Similar shaped vessels of the Greeks, Romans and other people might be easily produced.
Fig. 53.—Water-Clock.
In Egypt, India, Chaldea and China the clepsydra or water-clocks date back beyond all records. Plutarch mentions them in his life of Alcibiades who flourished in the Fifth Century B. C. when they were employed in the tribunals at Athens to measure the time to which the orators were limited in their addresses to the judges. Julius Cæsar found the Britons in possession of them.
The clepsydra is a device for measuring time by the amount of water discharged from a vessel through a small aperture, the quantity discharged in a given unit of time, as an hour being first determined. In the earlier clepsydras the hours were measured by the sinking of the surface of the water in the vessel containing it. In others the water ran from one vessel to another, there being in the lower a cork or piece of light wood which as the vessel filled, rose and thus indicated the hour. In later clepsydras the hour has been indicated by a dial.
Fig. 53 shows a water-clock described by Hero of Alexandria, Egypt, made to govern the quantities of fluids flowing from a vessel. The note below gives the exact wording of the description which has come down to us.
“A vessel containing wine, and provided with an open spout, stands upon a pedestal: it is required by shifting a weight to cause the spout to pour forth a given quantity,—sometimes, for instance, a half cotyle (1⁄4 pint), sometimes a cotyle (1⁄2 pint), and in short, whatever quantity we please. A B (fig. 53), is the vessel into which wine is to be poured: near the bottom is a spout D: the neck is closed by the partition E F, and through E F is inserted a tube, G H, reaching nearly to the bottom of the vessel, but so as to allow of the passage of water. K L M N is the pedestal on which the vessel stands, and O X another tube reaching within a little of the partition and extending into the pedestal in which water is placed so as to cover the orifice O, of the tube. Fix a rod, P R, one-half within, and the other without the pedestal, moving like the beam of a lever about the point S; and from the extremity P of the rod suspend a water-clock, T, having a hole in the bottom. The spout D having been first closed, the vessel should be filled through the tube G H before water is poured into the pedestal, that the air may escape through the tube X O; then pour water into the pedestal, through a hole, until the orifice O is closed, and set the spout D free. It is evident that the wine will not flow, as there is no opening through which air can be introduced: but if we depress the extremity R of the rod, a portion of the water-clock will be raised from the water, and, the vent O being uncovered, the spout D will run until the water suspended in the water-clock has flowed back and closed the vent O. If, when the water-clock is filled again, we depress the extremity R still further, the liquid suspended in the water-clock will take a longer time to flow out, and there will be a longer discharge from D: and if the water-clock be entirely raised above the water, the discharge will last considerably longer. To avoid the necessity of depressing the extremity R of the rod with the hand, take a weight Q, sliding along the outer portion of the rod, R W, and able, if placed at R, to lift the whole water-clock; if at a distance from R, some smaller portion of it. Then, having obtained by trial the quantities which we wish to flow from D, we must make notches in the rod R W and register the quantities; so that, when we wish a given quantity to flow out, we have only to bring the weight to the corresponding notch and leave the discharge to take place.”
THE SYPHON.
The Syphon is a bent pipe or tube with legs of unequal length, used for drawing liquid out of a vessel by causing it to rise in the tube over the rim or top. For this purpose the shorter leg is inserted in the liquid, and the air is exhausted by being drawn through the longer leg. The liquid then rises by the pressure of the atmosphere and fills the tube and the flow begins from the lower end.