ING FORCEShortly after Professor Tait's researches were published, other investigators began to study the effect of the surface roughness on the lift on a spinning ball in flight. The markings on balls went through extensive development before the present dimpled surface was considered to be near the optimum design. While Professor Tait was apparently the first to study the lift on a spinning ball in the game of golf, others before him had studied this effect. Sir Isaac Newton, some 220 years before Tait, recognized that a tennis ball "struck with an oblique racket" would move on a curved path. This effect is sometimes called the Magnus effect because Magnus did some careful experimental work on it. Students of elementary physics study a more general effect discovered by Daniel Bernouilli in 1738. Bernouilli found that whenever the speed of a fluid, a liquid, or a gas is caused to increase, its pressure decreases. This curious, almost paradoxical, effect can be shown to follow from Newtonian mechanics quite simply for the special case of a nonviscous incompressible fluid in streamline flow in a pipe. EFFECTS OF AERODYNAMIC FORCESRather than discuss the Bernouilli effect in a theoretical fashion, it is probably better to suggest that those interested try a very simple experiment to demonstrate the reality of this effect. Cut a small square piece of light cardboard, about 2 in. on a side, stick a pin or a thumbtack through its center, and place it over the end of a spool with the pin in the hole of the spool. Blow through the spool as hard as you are able. Practically everyone expects the card to be blown away, but actually the harder you blow the tighter the card clings to the end of the spool. The pin or thumbtack is essential to keep the card from slipping sideways from the spool. The behavior of the card can be understood in terms of Bernouilli's Principle. As the air flows between the card and the end of the spool its speed increase...
