173. =Uses of the Syphon.=--The syphon is used chiefly for discharging liquids from one barrel or vessel into another. For convenience, it is often constructed after the plan of Fig. 120. To the long branch, B C, is attached the tube ED. It is used in this way: The end of the short branch, A, being introduced into the liquid to be drawn off, you place your finger upon C, and after filling the syphon by suction at E, you remove the finger and let the liquid run. The syphon has sometimes been used to drain pits and mines. It of course can never be used where the elevation over which the tube is to bend is over 34 feet from the surface of the water to be discharged, for then the air would not press the water up to the bend of the syphon. [Illustration: Fig. 121.] 174. =Cup of Tantalus.=--This cup, Fig. 121, has a syphon in it, the short branch, _b_, opening into the cup, and the long branch, _d_, having its outlet in the bottom. As you pour water into the cup it will remain there until you pour enough in to cover the bend of the syphon. As soon as this is done, the syphon being filled, the water suddenly flows out from the outlet, _a_, of the long branch. [Illustration: Fig. 122.] 175. =Intermitting Springs.=--The operation of an intermitting spring is essentially the same with that of the cup of Tantalus. You have a representation of such a spring in Fig. 122. There is a cavity in a hill, supplied with water from a passage above. There is also a passage from it which takes a bend upward like a syphon. Now when the water in the cavity is low it will not run out from the syphon-like channel; but when the cavity becomes filled above the level of the bend the water will at once flow out, just as it does from the cup of Tantalus as soon as the bend of its syphon is covered. [Illustration: Fig. 123.]