Most visitors to Sydney for the current razzmatazz will take a bath at the earliest opportunity. No doubt they will take several, because only by repeated experiment will they be able to determine if there is truth in the persistent whisper that the water always exits from the bath Down Under in a clockwise swirl; in contrast to the northern hemisphere where, it is said, it always rotates in an anti-clockwise sense.
The theory has been tested in laboratory conditions. Ascher Shapiro, in 1962 in Massachusetts, used a circular tank 6ft in diameter and 6in high, with the drain hole connected to a hose 20 feet long, through which the water was allowed to drain away.
After filling, the tank was covered with a plastic sheet to eliminate the effect of any currents in the air above, and the room was carefully maintained at constant temperature. It was found, after dozens of experiments, that when the water was allowed to settle for more than about 24 hours, it exited the tank invariably in an anti-clockwise spiral; if significantly less time was allowed for settling, the result was unpredictable.
In 1964 an English scientist called Binnie obtained the same results at Cambridge, but the acid test came in 1965 when an Australian team called Bilger, Fink and Luxton repeated the experiment in Sydney. Sure enough, when allowed to rest for an extended period, and protected from extraneous effects, the water always left the tank, as theory would predict, rotating in a clockwise sense.
This inclination of a fluid moving near the Earth's surface to display a tendency twirl comes about because the Earth itself rotates; it is known as the Coriolis effect, named after the French mathematician Gustave de Coriolis who studied it in 1835. But it is normally significant only in the case of relatively large-scale movements of a fluid, and cannot be assumed to affect the modest amount of liquid sloshing in your bath. In this case, the Coriolis effect is insignificant compared to pre-existing eddies in the water or the interfering geometry of the tub itself. The direction of rotation of the whirlpool when you pull the plug depends mainly on what you have been doing in the bath.
If you have swished your loofah in a clockwise direction as you scrub your back, thereby imparting clockwise vorticity to the bath-water, it will rotate clockwise as it gushes out of sight. Conversely, whether you be in Sydney or Ringaskiddy, if you have been swishing the loofah anti-clockwise, the water will whirl in that direction as it disappears from view.