As we noted yesterday, the Coriolis effect results in the general rule that air moving over the surface of the Earth in the northern hemisphere is always deflected to the right.
It may start to flow directly from high to low pressure, but gradually changes direction until it ends up moving along the isobars with low pressure on its left hand side. Or to put it another way, in the northern hemisphere, the air spirals in an anti-clockwise direction around an area of low pressure.
Now this rule applies, not just to air, but to any fluid that happens to be moving. And this in turn gives rise to the great bathtub controversy, the heated debate as to whether water draining from a bath should be influenced by the Coriolis force and, therefore, disappear down the plug-hole in an anti-clockwise in the northern hemisphere, and clockwise in Australia or other places south of the equator.
Now casual observation makes it clear that this, in fact, is not the case. The sense of rotation of the vortex of the domestic kitchen sink or bath-tub varies widely - but this is understandable. On such a tiny scale, the Coriolis force is very weak, and dwarfed by residual motions in the water, by currents induced by thermal inhomogeneity, or by viscous forces emanating from irregularities on the internal surface of the bath itself.
But if these could be eliminated, the Coriolis force should then be free to dictate the direction of the liquid pirouettes.
Ascher Shapiro, in 1962 in Massachusetts, was one of the first to test these theories in controlled conditions. He used a circular tank 6 ft. in diameter and 6 ins. high, with the drainhole connected to a 20 ft. hose 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 with the unlikely names of Bilger, Fink and Luxton repeated the experiment in Sydney.
There was now no room for doubt: when allowed to rest for an extended period, and protected from extraneous complications, the water left the tank, as theory would predict, always rotating in a clockwise sense.