The clepsydra, in one of its earlier forms, was used as an astronomical instrument, by the help of which the equator was divided into twelve equal parts, before the mathematical division of a circle was understood ; it was deemed of more value than a sun-dial, on account of its dividing the hours of the night as well as of the day. It was introduced into Greece by Plato, and into Rome by P. Cornelius Scipio Nasica, about 157 years before Christ.

Pliny says (lib. xxxvii.) that Pompey brought a valuable one among his spoils from the Eastern nations ; and Caesar is said to have met with an instrument of this kind in England, by the help of which he observed that the summer nights his of this climate are shorter than they are in Italy. The life which Pompey made of his instrument was to limit the speeches of the Roman orators ; which Cicero alludes to when he says "latrare ad clepsydram."

Besides the ancient clepsydra, above described, F. Berthoud mentions another (Histoire de la Mesure du Temps, tom. I. p. 20.), which was called the anaphoric, on the dial-plate of which were projected the circles of the sphere, including the parallels of the sun's altitude, with the semi-diurnal and semi-nocturnal arcs, to which an adjustable bead, as the sun's representative, pointed as an index to shew the hours, parallels, &c. as the dial-plate revolved daily by means of wheel-work, which was impelled by water. It does not seem certain at what period this instrument was invented and used ; but Berthoud thinks that tables of the sun's motion must have existed previously to its invention, and also a knowledge of projections of the sphere on a plane surface, whence he fixes the date posterior, to the time of Hipparchus, who, according to Pliny, died about 125 years B.C. The name anaphoric derived from anaphora, which was the second house in the heavens, according to the doctrine of astrology, which prevailed about the time here specified.

In Athenaeus, lib. iv. p. 174, we have a history and description of all ancient instrument. He tells us that it was invented in the time of the second Ptolemy Euergetes, by Ctesibius, a native of Alexandria, and by profession a barber : or rather, that it was improved by him, for Plato furnished the first idea of the hydraulic organ, by inventing a night-clock, which was a clepsydra, or water-clock, that played upon flutes the hours of the night at a time when they could not be seen on the index.

The anecdote in Athenaeus concerning the mechanical amusements of the great ideal philosopher, is curious. What a condescenslon in the divine Plato to stoop to the invention of any thing useful ! This musical clock must have been wholly played by mechanism.

In describing it, Athenaeus says, it resembled in appearance a round altar ; but was not to be ranked with stringed but wind instruments, composed of pipes ; the orifices of which being towards the water, when it was agitated, produced from the pipes, by its fall, a soft and pleasing sound.

Modern Clepsydra.--The modern method of dividing the natural day into 24 solar hours of equal length, has rendered the preceding constructions of the clepsydra useless for some centuries back ; and, notwithstanding the science of hydrostatics is much better understood by the modern than it was by the ancient philosopher, to that a scale of altitudes corresponding to the variable velocities of the efflux of a fluid out of a given aperture can be ascertained by calculation for a containing vessel of any capacity or figure, yet, since the happy inventions of the balance and pendulum, as regulators of watches and clocks, horological machines, actuated by the motion of water, have become so rare, as to be considered as objects only of curiosity.

Beckmann, in his "History of Inventions," vol. i p. 136, attributes the contrivance and introduction of a water-clock to some time between 1643 and 1663, and gives nearly the same brief account of one as we meet with in " Bion, or Mathematical Instruments," and also in "Ozanam's Recreations," edited by Dr. Hutton, the last of which authors said, in the year 1693, that the first water-clock brought to Paris about that time was from Burgundy. He also says, that father Timothy, a Barnabite, had given the machine all the excellence it was capable of, by constructing it so as to make it go a month at one winding up, and to exhibit not only the hours on a dial-plate, but also the sun's place, day of the month, and festivals throughout the year.

How these and similar particulars might be indicated, will be easily apprehended from the following description, which is agreeable to the accounts given of a water-clock of the 17th century by the authors already named.


In fig. 1, of Plate I I. of Horology, A B C D is an oblong frame of wood, to the upper part of which two cords, A a and B b, are fixed at their superior extremities, and at their inferior, to the metallic arbor, a b, of the drum, E, which contains distilled water ; this water is confined in cells so peculiarly constructed, that they regulate the velocity with which the drum shall descend by the force of gravity from the top to tile bottom of the frame, and the ends of the arbor indicate the hours marked on the vertical plane of the frame during the time of descent. An observer, who knows not the nature of the interior cells of the drum, is surprised to see that its weight does not make it run down rapidly, when mounted to the top of the frame by merely folding the strings round the arbor, there being, apparently no mechanical impediment to the natural action of gravity.


To explain how this phenomenon is produced, we must refer to fig. 2, which is a section of the drum at right angles to its arbor ; this circular plane we will suppose to be six inches, which is about the usual size, in diameter, and to represent the inner surface of either of the two ends of the drum, which may be made of any of the unoxidable metals ; then, if we conceive seven metallic partitions, F f, G g, H h, I i, K k, L l, and M m, to be closely soldered to both ends of the drum, in the sloping direction indicated by the figure, where the black lines are equidistant tangents to the small dotted circle of an inch and half diameter at the points f, g, h, &c. ; it is evident, that any small quantity of water introduced into the drum would fall into two, or at most three, of the lower compartments, and would remain there until some external force should alter the position of the drum, supposing in this case the cords tied fast to the arbor ; but we have said that they are wound round the circumference of an arbor, that has a sensible diameter, suppose one-eighth of an inch ; therefore, they are removed one-sixteenth of an inch, or upwards if we take their thickness into the account, from the centre of the drum, which would also be its centre of gravity, if it were empty, on which account, it would, in that case, revolve to the left, in the direction F G H downwards, from the cord being at the remote side of the centre, as represented by N O ; but conceive the water to be included now and then, it would be elevated to the right, till its weight became a counterpoise to the gravity of the heavier side of this drum, in which situation all motion would cease, and the drum would remain, suspended, indeed, by the cords, but in state of equilibrio. Conceive again a small hole perforated in the partition pressed upon by the water near the circumference of the large circle, and also at the points F, G, H, I, K, L, M, and the consequence will be, that the water will first force its way slowly through the perforation at K, from the more elevated to the lower compartment, which effect will diminish its power as a counterpoise, and give such an advantage to the heavy side, F G H, of the drum, considered as empty, as will occasion a small degree of motion towards the left, and consequently carry the water once more towards the right ; but now the water passes through the perforation of the next partition also at I, and produces again the same effect, as has been described with respect to K, and will continue to do so, at the successive perforations, till all the compartments have been filled and emptied by means of these perforations, in succession, which kind of motion of the drum, contrary to that of the water, it is now not difficult to conceive will be pretty regular, if all the partitions are perforated exactly alike. The difference of the pressures of' the water in cells, nearly full and nearly empty, will occasion some little deviation from regularity ; but these will be periodic, and must be allowed for in the hour divisions, which ought to be made by a comparison of the spaces fallen through, with the time indicated by a clock or watch. About nine ounces of distilled water will suffice for a clepsydra of six inches diameter, and two inches depth, and the velocity of the fall may be limited, either by varying the quantity of water, or by hanging a small metallic cup, F, to receive weights, by a cord wound in a direction contrary to the cords of suspension, to act as a counterpoise in aid of the water, if the fall be too rapid, or vice versa.

It is absolutely necessary that the arbor should fit the central square hole so well as to prevent the escape of water from the drum, otherwise the instrument would continue to gain velocity, till at length it would no Ionger afford a true indication of time. Sometimes a cord, c d, with a weight, P, is made to pass round a pulley fixed to an arbor at the top of the frame, with a noose passing over the axis near a, as is seen in the same figure, which arbor, projecting through a dial-plate or face, turns round and carries a hand to indicate the hours like an ordinary clock ; when this construction is preferred, it is an indispensable requisite that the circumference of the pulley's groove be exactly of the same dimensions as the fall of the drum in 12 or 24 hours, accordingly as the dial is divided.

This clepsydra, it is said, goes faster in summer than in winter, which is owing to the drum being relatively heavier in rarefied than in dense air; we can hardly suppose that any alteration in the fluidity of the water, as formerly supposed, would make any difference. The minute hand and also the striking part of a common clock might easily be superadded to this clepsydra.