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That depends on which way the water spills into the bathtub, and how the bathtub is built. Contrary to popular opinion, it doesn't depend on the bathtub's latitude. The inertial force with which the rotating Earth affects air and water masses is known as the Coriolis effect or Coriolis force. It's the reason why cyclones rotate clockwise in the southern hemisphere, and counterclockwise in the northern. However, you need huge amounts of moving air and water for it to take effect. In the case of a bathtub, the water drain is primarily affected by the shape of the drain and residual currents in the plumbing. According to this Scientific American piece, rumor has it that some entrepreneurs in an African country have set up toilets on either side of the equator. For a price, they let the curious watch the toilets flush in opposite directions. But it's bunk. The Guardian hosts a wonderful column called "Speculative Science" that recently featured several no-nonsense types responding to the same question. Reader William Dunlap roundly debunks the Coriolis Drain Myth and refers us to the Bad Science Page for more shattering revelations. You'll
also find a Coriolos entry in the ever-helpful Urban Legends Reference Pages that sums up the issue: "Toilets and sinks drain in the directions they do because of the way water is directed into them or pulled from them."
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