Even though air pressure would drop subtle hints, its existence remained the atmosphere's own little secret until the Renaissance. An example of how pre-Renaissance thinkers had their collective heads buried in the sand on the idea of air pressure was the blind acceptance of how a suction tube operated (see Figure 4.1). If a Renaissance piston was drawn upward in a tube which sat in a tub of water, water would rise into the tube. Before the secret of air pressure was revealed, the popular explanation for a suction tube was scientifically naive: nature "abhorred a vacuum" and would do anything, legal or illegal, to prevent such a terrible thing from happening.

Fortunately, the age of blind acceptance ended with the enlightened era of the Renaissance, but it didn't occur overnight. Even scientific giants like Galileo were clueless for a while. No matter how high Galileo raised the piston, water would not rise more than about 34 feet in the tube. It took Torricelli, an insightful student of Galileo, to answer the $64,000 question: the water was forced upward in the tube by the force exerted by air pressing on the surface of the water in the tub, a force which obviously had physical limits. Torricelli had stumbled upon the secret of air pressure. He further reasoned that 34-foot high tubes of water were just a bit cumbersome to measure air pressure. Torricelli realized that a thicker, heavier liquid such as mercury would require a much shorter tube. A volume of mercury is about 13.6 times heavier than an equal volume of water, so mercury would rise about 34/13.6 = 2.5 feet, or 30 inches up the tube (see Figure 4.2). Torricelli had invented the mercury barometer!