CLEPSYDRA

CLEPSYDRA, Latin, from Greek klepsudra : kleptein, kleps-, to steal + hudōr water, was an horological instrument of great antiquity, among the Egyptians and other eastern nations, probably before sun-dials were invented; though the name of the original inventor is not handed down to us ; the construction has been varied in different ages and countries, according to the variation of the different modes of reckoning time, but one principle is the basis of all the forms it has undergone, namely, the constant dropping, or running of water through a small aperture, out of one vessel into another. At first the indication of time was effected by marks corresponding to either the diminution of the fluid in the containing vessel, during the time of emptying, or to the increase of the fluid in the receiving vessel during its time of filling; but it was soon found, that the escape of the water was much more rapid out of the containing vessel when it was full, than when it was nearly empty, owing to the difference of pressures at different heights of the surface ; this irregularity in the dropping, presented an obstacle which required much ingenuity to correct. In our account of the different constructions of clepsydrae, we will class them under the two heads of ancient and modern.

Ancient Clepsydrae.--According to M. Vitruvius Pollio, the first improver of the ancient clepsydra, or water-clock, was Ctesibius of Alexandria, the son of a barber, who, about 245 years before Christ, spent much time in devising mechanical contrivances for removing not only the obstacle in question, but also another equally formidable one, which arose from the daily inequality of the Egyptian hours. As one-twelfth part of the time elapsed from sun-rise to sun setting on any day, was called an hour of that day ; and as one-twelfth part of the time that passed from full setting to sun-rise was called an hour of the night; not only did the hours of day differ from the hours of night, but from one another, at all times, except at the vernal and autumnal equinoxes ; hence it became necessary, either to make the water fall irregularly into a receiving vessel, with equidistant hour-marks, or to have varying hour-marks for a regular efflux ; the first of these methods (which probably preceded that of Ctesibius) was thus effected, viz. 1. A conical hollow vessel, A, was inverted, or placed like a funnel in a frame C C (Plate I. fig. 1. of Horology)

there being a very small aperture at the apex of the cone, and another solid cone, B, every way similar as to dimensions, was plunged into the hollow one when filled with water to a greater or a smaller depth, accordingly as the efflux was wanted to be more or less rapid, and then adjusting marks, corresponding to every day and night in the year, were put on a long Item D, inserted into the broad end of the solid cone B, and kept in its position by the frame, as represented in the figure, to show how much the inner cone was to be depressed or elevated, to accelerate or retard the issue of the fluid for the corresponding time ; H was the spout which supplied a constant influx of water, and I the waste pipe, connected with the top of the conical vessel, which carried off the superfluous water ; hence the constant influx of water preserved an unvarying height of the surface from the aperture, which aperture was varied at pleasure, by the elevation or depression of the inner cone ; if now we suppose the subjacent vessel to be a cube, cylinder ; or any other regular figure, and equidistant hour-marks to be properly made on its side, the surface of the water or an index borne by it on a piece of cork, would, as it rose, indicate the hours corresponding to those marks.

The imperfections of this clepsydra were these :
1. It required two daily manual adjustments, one in the morning, and the other in the evening ; and, 2. It made no allowance for the variation of fluidity, in different degrees of temperature, which, it is asserted (but perhaps without proof), greatly influenced the isochronism of the drops. As an improvement, or rather appendage, to this construction of the clepsydra, a bar, E E with rack-work at the upper end, as shewn by the dotted lines, was made to float on the surface of the lower vessel by means of an affixed piece of cork, F, so that as the cork and its bar rose in the vessel, the teeth of the bar turned a small wheel, G, fixed to the upper part of the frame by a cock, on the arbor of which wheel a hand was put, which revolved and indicated the hours on a fixed dial-plate. This addition, however, did not render the instrument a more accurate measure of time, but only indicated the hours, such as they were, in an improved manner. It may be worthy of remark here, that water was at once the regulator and the maintaining power of the instrument before us ; the interval between two successive drops was to the clepsydra what one vibration of the pendulum is to a clock, or one oscillation of the balance is to a watch ; and the floating of the indented bar was in place of a weight or spring to move the wheel to which the hand was attached ; consequently it might be said to be an horological machine of the simplest construction possible. The adjustment of the two cones was regulated by the latitude of the place, owing to the manner in which the hours were divided ; at Alexandria, for instance, the greatest and least velocity of the drops were required to be to each other as 70 to 50, the longest and shortest hours in that latitude being respectively 1h 10m and 50m of equable time ; and in higher latitudes the disparity is still greater.

The next attempt to improve the clepsydra was by constructing it so that its aperture was adjusted, as the year advanced, by the putting of an index to the sun's place in an ecliptic circle ; which attempt, of course, rendered the instrument more complex. Perrault conceives the parts to have been thus adapted, according to the description given of it by M. Vitruvius Pollio, in his book "De Architectura" (cap. ix. lib. ix.).

Fig. 2. of Plate I. represents an ancient clepsydra with an horary circle and a variable aperture : A is a reservoir, to the top of which is attached a water-pipe, not seen in the drawing, to preserve an equal pressure by carrying off the superfluous water ; B is a pipe projecting from the reservoir into the upper part of the drum, M N, on the front of which drum the ecliptic circle is marked ; O D L is a smaller inner drum, which revolves on a tubed arbor, F, and which is represented as drawn out of its place ; this small drum has a thorough groove, a b varying in breadth all round it, like a hoop tapering throughout from the broadest part both ways to its opposite point, and is of such a diameter that the middle of the groove just reaches to, and coincides with, a perforation under the tube, B, at the upper part of the great drum, so that, as the little drum, which carries the diurnal index, L, and nocturnal index, O, opposite to the former, is turned round by hand, the variation in the breadth of the groove occasions a corresponding variation in the velocity of the efflux of water, by making a larger or smaller aperture, accordingly as the sun's place is more or less advanced in the ecliptic, the largest aperture being when the diurnal index is at the beginning of Capricorn ; a little bason or funnel attached to the upper part of the fixed tube or hollow arbor, F, (not visible), receives the water in its fall within the drum, and transmits it through the said tube by G into the receiving vessel, H, in which is floated the piece of cork, I ; this floating-piece is connected, by a chain, with the counterpoise, K, after it is folded round the arbor, P, which carries the hour-hand of the dial-plate ; consequently, as the water rises in the vessel, H, the piece, I, is raised, and its counterpoise, K, at the same time falling gives motion to the arbor and hour-hand, and the hours are longer or shorter according to the breadth of the groove which is at any time under the perforation of the tube, B, i. e. according to the place in the ecliptic to which the proper index is put.

This clepsydra, like the preceding one, composed of two cones, requires two manual adjustments, one in the morning and the other in the evening, and makes no allowance for the (supposed) variation of fluidity occasioned by the different states of the weather ; and the variation in the breadth of the groove or slit, it is presumed, was more plausible in theory, than feasible in practice ; the contrivance, however, was ingenious, and bespoke the inventor's acquaintance with astronomy.

The next improvement in the ancient clepsydra was probably that of Ctesibius which was an automaton, or self-adjusting machine, and is represented by fig. 3,

which, according to Perrault and Ferd. Berthoud, exhibits the interior construction of this machine ; A is the end of a tube over which an image stands, which is connected with a full reservoir, and from the eyes of which, considered as invariable apertures, the water continually flows or drops in a regulated manner into it ; this tube conveys the water from M towards B into the top of a long regular vessel, B C D F, which it gradually fills, and raises the cork, D, with its attached light pillar, C D ; on the top of this pillar is surmounted another image holding an index which points to the divisions on the large column above. Now, when the water rises in the vessel that contains the cork, it also rises in the small tube, F B, which constitutes one leg of a syphon, F B E, that is connected with the bottom of the cubic vessel ; consequently, when the index has mounted to the uppermost division on the large column of hour lines, consulting of twice twelve, the water flows over the bent part, B, of the syphon, and, immediately empties the vessel into one of the six troughs or divisions of the water-wheel, K, which is thus turned one-sixth part of a revolution, during which time the image falls with its index to the bottom of the column, to be ready for the next day. This portion of the mechanism would have been sufficient to constitute the machine, if the hours had been considered as of equal length throughout the year, but the Egyptian mode of dividing and reckoning time made it requisite that the hour lines should slope out of an horizontal direction on the surface of the column, so as to make variable spaces, and also that the column should revolve once in a year, to present all the variations of space to the index. This annual motion of the column is said to be effected by wheel-work in the following manner:¾on the arbor of the water-wheel, K, is fixed the pinion, N, of six leaves, which impels the contrate-wheel I, of 60 teeth in 6 x 60/6 = 60 days, then on the perpendicular arbor of I is another pinion, H, of ten leaves, which drives the wheel, G, of 61 teeth round in 60 x 61/6 = 366 days, and along with it the horary column, into which its arbor is inserted at L. On the bottom of the column is marked an ecliptic circle ; and 12 perpendicular lines drawn lengthwise down the column divide it it into the respective signs, which are serviceable for ascertaining the requisite slope of the hour lines in any month. The writer of this article, however, suspects, that the above train of wheel-work is only what Perrault, the translator of Vitruvius, supposed to be that of Ctesibius; for, on referring to the original account of Vitruvius, the year in which the column revolved is stated to be 365 days, a period which might be effected thus :

Let the water-wheel have only five compartments instead of six, and let an endless screw be cut on its arbor to impel a wheel of 73 teeth, with a perpendicular arbor, to be inserted into the column of hours, which will, by such a simple construction, revolve in 5 x 73/1 = 365 days, agreeably to the original account.