This instrument, called in Latin _hydrometrum_, _hygroscopium_,
_hygrobaroscopium_, _hydroscopium_, _areometrum_, and _baryllion_,
serves to determine the weight or specific gravity of different fluid
masses, by the depth to which it sinks in them.
The laws respecting the comparative specific gravity of fluids and
solid bodies immersed in them were discovered by Archimedes, when he
tried the well-known experiment, by order of Hiero king of Sicily,
to find the content of a golden crown, made for that sovereign. Upon
these is founded the construction of the hydrometer; and it is not
improbable that Archimedes, who was killed in the year 212 before the
Christian æra, was the inventor of it, though no proofs to warrant this
conjecture are to be found in the writings of that great man, or in
those of any other author.
The oldest mention of the hydrometer occurs in the fifth century, and
may be found in the letters of Synesius to Hypatia. Of the lives of
these two persons I must here give some anecdotes, as they deserve
to be known on account of the singular fate which attended them.
Hypatia was the daughter of Theon, a well-known mathematician of
Alexandria, some of whose writings are still extant. By her father she
was instructed in mathematics, and from other great men, who at that
time abounded in Alexandria, she learned the Platonic and Aristotelian
philosophy, and acquired such a complete knowledge of these sciences,
that she taught them publicly with the greatest applause. She was young
and beautiful, had a personable figure, was sprightly and agreeable
in conversation, though at the same time modest; and she possessed
the most rigid virtue, which was proof against every temptation. She
conducted herself with so much propriety towards her lovers, that
they never could obtain more than the pleasure of her company and of
hearing her discourse; and with this, which they considered as an
honour, they were contented. Those who wished to intrude further were
dismissed; and she destroyed the appetite of one who would not suffer
her to philosophise, by means of some strong preparation, which, as far
as I know, remained a secret. She was not baptized, and with all her
knowledge, adopted the blind superstition of paganism. Had she been a
Christian, and suffered a cruel death from heathen persecution, she
would have merited a place in the martyrology of the saints: but the
case was reversed; for, by the conduct of the Christians towards her,
she became entitled to have her name enrolled in the martyrology of the
philosophers.
The patriarch of Alexandria, at the time when she lived, was Cyril,
whose family for a hundred years before had produced bishops, who were
of more service to their relations than to the church. This prelate
was a proud, litigious, vindictive and intolerant man, who thought
every thing lawful which he conceived to be for the glory of God; and
who, as prosecutor and judge, condemned Nestorius without hearing his
defence. In the city of Alexandria, which was then very flourishing on
account of its commerce, the emperor allowed greater toleration than
he imagined could be justified to the clergy in any other place; and
it contained a great many Jews, who carried on an extensive trade, as
well as a number of pagan families who were of service to the city, or
at least did it no harm. This, in the eyes of Cyril, was not proper;
he would have the sheep-fold clean, and the Jews must be banished.
Orestes, however, the governor, who was a man of prudence, and better
acquainted with the interests of the city, opposed a measure that was
likely to be attended with mischief, and he even caused to be condemned
to death a Christian profligate, who had done some injury to the Jews.
This malefactor was, by the order of Cyril, buried in the church
as a martyr; and he immediately collected five hundred monks, who
ill-treated Orestes in the streets, and excited an insurrection among
the people, who plundered the unfortunate Jews, and expelled them from
a city in which they had lived since the time of Alexander the Great.
Cyril, observing one day a great number of horses and servants
belonging to persons of the first rank, before a certain house in the
city, inquired the cause of their being assembled in that manner.
He was informed that the house was the habitation of the celebrated
female philosopher Hypatia, who, on account of her extensive learning
and eminent talents, was visited not only by people of the highest
distinction, but even by the governor himself. This was sufficient to
excite the bishop’s jealousy against the unbelieving Hypatia, and he
resolved to effect her ruin. As he had instigated the people against
the Jews, he in like manner encouraged them to attack Hypatia. They
seized her in the street, hurried her to the church, stripped off her
clothes, tore her flesh to pieces with potsherds, dragged her mangled
limbs about through the city, and at length burned them. This bloody
tragedy, which took place in the year 415, could tend only to inspire
the heathens with a greater hatred to Christianity, and to make
sensible Christians ashamed of the conduct of their brethren. To Cyril,
however, it occasioned no shame; on the contrary, he endeavoured to
divert the emperor from punishing those who had been guilty of so gross
a violation of the principles of justice, and in this he was assisted
by his numerous adherents and friends. In some circumstances of this
relation historians are not agreed, but they all concur in bestowing
praise on Hypatia, whose memory was honoured and preserved by her
grateful and affectionate scholars[413].
Among these was Synesius, of a noble pagan family, who cultivated
philosophy and the mathematics with the utmost ardour, and who had
been one of her most intimate friends and followers. On account of his
learning, talents, and open disposition, he was universally esteemed,
and he had been employed with great success on public occasions of
importance. The church at Ptolemais at length wished to have him
for their bishop. After much reluctance he accepted the office, but
on condition that they should not require him to acknowledge the
resurrection of the dead, which he doubted. The people having consented
to allow him this indulgence, he suffered himself to be baptized,
and became their bishop. He was confirmed by the orthodox patriarch
Theophilus, the predecessor of Cyril, to whose jurisdiction Ptolemais
belonged; and he afterwards renounced his errors, and declared himself
convinced of the truth of the resurrection. This learned man showed his
gratitude to Hypatia, by the honourable mention which he made of her in
some letters that are still preserved among his writings.
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 Mathematica[414]; 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 doubt[415] 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 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.
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
1644[421]; but he confesses that he acquired his information from a
German treatise by Tholden, whom Kircher[422] 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[423],
Sturm[424] 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 1668[425]; whereas Boyle
did not make his known till 1675[426]. Besides these the following
also are worthy of notice: Feuille[427], Fahrenheit, Clark[428], and
Leutmann[429], whose improvements have been described by Wolf[430],
Leupold[431], 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.]