Hydrodynamics: The Bathtub Vortex

HYDRODYNAMICS Busy as they are sending a man to the fringes of space and the to bottom of the sea, routing disease and building bigger and better nuclear bombs. 20th centry scientists still find time for smaller, more mundane problems. One of the smallest: Which way does the water spin when it swirls down the bathtub drain?

Scientists have known—or thought they have known—the answer ever since man has had bathtubs. backgroud information on the subject has been building ever since the greeks advanced the notion that the earth rotates on its axis. Left to itself, a tub of water should theoretically be influenced by the rotation of the earth and go down a drain in the tub’s bottom in the same direction as the earth is spinning—which would look clock wise to an observer hovering in space hovering in space above the Southern Hemisphere, Counterclockwise to an observer in the north. The theory was convincing enough, but so difficult was the proof that only recently has the direction of the bathtub vortex been verified in the laboratory.

Movement casued by the earth’s rotation is so slight that even the smallest countermovement can overwhelm it: in the ordinary bathtub, water will swirl out either way—usually in the same direction it was swirled in. Even if allowed to sit enough to stop the force of its inward swirl, the water’s natural rotational movement will often be overcome by air currents, uneven heating, surface tension or irregularities in the shape of its container.

Three years ago, Professor Ascher H. Shapiro, head of the Department of Mechanical engineering at the Massachusetts Institute of Technology, finally proved the counterclockwise movement for the Northern Hemisphere by constructing a perfectly symmetrical tub filling it with with a clockwise swirl, then letting it sit for 24 hours in still air. After that, he carefully pulled the plug and filmed the results. Using Shapiro’s technique, five persistent investigators at the University of Sydney have now duplicated his experiment, demonstrating that Down Under water will drain clockwise. To be sure they report in the magazine Nature, they cannot conclusively prove that it was the earth, not the swirl. But, with proper scientific caution, they add: “We have acquired confidence in the Hypothesis.” Shapiro is unsurprised. He sees a certain “esthetic symmetry” in the results, points out that with the proper tub and reasonable patience, man could have proved the rotation of the earth 1,000 years ago.