These high quality reproduction Sand timers and Hourglassess are replicas of the original sandtimers used by mariners over the ages to measure time against distance to determine the rate of knots sailed The are made of the finest Brass.These Nautical hourglasses make exciting nautical gifts and executive presents to anyone who loves the sea. It is only with the introduction of mechanical type clocks that time began to be measured in units.Prior to 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 aperature was unaffected by weather, so the hourglass sand timer became the ultimate expression of flowing time measuring a unit. and 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 speed at which the knots ran out was measured by the 28 second glass, giving nautical speed in "knots."