Clepsydra
A water clock. A machine used anciently for measuring time by means of the discharge of water through a small aperture. The Egyptians divided the space between sunrise and sunset into twelve hours, known as day, and between sunset and sunrise into twelve others, known at night. The days and nights therefore varied according to the seasons, so that the artificial divisions varied in like proportions, rendering the task of inventing a mechanism capable of being retarded or accelerated quite a formidable one for the mechanic of that day. The clock illustrated in Fig. 86 was so constructed that its aperture was adjusted as the year advanced by the attachment of an index to the sun's place in an ecliptic circle. It consists of a reservoir, A, at the top of which will be seen a waste pipe, to carry off the superfluous water and thus keep the level the same at all times. From this reservoir projects a pipe, B, which connects with the rim of a drum, M, N, on the front of which is a circle with the signs of the ecliptic engraved upon its dial. Fitting inside this large drum is a smaller one, O F, having an index attached to it. This drum has a groove or slot, a b, cut through it, tapering in breadth both ways to a point. This tapering groove, when the parts are in their places, comes just under the end of the pipe leading from the reservoir. This smaller drum turns upon the pipe F, which is continued within and has a funnel attached for receiving the water as it drops through the groove in the drum. The index or hand is double, L for day and O for night, and it will be evident that as it is turned the capacity of the orifice is altered and the water is regulated to pass more or less rapidly through the pipe. The ecliptic being properly divided, the hand was set to the proper sign in which the sun then was and was altered as it shifted around the ecliptic. The water, after passing through the regulator and the pipe C, dropped into the cylinder H, in which was a float I, connected by a chain passing over a pulley on an arbor P, and having a counter-poise weight K attached to its other end. To the pulley was attached a hand which pointed to the hour on the circular dial. A water clock, made by Ctesibius, is illustrated in Fig. 87. The water dropped into a funnel A, and was conveyed to the reservoir by means of the pipe M. In this cylinder was located a float, to which was attached a light pillar, on top of which was a figure pointing to the hour upon a column opposite. Attached to the bottom of the water cylinder was a small pipe, bent in the form of a syphon, as shown at E B F. As the water rose in the cylinder it also rose in the small pipe until it reached the top, when it flowed over the bend, thus filling the syphon, and by a well known law it quickly emptied the cylinder, the float and figure falling as the water receded. In order to overcome the obstacle of hours of a varying length, the inventor very cleverly drew the divisions on the column out of horizontal, so as to vary in their distance on different sides of the column. As the water came from the syphon it fell into receptacles in a wheel, shown at K, which, turning with the weight, as each compartment filled, caused the cylinder to revolve by the action of the pinion on its axis, taking into the contrate wheel I, which by another pinion H, turned the wheel and shaft G and L. In this way a variable scale of divisions was presented to the index, the space being regulated by the number of teeth in the wheels. Authorities differ as to the date of the revival of the clepsydra, one authority placing the date at 1646, while another gives it as 1693. A clepsydra of the seventeenth century is illustrated in Fig. 88. It consisted of an oblong frame of wood, A B C D, to the upper part of which two cords A a, B b, are fixed at their superior extremities and at their inferior are wound around the axis of the drum E. This drum was divided into several water-tight compartments, as shown in Fig. 89. The cord was wound around the axis until the drum was elevated to the top of the frame and it was then left to obey the force of gravity. A hole was pierced near the bottom of each compartment, allowing the water to slowly ooze from one compartment to the other, thus causing the drum to revolve with a certain degree of accuracy. The rate of motion was regulated by altering the size of the apertures. The hours were indicated in two ways; one by the axis pointing out the hours on the side of the frame as it revolved, and another by passing a cord, c d, over a pulley attached to an arbor, having an index or hand to point out the hours on a dial, a weight, F. being fastened to the other end of the cord.