A History of Inventions, Discoveries, and Origins, Volume II (of 2)

In his fifteenth letter, he tells Hypatia that he was so unfortunate, or found himself so ill, that he wished to use a hydroscopium, and he requests that she would cause one to be constructed for him. “It is a cylindrical tube,” adds he, “of the size of a reed or pipe. A line is drawn upon it lengthwise, which is intersected by others, and these point out the weight of water. At the end of the tube is a cone, the base of which is joined to that of the tube, so that they have both only one base. This part of the instrument is called baryllion. If it be placed in water, it remains in a perpendicular direction, so that one can discover by it the weight of the fluid.”

Petavius, who published the works of Synesius in the year 1640, acknowledges in his annotations, that this passage he did not understand. An old scholiast, he says, who had added some illegible words, seemed to think that it referred to a water-clock; but this he considers improbable, as a clepsydra was not immersed in water, but filled with it. He conjectures, therefore, that it may allude to some such instrument as that which Vitruvius calls chorobates. The latter however was employed for leveling; and it appears that Synesius, who complains of the bad state of his health, could not think of leveling. Besides, no part of the description in Vitruvius agrees with that which is given in so clear a manner by Synesius.

Petau published his edition of the works of this philosopher in the time of Peter de Fermat, conseiller au parlement de Toulouse, a man of great learning, who was an excellent mathematician, and well-acquainted with antiquities and the works of the ancients. We have by the latter a commentary upon some obscure passages of Athenæus, annotations on the writings of Theon of Smyrna, and emendations from a manuscript to the Stratagemata of Polyænus, which may be found also in his Miscellanies. Mursinna, in his edition of the same author, has added them to the end of the preface. As Fermat was often consulted respecting difficult passages of the ancients, he could not be unacquainted with that in the new edition of Synesius. He drew up an explanation of it, and gave it to a friend who was then about to publish a French translation of Bened. Castelli’s book, Della Misura dell’Acque Correnti, and who caused it to be printed along with that work. Fermat died in the year 1665. After his death his son published some of his writings under the title of Varia Opera Mathematica414; and in this collection is inserted his short treatise on the hydroscopium, from which I have extracted the following explanation.

It is impossible, says he, that the hydroscopium could be the level or chorobates of Vitruvius, for the lines on the latter were perpendicular to the horizon, whereas the lines on the former were parallel to it. The hydroscopium was undoubtedly a hydrometer of the simplest construction. The tube may be made of copper, and open at the top; but at the other end, which, when used, is the lowest, it must terminate with a cone, the base of which is soldered to that of the tube. Lengthwise, along the tube, are drawn two lines, which are intersected by others, and the more numerous these divisions are, the instrument will be so much more correct. When placed in water, it sinks to a certain depth, which will be marked by the cross lines, and which will be greater in proportion to the lightness of the water. A figure, which is added, illustrates this explanation more than was necessary. When a common friend of Fermat and Petavius showed it to the latter, he considered it to be so just, that he wished to have an opportunity of introducing it in a new edition.

Mersenne, on the other hand, entertains some doubt415 respecting this instrument, though he does not mention Fermat, with whom he was well-acquainted; for in the dispute which the latter had with Descartes, Mersenne was the bearer of the letters that passed between them, as we learn from the Life of Descartes, by Baillet. His objections however are of little weight. Why should Synesius, asks Mersenne, consider himself unfortunate, because he had not a hydrometer? It may be here replied, that he was in an infirm state, and that the physicians seem to have ordered him to drink no water but what was pure and light. We know that in former times, when so many artificial liquors were not in use, people were accustomed, more than at present, to good water. We read in the works of the ancient physicians, such as Galen and Celsus, directions how to examine the lightness and purity of water. He might have tried it, says Mersenne, with a common balance. He indeed might, but not so conveniently. That Synesius was in a bad state of health is apparent from several of his letters; otherwise one might say that in a letter many expressions may be only jocular, respecting some circumstance known to the friend to whom one writes; and that every expression is not to be taken according to its literal meaning. One might confess also, without weakening a received explanation, not to know what Synesius alludes to in the first line of his letter. But even if we allow that the instrument was not a hydrometer, but a water-clock, or a level; it may be asked how the want of these could make him unfortunate? Mersenne thinks further, that the cone, added to the end of the tube, would have been unnecessary in a hydrometer; but it serves to keep the instrument with more ease in a perpendicular direction in the water. Such is the opinion of H. Klugel, whom I shall soon have occasion to quote.

For the explanation of Fermat one may produce a still stronger testimony, with which he seems not to have been acquainted. It can be proved that this instrument was used in the next, or at least in the sixth century. Of that period, we have a Latin poem on weights and measures, which contains a very just description of a hydrometer. The author, in manuscripts, is called sometimes Priscianus, and sometimes Rhemnius Fannius Palæmon; but we know, from grounds which do not belong to this subject, that the former was his real name. Two persons of that name are known at present. The one, Theodore Priscian, was a physician, and lived in the time of the emperor Valentinian, towards the end of the fourth century. As more physicians have written on weights and measures, with which it is indispensably necessary they should be acquainted, one might conjecture that this Priscian was the author of the above poem. The rest of his writings, however, still preserved, are in so coarse and heavy a style, that one can scarcely ascribe to him a work which is far from being ill-written; especially as it is nowhere said that he was a poet. With much more probability may we consider as the author the well-known grammarian Priscian, who died about the year 528.

This poem has been often printed, and not unfrequently at the end of Q. Sereni Samonici De Medicina Præcepta. The best edition is that inserted by Wernsdorf in the fifth part of the first volume of his Poetæ Minores, where an account may be found of the other editions.

Be the author who he may, this much is evident, that he was acquainted with the hydrometer of Synesius, and has described it in a very clear manner.

“Fluids,” says he, “are different in weight, as may be proved by the specific gravity of oil and honey compared with that of pure water;” and the given proportion agrees almost with that found by modern experiments. “This,” adds he, “may be discovered by an instrument,” which he thus describes: – “It consists of a thin metallic cylinder made of silver or copper, about as large as the joint of a reed between two knots, to the end of which is added a cone. This cone makes the lower end so heavy, that the instrument, without sinking or floating on the surface, remains suspended perpendicularly in the water. Lengthwise, upon the cylinder, is drawn a line, which is divided by cross lines into as many parts as are equal to the weight of the instrument in scripla. If placed in light fluids, more of the divisions will be covered than when put into heavy fluids; or it sinks deeper into those which are light than into those which are heavy. This difference of gravity may be found also,” continues he, “by filling vessels of equal size with the fluids and weighing them; for the heavier must then weigh most; but when one takes an equal weight of two fluids, the lighter will occupy more space than the heavier. If twenty-one divisions of the instrument are covered in water, and twenty-four in oil, and if one take twenty-four scripla of water, twenty-one scripla of oil only can be contained in the space occupied by the water.” Such is the manner in which Professor Klugel has conjectured the meaning of the author from hydrostatic principles; though neither he nor Wernsdorf has ventured to give a literal translation of the words which ought to convey this explanation. But however obscure they may be, it evidently appears that they allude to a hydrometer.

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.

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 dixerunt416.” 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 bodies417; by the second, where he says that the weight of water was ascertained by a common balance418; 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 floated419. 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 springs420, 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 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 1644421; but he confesses that he acquired his information from a German treatise by Tholden, whom Kircher422 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 Schott423, Sturm424 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 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 1668425; whereas Boyle did not make his known till 1675426. Besides these the following also are worthy of notice: Feuille427, Fahrenheit, Clark428, and Leutmann429, whose improvements have been described by Wolf430, Leupold431, 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 instrument.

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.]

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 well-founded; 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 danger432, 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 convincing433. That Naples was not lighted, appears from the return of Gito in the night-time, mentioned by Petronius434. 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.

Libanius, who lived in the beginning of the fourth century, says in his Panegyric435, where he praises his native city Antioch, “The light of the sun is succeeded by other lights, which are far superior to the lamps lighted by the Egyptians on the festival of Minerva of Sais. The night with us differs from the day only in the appearance of the light: with regard to labour and employment everything goes on well. Some work continually; but others laugh and amuse themselves with singing.” I cannot allow myself to imagine that the sophist here considers it as a subject of praise to his native city, that the inhabitants after sun-set did not sit in darkness, but used lights to work by. It appears, therefore, that he alludes to the lighting of the streets.

In another passage, in the oration to Ellebichus436, the same author tells us, that the ropes from which the lamps that ornamented the city were suspended, had been cut by some riotous soldiers, not far from a bath. “Proceeding,” says he, “to a bath not far off, they cut with their swords the ropes from which were suspended the lamps that afforded light in the night-time, to show that the ornaments of the city ought to give way to them.” This quotation indicates, at any rate, that there were lamps suspended from ropes near the baths and places of greatest resort. The following passage of Jerome, however, seems to make it probable, or rather certain, that the streets of Antioch were lighted. In the altercation between a Luciferan and an Orthodox, he relates that an adherent of the schismatic Lucifer disputed in the street with a true believer till the streets were lighted, when the listening crowd departed; and that they then spat in each other’s face, and retired.

In the elegant edition of the works of that father, by Dominicus Vallarsius, we have a short dissertation on the time when this unmannerly dispute took place; and the editor shows that it happened at Antioch in the year 378437.

Basilius the Great, in a letter to Martinianus, giving an account of the miserable situation of his native city Cæsarea, in Cappadocia, in the year 371, says they had nights without lights (noctes non illustratas). Most commentators explain this passage as if it meant that the lamps in the streets had not been lighted438.

That the streets, not only of Antioch, but also of Edessa, in Syria, were lighted in the fifth century, seems proved by a passage in the History of Jesue Stylites. It is there expressly said, that Eulogius, governor of Edessa, about the year 505, ordered lamps to be kept burning in the streets during the night; and that he employed for that purpose a part of the oil which was before given to the churches and monasteries439.

With regard to the public lighting of whole cities on festivals, and particularly on joyful occasions, which we call illuminations, that practice seems to be of great antiquity. Of this kind was a particular festival of the Egyptians440, during which lamps were placed before all the houses throughout the country, and kept burning the whole night441. During that festival of the Jews, called festum encæniorum, the feast of the Dedication of the Temple, which, according to common opinion, was celebrated in December, and continued eight days, a number of lamps were lighted before each of their houses. A passage in Æschylus shows that such illuminations were used also in Greece. At Rome, the forum was lighted when games were exhibited in the night-time; and Caligula, on a like occasion, caused the whole city to be lighted442. As Cicero was returning home late at night, after Catiline’s conspiracy had been defeated, lamps and torches were lighted in all the streets in honour of that great orator. The emperor Constantine caused the whole city of Constantinople to be illuminated with lamps and wax candles on Easter eve443. The fathers of the first century frequently inveigh against the Christians, because, to please the heathens, they often illuminated their houses, on idolatrous festivals, in a more elegant manner than they. This they considered as a species of idolatry444. That the houses of the ancients were illuminated on birth-days, by suspending lamps from chains, is too well known to require any proof445.

Of modern cities, Paris, as far as I have been able to learn, was the first that followed the example of the ancients by lighting its streets. As this city, in the beginning of the sixteenth century, was much infested with street robbers and incendiaries, the inhabitants were, from time to time, ordered to keep lights burning, after nine in the evening, before the windows of all the houses which fronted the street. This order was issued in the year 1524, and renewed in 1526 and 1553446; but in the month of October 1558, falots were erected at the corners of the streets, or, when the street was so long that it could not be lighted by one, three were erected in three different parts of it. These lights had, in a certain measure, a resemblance to those used in some mines; for we are told, in the Grand Vocabulaire François447, that falot is a large vase filled with pitch, rosin, and other combustibles, employed in the king’s palace and houses of princes to light the courts. At that period there were in Paris 912 streets; so that the number of lights then used must have been less than 2736.

In the month of November, the same year, these lights were changed for lanterns of the like kind as those used at present. The lighting of the streets of Paris continued, however, for a long time to be very imperfect, till the abbé Laudati, an Italian of the Caraffa family, conceived the idea of letting out torches and lanterns for hire. In the month of March 1662, he obtained an exclusive privilege to this establishment for twenty years; and he undertook to erect, at certain places, not only in Paris, but also in other cities of the kingdom, booths or posts where any person might hire a link or lantern, or, on paying a certain sum, might be attended through the streets by a man bearing a light. He was authorised to receive from every one who hired a lantern to a coach, five sous for a quarter of an hour; and from every foot-passenger, three sous. To prevent all disputes in regard to time, it was ordered that a regulated hour-glass should be carried along with each lantern.