1911 Encyclopædia Britannica/Hour-glass

HOUR-GLASS, a device for measuring intervals of time, also known as sand-glass, and as 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 apices and having a minute passage formed between them. A quantity of sand (or occasionally of mercury) is enclosed in the bulbs, and the size of the passage is so proportioned that this sand will completely run through from one bulb to another in the time it is desired to measure—e.g. an hour or a minute. Instruments of this kind, which have no great pretensions to accuracy, were formerly common in churches. In the English House of Commons, as a preliminary to a division, a two-minute sand-glass is still turned, and while the sand is running the “division bells” are set in motion in every part of the building, to give members notice that a division is at hand.