What's a diving bell?

The simplicity of the diving bell actually makes it believable that Alexander the Great may have used an early version, as legend suggests. Next time you're in the kitchen get a cup, fill up your sink and push the open end of the cup straight down into the water. You'll feel some resistance, but as long as you keep the cup straight up and down there will be a pocket of air inside the cup. This is the concept behind the diving bell.

The bells were cast in all kinds of shapes made from all kinds of materials. There were wooden bells that weren't shaped like a bell at all, but barrel-shaped. Cast iron bells were square or round, bell-shaped or conical. Some looked like whisky bottles while others resembled inverted wine glasses. The thing that each had in common was that they were all very heavy. This is why iron was commonly used. Lighter materials were weighted down and balanced with ballasts and weights. The reason it needed to be so heavy was because of the resistance you feel when you push the cup into the water.

The force of the water pushes the air up, compressing it as water enters the bottom of the open bell. While there is some water in the bottom of the bell, the bulk of it remains packed with breathable air. But there was one key limitation to this -- the bell could only go so deep and still have a usable pocket of air. A 10-foot tall (3-meter) diving bell that dove to 325 feet (100 meters) would only leave about 11 inches (30 centimeters) of air. For a while, divers simply tried shallow waters and ascended when the air was used up.

There was also the matter of decompression sickness, or blood. Most of the oxygen is consumed by tissue, the nitrogen remains. This dissolved nitrogen is what causes the bends. If you ascend too quickly, the nitrogen leaves your blood too fast and forms bubbles. These bubbles block tiny blood vessels and can lead to strokes, heart attacks, ruptured blood vessels in the lungs and joint pain.

Divers endured these limitations while continuing to use versions of the diving bell during the Renaissance and into the 16th century. It would take some key innovators in the late 1600s and into the 1700s to improve the limitations of the diving bell.

The first real innovation to the diving bell came when Frenchman Denis Papin figured out how to get fresh air into them in 1689. Flexible hoses attached to bellows ran into the water and up inside the bell, providing fresh, breathable air. While this helped divers stay submerged longer, it did nothing to combat the pressure and to allow for deeper dives. The next obstacle was to figure out how to get pressurized air into the bell, something Englishman Edmund Halley did just a year later.

Halley attached weighted wooden barrels to the diving bell. At the bottom of each was a hole in the side that allowed water to come in, forcing the air up. At the top, a hose ran from the barrel and led to the bottom side of the bell, where it was attached with a faucet head. The divers inside could simply turn the faucet to allow the fresh air to flow in. The barrels were alternately pulled to the surface for "refilling," and dropped back in again. By being able to control the amount of air in the bell, the divers were able to match the pressure of the displaced water. Halley's innovation made the pressure of the interior of the bell equal to that of the surrounding water.

Things went on like this for a while, until English scientist John Smeaton invented the diving air pump in 1788. The surface pump was attached to the roof of the bell and operated by four men. In the end, it was, in concept, much like Denis Papin's original plan 100 years earlier. Using this new technology, workers were able to descend in a diving bell and repair the foundation of the Hexham Bridge in England. Smeaton also was one of the first to make the bells square, calling them "diving chests."

Other improvements over the years included adding electric lighting inside, thick convex lenses as windows, and increasing the interior space so that as many as 12 men could descend at a time. The earliest applications of the bells were to transport workers under water so that they could repair or build docks, lighthouse foundations and bridges. Early enthusiasts also tried their hand at deep shipwreck salvage, something the bells are still used for today.