The hourglass is a contrivance for measuring time. In making, the glass-blower closes a cylinder at each end, and draws the walls together to form a small passage or waist at the middle. When complete the glass has the form of two cones placed point to point. In use, as much sand or mercury is inclosed in one end as will run through the passage in an hour. By turning the glass the other end up, the passage of another hour can be marked, and so on. If a person were set to do a task, or wished to speak for an hour, the hourglass reminded him of the passage of time, and also when his "hour was up." Its use gave rise to the expression, "the sands of time." The hourglass continued in use for a long time after the invention of clocks.
