Hourglass
What is an hourglass?
What are some drawbacks of using an hourglass?
What materials have been used in hourglasses?
Hourglass, an early device for measuring intervals of time. It is also known as a sandglass or a log glass when used in conjunction with the common log for ascertaining the speed of a ship. It consists of two pear-shaped bulbs of glass, united at their apexes and with a minute passage formed between them. A quantity of sand or other granular filling (or, rarely, liquid mercury) is enclosed in the bulbs, and the size of the passage is so proportioned that the material will completely run through from one bulb to the other in the time it is desired to measure—for example, an hour or a minute.
Instruments of this kind make no claims to accuracy and have had limited applications, even in their heyday. One major drawback is the difficulty of marking shorter intervals within their full running time. Unlike water, sand does not maintain a level surface as it flows, making it difficult to relate the amount remaining in the upper bulb to evenly spaced time divisions. Most hourglasses therefore measure a single fixed interval. To provide a range of intervals, hourglass makers sometimes affix several glasses with different running times in a single frame. More sophisticated designs employ vessels with steeply sloping walls, which encourage the sand to flow more uniformly and allow shorter intervals to be marked with greater precision. In the past, however, the stocky, stunted varieties dominated the market because of their aesthetic appeal, despite being less efficient than the leaner, steep-walled designs.
Evolution
The hourglass uses a principle similar to that of the ancient water clock (or clepsydra), replacing flowing liquid with fine granular material, usually sand. The water clock was in use in ancient Egypt, Rome, and Greece, but the hourglass, specifically a sandglass, does not appear in the historical record until the medieval period. The long delay between the development of water clocks and hourglasses may have resulted from the technical difficulties of making granular material flow at a steady rate.
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The earliest evidence of an hourglass dates to about 1338, when it was depicted in an Italian fresco. By the late Middle Ages, hourglasses were used to time scholarly work and religious services before becoming more widely adopted in everyday life. Hourglasses were particularly useful aboard ships for managing sailors’ daily activities because, unlike water clocks, their accuracy was relatively unaffected by the ship’s motion.
The design and components of the hourglass have undergone modifications over time as makers experimented with different materials and refined the construction of the device. Before about 1760, the two glass bulbs forming the body of the hourglass were made separately and then joined and sealed together. Thereafter the bulbs were increasingly blown as a single piece, which reduced problems associated with moisture and wear.
Granular fillings used
Various materials have been used to provide a steady flow through the narrow opening of the hourglass. The quality of the materials determines both the accuracy of timekeeping and the structural integrity of the device, as coarse materials tend to abrade the glass. Ordinary river sand was often used, although its uneven grain size and sensitivity to dampness affected the timing. Hourglass makers have also tried other substances, including powdered marble, fine particles of silver, tin, copper, and iron, as well as powder made from burned lead. Ground cinnamon, ground eggshells, and fine emery have also been used. Historically, ground eggshells were considered particularly suitable for use at sea, possibly because they resist moisture, and a dense mixture made from burned tin and lead, called Venetian sand, allowed a much longer running time without requiring a proportionally larger glass.