WHAT gives an airplane its lift? Near the beginning: of the eighteenth century, a Swiss mathematician, Daniel Bernoulli, discovered a physical principle which accounts for the greater part of the answer. In the course of experimenting with fluids, both liquids and gases, he found that pressure decreases as velocity increases.

The Bernoulli principle explains the increased draft of a chimney when wind blows across its top, the bursting of a house near the path of a tornado, the curve of a ball that is spun as it is thrown. It is also one of the most vital factors in aerodynamics, accounting for nearly 75 percent of the total lift exerted by an airplane wing. Air passing rapidly over the curved upper surface of a wing reduces pressure there, causing a partial vacuum. The relatively greater pressure beneath the wing then buoys it up. This may sound mysterious, for the idea that lift comes from a partial vacuum above a wing is not the most obvious explanation. But by performing the accompanying simple experiments, you can prove to yourself how the Bernoulli principle plays a part in modern aerodynamics.