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About hourglass
An hourglass is a device used to measure the passage of time. It comprises two glass bulbs connected vertically by a narrow neck that allows a regulated flow of a substance from the upper bulb to the lower one due to gravity. Typically, the upper and lower bulbs are symmetric. The specific duration of time a given hourglass measures is determined by factors including the quantity and coarseness of the particulate matter and the neck width.
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