The erroneous bit of folk wisdom you refer to says that water always drains in a clockwise direction in the Southern Hemisphere, and in a counterclockwise direction in the Northern Hemisphere. The supposed reason for this "fact" is the Coriolis effect, which has to do with the effect of the earth's rotation on moving objects.
Well, there is such a thing as the Coriolis effect. It explains why macroevents such as hurricanes rotate in a clockwise direction in the Southern Hemisphere and counterclockwise in the Northern Hemisphere. However, when you get down to itty-bitty phenomena such as the water draining out of your bathtub, the Coriolis effect is insignificant, amounting to roughly three ten-millionths of the force of gravity (in Boston, at least, which is where they happened to do the measuring).
The boring truth is that water drains every which way no matter what hemisphere you're in, for reasons which have to do mostly with the shape of the drain, the way you poured in the water in the first place, and so on.
All this was demonstrated way back in 1962 by one Ascher Shapiro, a researcher at the Massachusetts Institute of Technology. Shapiro filled a circular tank six feet in diameter and six inches high in such a way that the water swirled in a clockwise direction. (Remember, now, that Coriolis forces in the Northern Hemisphere act in a counterclockwise direction.)
Shapiro then covered the tank with a plastic sheet, kept the temperature constant, and sat down to read comic books or whatever scientists do while they wait for their experiments to percolate. When he pulled the plug after an hour or two, the water went down the drain clockwise, presumably because it still retained some clockwise motion from filling.
On the other hand, if Shapiro pulled the plug after waiting a full 24 hours, the draining water spiraled counterclockwise, indicating that the motion from filling had subsided enough for the Coriolis effect to take over. When the plug was pulled after four to five hours, the water started draining clockwise, then gradually slowed down and finally started swirling in the opposite direction.
Needless to say, unless you are a consummate slob, you do not wait 24 hours (or even 4-5 hours) to drain your bathtub. Hence the influence of the Coriolis effect may be safely described as slight.
But I'm sure the myth of the bathtub spirals will endure. Shapiro did his work in 1962 and I proclaimed it to the world in 1983. Yet next to the mystery of where all the baby pigeons are, this remains the commonest question I get.
--CECIL ADAMS
