The hourglass was invented in the 14th century when the 'sand' was originally powdered eggshell. As it takes a fixed amount of time for the sand to seep through from the top bulb to the bottom bulb, hourglasses are used for measuring periods of time.
© Islington Artefact Library
The length of time it takes for the sand to move from the top to the bottom bulb can take anything from 3 minutes to 1 hour, depending on what the hourglass was used for and who it was made by. Before the advent of clocks, hourglasses were used in a wide variety of situations, such as law courts, where they were used to make sure that people didn't speak for too long. Doctors used smaller ones like the one in the picture to measure pulse rates. Sailors used them to time 'watches' (the period when a sailor is on duty). Sailors keeping watch at night would sometimes 'flog the glass' and turn it early so that they could shorten their watch while everyone else was asleep!
Sailors also estimated the ship's speed using 'dead reckoning'. A log with a lead weight along its length (causing it to float upright and resist towing) was tied to the end of a long rope in which knots had been tied at equal intervals. The log was thrown overboard and an hourglass was turned over. The log remained more or less stationary in the water while the rope was let out behind the boat as it moved on until the hourglass ran out. The line and log were then hauled aboard and the speed of the ship was determined by dividing the number of knots let out by the amount of time measured by the hourglass. The speed of a ship is still measured in knots today.