An hourglass consists of a pair of glass bulbs joined together by a narrow neck, the entire structure often surrounded by a protective frame. Sand or similar granules flow between the bulbs to mark the passage of time. While the hourglass may seem simple, its accuracy depends on precision crafting and design.
Depending on how it's built, an hourglass may measure periods of time ranging from just a few minutes to an hour or more. The ability of the device to accurately measure time depends on the shape and size of the bulbs, the width of the neck and the type of sand used within. Hourglass makers have historically tinkered with the sand in an hourglass, swapping regular sand for eggshells, powdered marble and other alternatives to find a material that flows through the neck without gradually wearing it down and widening it, which changes the function of the device.
The Guinness Book of World Records suggests that the oldest hourglass dates back to a monk named Liutprand between 700 and 800 AD. While there is little evidence to support this fact, ships' records from Britain suggest the hourglass was in common use by the 14th century.