Hourglass

An hourglass (or sandglass, sand timer, sand watch, or sand clock) is a mechanical device used to measure the passage oftime. It comprises two glass bulbs connected vertically by a narrow neck that allows a regulated trickle of material (historically sand) from the upper bulb to the lower one. Factors affecting the time interval measured include the sand quantity, the sand coarseness, the bulb size, and the neck width. Hourglasses may be reused indefinitely by inverting the bulbs once the upper bulb is empty.
Little written evidence exists to explain why its external form is the shape that it is. The glass bulbs used, however, have changed in style and design over time. While the main designs have always been ampoule in shape, the bulbs were not always connected. The first hourglasses were two separate bulbs with a cord wrapped at their union that was then coated in wax to hold the piece together and let sand flow in between. It was not until 1760 that both bulbs were blown together to keep moisture out of the bulbs and regulate the pressure within the bulb that varied the flow.
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