A standard chore in biological laboratories is weighing the experimental animals (e.g., mice and guinea pigs) to record their rate of growth. Biologist David Marshall Prescott, 27, of the University of California does this chore too, but his experimental animals are amoebas, and they weigh only ten billionths of a gram each.
The weighing is done in a complicated apparatus whose essential part is a tiny glass tube closed at one end and trapping a bubble of air. When this "Cartesian diver" is properly weighted, it floats midway in a column of water, and slight changes in water pressure can make it...
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