Richard Serpo, age 12, of Guadalupe, California, for his question:

What is the story of the Aqua Lung?

All of us have seen this handy device in those fascinating underwater movies and TV shows. It is that tank and hose equipment strapped onto a deep sea diver. With its help, he can stay below until the compressed air in his tanks is exhausted.

The term "Aqua Lung" is a trade name patented by the designer. "Aqua" means water and a lung, of course, is for breathing air. This portable lung is a sturdy metal canister of compressed air with two hoses attached to a mouthpiece. A diver can strap one or two of the air tanks to his back, fix the mouthpiece where it belongs and plunge freely into the deep blue sea. Watertight valves allow him to breathe without having to surface to fill his lungs. The device sounds so simple that you would think that it was invented by the earliest deep sea divers. But the first reliable model was perfected only 25 years ago.

Deep water diving began ages ago when people discovered the beauty of pearls. The oriental pearl oyster creates his masterpieces in the Persian Gulf and .other mild stretches of ocean water. The pearl hunter had to dive for his treasure and he could stay below only as long as he could hold his breath. A practiced diver might stay below for three or four minutes. Persian divers invented a face mask of transparent shell and in 1300 A.D. this was the last word in diving equipment.

Later in history, divers were tempted to ransack sunken ships. This required longer and deeper dives. In the 18th century, English salvage divers used a great leather suit to hold air and keep out the cold, and in the next century a snug diving helmet was invented. Various devices were used to pipe air down to a diver from tanks on the deck of a salvage ship, but of course the diver was limited in his range by the length of the air hose. By the 1930's, a diver could equip himself with a warm, watertight suit, a snug face mask, goggles and a pair of fins to boost his underwater swimming. He could carry a short snorkel tube for sucking air from above the surface. This meant he could swim freely, but he had to surface for air.

Many inventors tried to perfect a device to enable a diver to take down a supply of extra air. One of them was the famous French deep sea scientist, Jacques Cousteau. He worked with designer Emile Cagnan and together they perfected their reliable Aqua Lung. In their invention was tested in the Mediterranean, With its help, a diver could make long, deep dives, swim freely underwater and stay below until he used up the air in his tanks. For shallow dives, usually the tanks are filled with compressed air. For dives below 200 feet, the mixture may be oxygen and helium.

The key to the Aqua Lung's success is a special watertight valve called the demand regulator. It opens and closes automatically with each breath taken from the tanks. In the usual model, the exhaled breath goes into the water and rises with a tell tale stream of bubbles. Divers in the armed services may use a special model that reconverts the exhaled carbon dioxide back into breathable oxygen. This lengthens the underwater limit and also does away with the tell tale bubbles, but it is not as good for deeper level sea exploration as the compressed air Aqua Lung.