An hourglass (or sandglass, sand timer, sand watch, or sand clock) is a mechanical device used to measure the passage of time. It comprises two glass bulbs connected vertically by a narrow neck that allows a regulated trickle of material (historically sand) from the upper bulb to the lower one. Factors affecting the time interval measured include the sand quantity, the sand coarseness, the bulb size, and the neck width. Hourglasses might be reused indefinitely by inverting the bulbs once the upper bulb is empty.

History

Antiquity

Sarcophagus dated c. 350 AD, representing the wedding of Peleus and Thetis (observe the magnification with the hourglass held by Morpheus in his hands)

The origin of the hourglass is unclear. Its predecessor the clepsydra, or water clock, might have been invented in ancient India. According to the American Institute of New York, the clepsammia or sand-glass was invented at Alexandria about 150 BC. According to the Journal of the British Archaeological Association the so-called clepsammia were in use before the time of St. Jerome (335 AD), and the first representation of an hourglass is in a sarcophagus dated c. 350 AD, representing the wedding of Peleus and Thetis, discovered in Rome in the eighteenth century, and studied by Winckelmann in the nineteenth century, who remarked the hourglass held by Morpheus in his hands.

Reappearance in the Early Middle Ages

Temperance bearing an hourglass; detail Lorenzetti's Allegory of Good Government, 1338

There are no records of the hourglass existing in Europe prior to the Early Middle Ages, such as invention by the Ancient Greeks; the first supported evidences appears from the eighth century AD, crafted by a Frankish monk named Luitprand who served at the cathedral in Chartres, France. But it wasn't until the fourteenth century that the hourglass was seen commonly, the earliest firm evidence being a depiction in the 1338 fresco Allegory of Good Government by Ambrogio Lorenzetti.

Use of the marine sandglass has been recorded after the fourteenth century. The written records about it were mostly from logbooks of European ships. In the same period it appears in additional records and lists of ships stores. The earliest recorded reference that can be said with certainty to refer to a marine sandglass dates from c. 1345, in a receipt of Thomas de Stetesham, receptionist of the King's ship La George, in the reign of Edward III of England; translated from the Latin, the receipt says: in 1345:

"The same Thomas accounts to have paid at Lescluse, in Flanders, for twelve glass horologes (" pro xii. orlogiis vitreis "), price of each 4½ gross', in sterling 9s. Item, For four horologes of the same sort (" de eadem secta "), bought there, price of each five gross', making in sterling 3s. 4d."

Marine sandglasses were quite popular on board ships, as they were the most dependable measurement of time while at sea. Unlike the clepsydra, the motion of the ship while sailing didn't affect the hourglass. The fact that the hourglass additionally used granular materials instead of liquids gave it more accurate measurements, as the clepsydra was prone to get condensation inside it throughout temperature changes. Seamen found that the hourglass was able to help them determine longitude, distance east or west from a certain point, with reasonable accuracy.

The hourglass additionally found popularity on land. As the use of mechanical clocks to indicate the times of events like church services became more common, creating a "need to keep track of time", the demand for time-measuring devices increased. Hourglasses were essentially inexpensive, as they required no rare technology to make and their contents weren't hard to come by, and as the manufacturing of these instruments became more common, their uses became more practical.

Hourglasses were commonly seen in use in churches, homes, and work places to measure sermons, cooking time, and time spent on breaks from labor. Because they were being used for more everyday tasks, the model of the hourglass began to shrink. The smaller models were more practical and quite popular as they made timing more discreet.

After 1500, the hourglass wasn't as widespread as it had been. This was due to the development of the mechanical clock, which became more accurate, smaller and cheaper, and made keeping time easier. The hourglass, however, didn't disappear entirely. Although they became relatively less useful as clock technology advanced, hourglasses remained desirable in their design. The oldest known surviving hourglass resides in the British Museum in London.

Not until the eighteenth century did John Harrison and his son James, come up with a marine chronometer that significantly improved on the stability of the hourglass at sea. Taking elements from the design logic behind the hourglass, they made a marine chronometer in 1761 that was able to accurately measure the journey from England to Jamaica accurate within five seconds.

19th century hourglass

Design

Little written evidence exists to explain 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 weren't 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 wasn't 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.

Material

While a few hourglasses actually did use sand as the granular mixture to measure time, a large number of didn't use sand at all. The material used in most bulbs was a combination of "powdered marble, tin/lead oxides, and pulverized, burnt eggshell". 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

3-minute egg timer

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 won't freeze in cold weather.

From the fifteenth 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, 18 hourglasses from Barcelona were in the ship's inventory, after the trip being authorised by emperor Charles V. 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 didn't depend on the glass, as the sun would be at its zenith. A number of sandglasses can be fixed in a common frame, each with a different operating time, e.g. as in a four-way Italian sandglass likely from the seventeenth century, in the collections of the Science Museum, in South Kensington, London, which could measure intervals of quarter, half, three-quarters, and one hour (and which were additionally used in churches, for priests and ministers to measure lengths of sermons).

Modern practical uses

The Timewheel in Budapest, Hungary.

While they're no longer widely used for keeping time, a few 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, hence the name "egg timer" for three-minute hourglasses. Egg timers are sold widely as souvenirs. Sand timers are additionally at times used in games such as Pictionary and Boggle to implement a time constraint on rounds of play.

Symbolic uses

Pirate Christopher Moody's "Bloody Red" jack, c. 1714

Unlike most additional methods of measuring time, the hourglass concretely represents the present as being between the past and the future, and this has made it an enduring symbol of time itself.

The hourglass, at times with the addition of metaphorical wings, is often depicted as a symbol that human existence is fleeting, and that the "sands of time" will run out for every human life. It was used thus on pirate flags, to strike fear into the hearts of the pirates' victims. In England, hourglasses were at times placed in coffins, and they have graced gravestones for centuries. The hourglass was additionally used in alchemy as a symbol for hour.

The former Metropolitan Borough of Greenwich in London used an hourglass on its coat of arms, symbolising Greenwich's role as the origin of GMT. The district's successor, the Royal Borough of Greenwich, uses two hourglasses on its coat of arms.

Modern symbolic uses

Recognition of the hourglass as a symbol of time has survived its obsolescence as a timekeeper. For example, the American television soap opera Days of Our Lives, after its first broadcast in 1965, has displayed an hourglass in its opening credits, with the narration, "Like sands through the hourglass, so are the days of our lives," spoken by Macdonald Carey.

Various computer graphical user interfaces might change the pointer to an hourglass throughout a period when the programme is in the middle of a task, and might not accept user input. During that period additional programs, for example in different windows, might work normally. When such an hourglass doesn't disappear, it suggests a programme is in an infinite loop and needs to be terminated, or is waiting for a few external event (such as the user inserting a CD). Unicode has an HOURGLASS symbol at U+231B (⌛).

Hourglass motif

Diagram of "hourglass" motif from carved stone tablet, Solomon Islands

Because of its symmetry, graphic signs resembling an hourglass are seen in the art of cultures which never encountered such objects. Vertical pairs of triangles joined at the apex are common in Native American art; both in North America, where it can represent, for example, the body of the Thunderbird or (in more elongated form) an enemy scalp, and in South America, where it is believed to represent a Chuncho jungle dweller. In Zulu textiles they symbolise a married man, as opposed to a pair of triangles joined at the base, which symbolise a married woman. Neolithic examples can be seen among Spanish cave paintings. Observers have even given the name "hourglass motif" to shapes which have more complex symmetry, such as a repeating circle and cross pattern from the Solomon Islands. Both the members of Project Tic Toc,from television series the Time Tunnel and the Challengers of the Unknown use symbols of the hourglasse representing either time travel or time running out.