snorkel, tube through which a submarine or diver can draw air while underwater. When in use, the top of the snorkel tube extends above the water surface into the air. The first snorkels were probably devised in ancient times out of the hollow reeds that are common to many lakes and marsh areas. Since they are mentioned by Pliny the Elder, a Roman naturalist of the 1st cent. A.D., it is certain that such devices were in use during the early years of the Roman Empire. The first modern snorkel was devised by Leonardo da Vinci at the request of the Venetian senate. It consisted of a hollow breathing tube that was attached to a diver's helmet of leather. The present-day diver's snorkel is typically a J-shaped tube that is open at the top and has a mouthpiece at the other end. Usually no more than 2 ft (61 cm) long, the snorkel can only be used as a breathing device when a diver is swimming face down near the surface. At greater depths, the diver must hold his breath and keep his tongue over the mouthpiece to prevent water seepage. When the diver nears the surface, a strong exhalation will clear the tube of water so that breathing can begin again. A common type of toy snorkel used by many children is S-shaped, with a plastic ball or cork that automatically rises into the upper part of the tube to prevent water seepage in subsurface dives. Because this simple valve works imperfectly, however, such snorkels are not used by skilled divers. The extensible snorkel of the submarine is usually used while the ship is submerged at about periscope depth. The air it supplies is necessary for the operation of the vessel's diesel engines. A submarine snorkel is normally equipped with safety devices that prevent seepage even if the water level should rise above the tube. When the devices are in use, the engines operate with air from the interior of the vessel. The engines are, in turn, equipped with devices that halt their operation when the air pressure in the submarine falls below about 12 psi (83×103N per sq m). Engine exhaust is discharged through a pipe somewhat shorter than the snorkel.