The first device that allowed man to descend underwater and observe the marine environment for an extended period was the diving bell. Named for their shape, these chambers began as open-ended metal-rimmed wooden barrels and have existed since antiquity. Legend claims that Alexander the Great (356-323 B.C.) used a diving bell in 332 B.C. to take a trip beneath the Bosporus. Whether this tale is true or not, Aristotle, a contemporary, told of a simple but functional diving bell. By 1620 Sir Francis Bacon (1561-1626) knew of a "sort of metal barrel" that could take men underwater. And in 1665, a diving bell was used to salvage guns from a shipwrecked Armada vessel. Although many eighteenth-century inventors worked on diving bells, the noted English astronomer Edmund Halley (1656-1743) is credited with the first modern version. Although still open at the bottom, his wooden bell had a capacity of 60 cubic feet and contained a bench and platform for its occupants. In 1716, he demonstrated a way to replenish the air inside so it could remain submerged for even more extended periods of time. While divers remained underwater inside the bell, lead containers filled with fresh air were lowered from the surface. The containers were encased in wooden barrels to which hoses were attached. When the barrel reached the bell, it was tipped in such a way that the air was forced upward and entered the bell. As the new air entered, the occupants released the spent air through a tap at the top of the chamber. Halley later developed a device he called the "cap of Maintenance," a kind of headgear that allowed divers to leave the bell yet remain attached to its air-supply system. By the 1720's developers added an air pump to the diving bell, ensuring a constant supply of fresh air. In 1788, British engineer John Smeaton built the first truly modern diving bell, which was both safe and well designed. Modern bells can hold up to four passengers and travel to depths of about 1,500 feet (460 m). These are primarily used for construction, maintenance, and repair of offshore oil platforms and for naval operations.