The sand of the hourglass

   Hourglass is a device that measures time. It has two glass bulbs joined together by a small opening. One of the bulbs contains grains of fine, dry sand. The sand takes exactly one hour to drain from the top bulb to the bottom bulb. When all the sand has run from the top bulb, the hourglass is turned over, and the sand begins to run into the empty bulb as before. Some earlier hourglasses contained mercury, but sand works better because it flows at an even rate, regardless of the amount of sand that the bulb contains.
   The Hourglass was used to measure time before clocks were invented.
Smaller glasses, such as the half-hour glass, measure shorter periods of time. Even smaller glasses meas­ure the time needed to boil eggs. These are called egg glasses. For many years, hourglasses were used to limit the amount of time a speaker could talk. Until the 1900's, sailors used a device like an hourglass that measured less than a minute while the log line was allowed to run out. In this way, they could deter­mine how fast the ship was traveling. Hourglasses were widely used before the invention of mechanical timepieces, but they have been replaced almost entirely by watches and clocks. Many writers have mentioned the hourglass both in poetry and prose to express the passage of time.