A device for measuring time consisting of two glass bulbs connected by a narrow neck, with sand flowing from the upper to lower bulb.
The compound 'hourglass' appeared in English around 1515, but hourglasses themselves date to the 8th century. The word combines 'hour' (from Greek 'hora' meaning season or time) with 'glass.' Interestingly, early hourglasses didn't always measure exactly one hour - they came in various time intervals, but the one-hour version became so standard that it gave its name to the entire category of sand timers.
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