Guglielmo's secret: The enigma of the first diving bell used in underwater archaeology

The earliest employment of a breathing apparatus in underwater archaeology took place in July 1535, when Guglielmo de Lorena and Francesco de Marchi explored a Roman vessel sunk in Lake Nemi near Rome using a one-person diving bell that Guglielmo had invented. Francesco's book on military architecture contains first-hand documentation of the exploration and of the diving bell; however, the author refrained from describing the bell's most intriguing feature, the air-supply mechanism. The diving bell was very small, so that the air it contained could support a diver for only a few minutes; however it had a mechanism that expelled breathed air while maintaining sufficient air pressure inside to prevent the water level from rising and this mechanism allowed the diver to work submerged for hours. Francesco did not describe this mechanism because he had taken an oath to keep Guglielmo's invention a secret. This article analyses Francesco's text to unveil this secret.