In addition to the two types of timekeeping devices—sundials and water clocks—there is also the hourglass (sablier), which is used for measuring short periods of time. Originating from Egypt, this type of clock is related to water clocks but was developed later. Today, hourglasses are often used to time conversations on the telephone or to measure the boiling time for eggs.
An hourglass consists of a small container with a narrow neck, where sand flows from the upper chamber to the lower chamber until it is empty. At that point, the hourglass is flipped over. The duration of sand flowing from the top to the bottom is consistent. There are various types of hourglasses: some measure one hour, others half an hour, and some are designed to measure shorter intervals, such as 10 minutes. The British Navy used these shorter hourglasses until 1839.
Additionally, the hour candles (chandelle horaire) were widely used during the Middle Ages. This type of candle measures time based on the principle that the passage of time is proportional to the amount of candle burnt.
Pham Van Tuan
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