sandglass

Sandglass, also known as an hourglass, is a device for measuring time. It consists of two transparent glass bulbs connected by a narrow neck, allowing fine sand to flow from the upper bulb to the lower one at a predictable rate. When the sand has transferred, the device can be inverted to measure another interval. The total duration is determined by the amount of sand and the size of the neck, and common durations range from several minutes to about an hour.

Origins of the sandglass are not precisely documented, but they appear in Europe in the late medieval period and became widely used on ships in the early modern era. Compared with water clocks and early mechanical clocks, the sandglass offered portable, relatively weather-insensitive timing and did not rely on gravity-driven weights. Over centuries, manufacturing and framing improved durability and readability, though the flow depends on sand quality and environmental conditions.

Construction typically involves two bulbs of clear glass sealed together, encased in a supporting frame made of wood, metal, or stone. The sand is fine and uniform to prevent clogging; variations include powdered marble or ground shells. The narrow neck controls the rate of flow, and the device is designed for easy inversion to start a new timing period.

In addition to practical timing tasks—such as cooking, speeches, or games—sandglasses have been used as decorative objects and as symbols of the passage of time in art and literature. In modern times they are often used in classrooms to demonstrate basic concepts of measurement and flow, while precision timing is handled by electronic or mechanical timers.