An hour glass , also known as a sandglass, sand timer, sand watch, sand clock, or egg timer, measures the passage of a few minutes to an hour of time.
It has two connected vertical glass bulbs allowing a regulated trickle of material, usually sand, from the top bulb to the bottom bulb.
Once the top bulb is empty, it can be inverted to begin timing again. Factors affecting the time measured include the amount of sand, the bulb size, the neck width, and the sand quality.
Alternatives to sand are powdered eggshells and powdered marble, although sources disagree on the best material.
In modern times, hourglasses are mostly ornamental, but are handy when used as a timer when an approximate measure suffices, such as egg timers for cooking or for timed board games.
The Design of the Hour Glass or Egg Timer
The shape behind the hourglass has hardly any written evidence of why its external form is the shape that it is. The glass bulbs used, however, have changed in style and design over time.
While the main designs have always been ampoule in shape, the bulbs were not always connected. The first hourglasses were two separate bulbs with a cord wrapped at their union that was then coated in wax to hold the piece together and let sand flow in between.
It was not until 1760 that both bulbs were blown together to keep moisture out of the bulbs and regulate the pressure within the bulb that varied the flow.
While some hourglasses actually did use sand as the granular mixture to measure time, many did not use sand at all. The material used in most bulbs was a combination of powdered marble, tin/lead oxides, and pulverized, burnt eggshells.
Over time, different textures of granule matter were tested to see which gave the most constant flow within the bulbs. It was later discovered that for the perfect flow to be achieved the ratio of granule bead to the width of the bulb neck needed to be 1/12 or more but not greater than 1/2 the neck of the bulb.
Practical uses of the hourglass in history
Hourglasses were an early dependable, reusable and accurate measure of time. The rate of flow of the sand is independent of the depth in the upper reservoir, and the instrument will not freeze in cold weather.
From the 15th century onwards, they were being used in a range of applications at sea, in the church, in industry and in cookery.
During the voyage of Ferdinand Magellan around the globe, his vessels kept 18 hourglasses per ship. It was the job of a ship’s page to turn the hourglasses and thus provide the times for the ship’s log.
Noon was the reference time for navigation, which did not depend on the glass, as the sun would be at its zenith.
More than one hourglass was sometimes fixed in a frame, each with a different running time, such as 1 hour, 45 minutes, 30 minutes, and 15 minutes.
Modern Uses for the Hourglass
While they are no longer widely used for keeping time, some institutions do maintain them. Both houses of the Australian Parliament use three hourglasses to time certain procedures, such as divisions.
The sandglass is still widely used as the kitchen egg timer; for cooking eggs, a three-minute timer is typical. Egg timers are sold widely as souvenirs. Sand timers are also sometimes used in games such as Pictionary and Boggle to implement a time constraint on rounds of play, and provide a sense of urgency to the game of Quicksand.