An hourglass, also known as a sandglass or sand timer, is a device for the measurement of time. It consists of two glass bulbs placed one above the other which are connected by a narrow tube. One of the bulbs is usually filled with fine sand which flows through the narrow tube into the bottom bulb at a given rate. Once all the sand has run to the bottom bulb, the device is inverted in order to measure another time period.

Factors affecting the amount of time that the hourglass measures include: the volume of sand, the size and angle of the bulbs, the width of the neck, and the type and quality of the sand. Alternatives to sand that have been used are powdered eggshell and powdered marble. It is still in use, but typically only ornamentally or when a relatively approximate measurement of time is needed (for example in cooking or board games).

History

Although the Ancient Greeks, Romans and other cultures undoubtedly had the glassmaking technology to construct hourglasses, there is no concrete evidence of their existence in ancient times. Since the hourglass was one of the few reliable methods of measuring time at sea, it has been speculated that it was in use as far back as the 11th century, where it would have complimented the magnetic compass as an aid to navigation.

However, it is not until the 14th century that we find evidence of their existence, appearing in a painting by Ambrogio Lorenzetti 1328.

The hourglass was often depicted on pirate flags where it symbolised the fact that human existence is fleeting, and in England hourglasses were sometimes placed in coffins to symbolise the fact that the "sands of time" had run out. In literature, references to time measuring devices can represent death. There are images depicting the grim reaper holding an hourglass.

Example use

During the voyage of Ferdinand Magellan around the globe, his vessels kept 18 hourglasses per ship. It was the job of a ship's page to turn the hourglasses and thus provide the times for the ship's log. Noon was the reference time for navigation, which did not depend on the glass, as the sun would be at its zenith.

See also

• water clock