The origin of sand clocks began in Italy during the 14th century. The earliest reference to the sand clocks comes from text dating back to 1346-1360. Sand clocks were invented because the previous way of keeping time, sun clocks, was not useful at night or when the weather was cloudy. The inventor is unknown. Sand clocks work by forcing sand through a small hole. By measuring how long it took for a certain amount of sand to get through the hole into the bottom half of the container, people could tell how much time had passed. This could be calibrated by changing the size of the sand, the size of the hole, the size of the container, or the amount of sand. This proved much more useful than water clocks, because water clocks would let more water go through depending on the temperature and humidity around them. Christopher Columbus is know to have passed time with the use of a half-hour sand clock that was turned is it emptied to keep track of the seven canonical hours. .
Because sand clocks remained basically unaffected by heat, cold, and moving around, they have a long history at sea. There are records of sand clocks in ships" inventories from about 1400 A.D. Small sand clocks were used as interval timers to measure speed in navigation. A log was thrown over the side with a line knotted every 47 feet attached to it. The speed in which the knots ran out was measured by the 28 second glass, giving nautical speed in "knots" clock.
