Description: 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.
Usage Dates: 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.
Ship's sandglass as shown at the Harvard Museum of Historical Scientific Instruments. The two glass halves are actually separate pieces, held in place by the outer protective casting, with rope around the center where sand would flow.
Close-up on the bottom of the same sandglass.
Sandglass in Art
Self-Portrait of Johann Zoffany, circa 1776. The painter holds a sandglass in his left hand and a human skull in his right. an inscription reads "Ars Longa, Vita Brevis": "Art is long, life is short."
How does an hourglass measure time? from the Library of Congress.
How Hourglasses are Made at MadeHow.com
The Museo Galileo's page on Hourglasses and Water clocks.
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