An hourglass consists of a pair of glass bulbs joined by a short passage, or neck. One bulb is partially filled with fine, dry sand. When the hourglass is turned to stand on the other bulb, the sand passes slowly through the neck into the empty bulb in a period of approximately one hour. This device was commonly used from the fourteenth through seventeenth centuries. Sailors continued to use them into the nineteenth century. Although marine chronometers were available, not all seafarers could afford to have them; a ship's chronometer was not always accessible to all those on board a vessel. Smaller glasses were made to measure smaller or larger time periods. Each hourglass was capable of measuring only a single unit of time and had to be constantly turned over to be maintained. They were also incapable of telling which hour it was. As with sundials, hourglasses have passed on to novelty status. They are still used in a small way as egg timers and as board-game timers. The old poetic adage "the sands of time" refers to the hourglass. As with all things, the hourglass has become a victim of the passage of time.