in ancient authors rather indicate the contrary; and according to my ideas, the Romans would not have considered the use of flambeaux and lanterns so necessary on their return from their nocturnal visits, as they seem to have done, had their streets been lighted; though I will allow that the public lighting of the streets in our cities does not render links or lanterns altogether superfluous. Whoever walked the streets of Rome at night without a lantern, was under the necessity of creeping home in perfect darkness, and in great danger1, like Alexis in Athenæus. Meursius endeavours to make it appear that the streets of Rome were lighted; and in support of this opinion quotes Ammianus Marcellinus, and the Life of Julius Cæsar in Suetonius; but his arguments to me are far from being convincing2. That Naples was not lighted, appears from the return of Gito in the night-time, mentioned by Petronius3. Some circumstances however related by ancient authors make it probable that Antioch, Rome and a few other cities had public lanterns, if not in all the streets, at least in those which were most frequented.

Athen. Deipn. vi. 8. p. 236.

2 Joh. Meursii Opera, ex recensione Joannis Lami. Florent. 1745, fol. v. p. 635.

3 Pet. cap. lxxix. That the author here speaks of Naples, I conclude from cap. lxxxi., where the city is called Græca urbs. Others, however, with less probability, are of opinion that Capua is meant.