Sand glasses, also known as hourglasses, sand clocks, and sand timers, are amongst the oldest time-measuring devices. A blown-glass structure allows sand to flow slowly from one bulb to another. Each glass may be slightly different, so proper calibration is important. Manufacturers would adjust the amount of sand inside, or mark appropriate times on the housing for the glass.
While sand glasses can be fairly accurate and reliable, they are not useful for long-term timekeeping. The simple need to keep turning them, and the introduction of human error into that process, necessitated the use and development of other timekeeping methods.
Sundial, Water Clock
In use at least as early as 350 CE, possibly invented in Alexandria in 150 BCE. Used for navigation from the 14th to 18th centuries. In modern times they only see occasional use in kitchens (as "egg timers") and in board games.
- How does an hourglass measure time? from the Library of Congress.
- How Hourglasses are Made at MadeHow.com
- Wikipedia's pages on the Marine Sandglass and Hourglass.
- The Museo Galileo's page on Hourglasses and Water clocks.