Hourglasses are rarely found intact on shipwreck sites. That they are made from blown glass alone makes them very fragile. This hourglass was recovered from the world's deepest wooden shipwreck, a 200-year-old merchant vessel, resting at a depth of 4,818 meters, almost 16,000 feet or 3 1/4 miles down, in the Blake Basin of the Atlantic Ocean. The wreck site lies deep in the heart of the infamous Bermuda Triangle and is known as the Atlantic Target Wreck Site. It has withstood not only the shipwreck itself, but has endured the pressures of such a deep wrecksite.
It was only with the introduction of the mechanical clock that time began to be measured in discrete units. Before the 15th century time was thought to be flowing. That concept resulted in the clepsydra, but heat and cold rendered water unreliable as a time medium. Dried sand passing from one container to another through a narrow aperture was unaffected by weather, so the hourglass sand timer became the ultimate expression of flowing time measuring a unit.
Because hourglass sand timers remain relatively unaffected by heat, cold and swinging about, they have a long history at sea. There are records of sandglasses in ships' inventories from about 1400 A.D. Small sandglasses were used as interval timers to measure speed in navigation. A log was thrown over the side with a line knotted about every 47 feet attached to it. The 28-second glass, giving nautical speed in "knots", measured the speed at which the knots ran out.