hourglasses

An hourglass, also known as a sandglass or sand timer, is a device for measuring a specific period of time. It consists of two glass bulbs connected by a narrow neck, through which a regulated flow of a granular material—traditionally fine sand—passes from the upper bulb to the lower one. When the upper bulb empties, the device has measured the intended interval. The hourglass is typically inverted to restart the timing.

Construction and operation: The two bulbs are sealed in a supporting frame. The flow rate depends on the neck diameter, the grain size and shape of the sand, humidity, and the geometry of the bulbs. Because these factors influence how quickly the sand passes, hourglasses are not universally standardized; different devices are designed to measure different intervals, from a few minutes to an hour or more. They are designed so that the sand’s flow is fairly constant for a given construction, though not perfectly precise.

History: Hourglasses emerged in medieval Europe and are often associated with maritime navigation, religious practices, and daily life. Earlier timekeeping methods included water clocks, but the hourglass gained popularity for its portability and ease of use. By the 14th to 16th centuries, hourglasses were common in ships, monasteries, and households, and they remained in use in various forms even after mechanical clocks became widespread.

Cultural significance and modern use: The hourglass has become a symbol of time, mortality, and the passage of life in art and literature. Today, it is often used as a decorative object, a kitchen or game timer, or as a teaching tool to illustrate concepts of flow and duration. Some specialty hourglasses are produced for demonstrations of physics or for aesthetic display.