Fig. 58 is designed to retain its contents when not in use, so that on plunging the short leg deep into a liquid the instrument will operate. This effect however will not follow if the end of the discharging leg descend below the bend near it, and if its orifice be not contracted nearly to that of a capillary tube. Fig. 59 is a syphon by which liquids may be drawn at intervals, viz., by raising and lowering the end of the discharging leg according to the surface of the liquid in the cistern. [Illustration: FIG. 60.] Figs. 60, 61, 62 are syphons described by Hero of Alexandria who lived 120 B. C.; the descriptions of the figures are the translation of the original. Let A B C D (Fig. 60) be a vessel open at the top, and through its bottom pass a tube, either an inclosed pipe as E F G, or a bent syphon G H K. When the vessel A B C D is filled, and the water runs over, a discharge will begin and continue till the vessel is empty, if the interior opening is so near the bottom of the vessel as only to leave a passage for the water. As before, let there be a vessel, A B (Fig. 61), containing water. Through its bottom insert a tube, C D, soldered into the bottom and projecting below. Let the aperture C of the syphon approach to the mouth of the vessel A B, and let another tube, E F, inclose the tube C D, the distance between the tubes being everywhere equal, and the mouth of the outer tube being closed by a plate, E G, a little above the mouth C. If we exhaust, by suction through the mouth D, the air in the tube C D, we shall draw into it the water in the vessel A B, so that it will flow out through the projection of the syphon until the water is exhausted. For the air contained between the liquid and the tube E F, being but little, can pass into the tube C D, and the water can then be drawn after it. And the water will not cease flowing because of the projection of the syphon below:—if, indeed, the tube E F were removed, the discharge would cease on the surface of the water arriving at C, in spite of the projection below; but when E F is entirely immersed no air can enter the syphon in place of that drawn off, since the air which enters the vessel takes the place of the water as it passes out. [Illustration: FIG. 61.]