A marine sandglass is a timepiece of simple design that is a relative of the common hourglass, a marine (nautical) instrument known since the 14th century (although reasonably presumed to be of very ancient use and origin). They were employed to measure the time at sea or on a given navigational course, in repeated measures of small time increments (e.g., 30 minutes). Used together with the chip log, smaller marine glasses were also used to measure the boat speed through the water in knots. Although vital to maritime navigation, marine glasses were not accurate measuring instruments for the passage of time; many design and environmental factors could affect the duration of sand's flow, and therefore its reported time. Their use continued through the early 19th century, when they were supplanted by reliable mechanical timepieces, and by other advances in marine navigation. Marine sandglasses were very popular on board ships, as they were the most dependable measurement of time while at sea. Unlike the clepsydra, the motion of the ship while sailing did not affect the hourglass. The fact that the hourglass also used granular materials instead of liquids gave it more accurate measurements, as the clepsydra was prone to get condensation inside it during temperature changes. Seamen found that the hourglass was able to help them determine longitude, distance east or west from a certain point, with reasonable accuracy. This video is targeted to blind users. Attribution: Article text available under CC-BY-SA Creative Commons image source in video

• published: 24 Nov 2014
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