hourglass
An hourglass (also: hourglass ) is a simple time measuring device that has been known since the beginning of the 14th century . Its earliest depiction can be found in the fresco "Allegory of the Good Government" created by Ambrogio Lorenzetti in 1338 in the Palazzo Pubblico (Siena) .
The hourglass is specifically mentioned in an inventory of Charles V of France in 1379 . However, earlier sources from the 14th century already refer to them as "glass clocks", which were used to measure time. Even if its exact origin remains unclear, it is certain that the hourglass was spread around the same time as the wheel clock .
Building an hourglass
The oldest hourglasses consisted of two separate glass flasks that were connected to one another at their necks. The sand flowed from one flask into the other through a perforated screen made of metal, glass, mica or wood. However, these perforated panels slowly wore off with the use of the watch and were widened by abrasion . As a result, the sand trickled faster through the cover and the running time of the hourglass was reduced. From 1750, however, hourglasses could be made from one piece, in which the perforated diaphragm was replaced by a durable narrowing between the glass bulbs. The sand was filled in through a small opening in the glass bottom, which then z. B. was sealed with wax or a cork. From around 1800, the hourglasses could be completely sealed by subsequently melting the filling opening.
Sand is usually unsuitable for filling. It needs a fine grain size , a grain size that is as homogeneous as possible, a grain shape that is as spherical as possible and abrasion resistance. The individual particles must not stick together and must be insensitive to fluctuations in temperature and humidity. In the late Middle Ages, finely ground eggshells were used alongside marble dust and tin or lead sand. Mixtures for the special preparation of the mixture are known from the 13th century . It was finely sieved, washed and classified and boiled according to the instructions. Today very fine glass beads are used for filling.
The glass body of the hourglass is very sensitive to impact. For protection and so that the hourglass can be set up, it is mounted in a holder. The hourglass on a ship is hung in such a way that the ship's movement and inclination affect it - and thus the accuracy of the measurement - as little as possible.
functionality
The working principle of the hourglass is similar to the water clocks known from ancient Egypt . The bulk material slowly trickles out of the upper flask through an open, narrow connection point between the two glass flasks and into the lower flask. On the basis of the amount passed through, time segments between a few seconds up to several hours can be measured, depending on the size of the hourglass.
The bulk material just trickles down without pressure. Above the constriction it loosens under the influence of gravity , the lowest grains begin to fall freely, others loosely follow. A cavity is formed which extends laterally to the glass surface of the funnel-shaped inlet to the constriction. This funnel angle must be significantly steeper than the angle of repose for the particles so that they cannot stay there. The flow of the isolated bulk material runs against the initially stationary gas phase, which is, however, pushed upwards from below as the trickle progresses through the bulk material and thus begins to flow slightly upwards. It is precisely this trickling fall of small, round particles through the viscous gas (dry air) that determines the speed; for a certain grain size, the balance of gravity and flow resistance on the particle results in a certain falling speed. In addition, if the bottlenecks are appropriately shaped, a certain throughput is established as a dynamic state of equilibrium, because too many particles make it difficult to fall through. The constant flow of the bulk material also dynamically causes a certain pressure increase in the lower gas volume. When the grain flow suddenly ends, a few extra small grains are sometimes visibly blown a few millimeters upwards by the gas, which is then momentarily relaxing below, before they finally disappear downwards.
Common areas of application
seafaring
In addition to the chronometer , the hourglass was an important instrument for measuring time in seafaring. A classic measure is the 30 minutes on the sailors' bell clock . Four hours (eight glasses ) was the usual duty of a guard. When the hourglass was turned, the ship's bell was struck twice for every hour since the start of the watch (double strike), followed by a single strike for half an hour. Four double blows marked the changing of the guard.
The hourglass was also used to determine the speed of ships relative to the water. Since a log is also thrown into the water when measuring time , this special hourglass is also called a log glass.
Since hourglasses wear out over time due to the effects of friction between bulk material and glass, they usually change their original duration by a few minutes. For this reason, small hourglasses on ships were also called runners.
Medicine and health
In the past (occasionally still today) small hourglasses with a running time of 15 seconds were used by doctors and in nursing to measure the pulse . These heart rate monitors (to carry a pen in your lab coat mostly in shatter-protective metal sleeve and a clip on the species) are still (as of 2017) at any pharmacy available. There are hourglasses for children to tell when it is time to brush their teeth. In saunas there are sauna hourglasses (15 minutes with 5-minute division).
Cost control
In the 1980s, telephone hourglasses with a running time of eight minutes came onto the market in order to give people a feeling for the length of the time intervals newly introduced in the local tariff (until 1979 a local call at the Deutsche Bundespost, regardless of the duration, only cost one fee unit).
The largest hourglasses
The length of an hourglass is not necessarily decisive for the running time. However, if its duration is several days or weeks, it must be quite long. Two such giants are the time wheel in Budapest and the hourglass in the sand museum in the Japanese city of Nima. With a height of eight and six meters and a duration of one year each, they are among the largest timepieces in the world. Another giant was in July 2008 as part of a marketing event on Red Square in Moscow. With a height of 11.90 m and a weight of 40 tons, it was probably the largest hourglass in the world. In contrast, the smallest hourglass in the world is only 2.4 cm high. It was made in Hamburg in 1992 and takes a little less than 5 seconds for one run.
Paradoxical hourglasses
There are also absurd or paradoxical hourglasses in which, as in the picture, after the overturning of the double piston, a specifically lighter liquid which, because it is optically dark and opaque, appears subjectively "heavier", in a water-clear, transparent light, but in Liquid denser in relation to gravity flows upwards. The light liquid can be based on paraffin oil, which does not mix with the water phase, the jet can be turbulent, wavering, depending on the construction and viscosity conditions, also trickling, but the counter-current of the transparent, heavy water downwards is hardly noticeable.
Such a paradoxical hourglass can in principle also be filled with air-filled glass balls as floats. With generally greater transparency, the spheres, which in turn can be partially filled with colored water, are clearly visible through their wall reflections. Here one thinks that solid bodies should rather sink and is happy to swim up piece by piece through the narrow point.
The hourglass in fine arts and literature
In art , the hourglass first appears in 1338 in a fresco by Ambrogio Lorenzetti in the town hall of Siena. In the 16th century it was often depicted as a vanitas symbol, a symbol for transience and death ( death symbolism ). An art historically important example is the death dance - wood sections (1538) of Holbein the younger .
Ernst Jünger wrote a monograph entitled The Hourglass Book (1954) on the art-historical and philosophical significance of the hourglass as a symbol . In the Discworld novels by British fantasy writer Terry Pratchett , Grim Reaper manages tall shelves full of hourglasses in a hall of his dark mansion, which represent the lives of the people on Discworld.
The cover of the 1990 album Piece Of Time by the American death metal band Atheist depicts an hourglass with Stonehenge in the lower half of the glass .
The hourglass as a symbol of transience
In artistic representations and as a decorative and functional object in living rooms, the hourglass reminds us that life is only short and that people should use their time, which flows like sand, sensibly. In the theory of archetypes in the sense of Carl Gustav Jung, it is an archetypal symbol for the transience of everything earthly, also called vanitas in Western tradition .
Other meaning of the hourglass symbol
The hourglass in graphical user interfaces was first introduced with the Apple Lisa in 1983. After pressing the Enter key , an hourglass-shaped mouse pointer symbolizes that the computer is currently processing the command entered and is therefore putting further entries in the queue or ignoring them.
Web links
- Sand Physics - Why you can measure time with sand at all
- hourglass
- Stroll through the history of hourglasses
- How hourglasses limited the length of the sermon , contribution to Monumente Online
Individual evidence
- ^ Chiara Frugoni: Pietro and Ambrogio Lorenzetti. Scala Books, New York NY 1988, ISBN 0-935748-80-6 , p. 83.
- ^ Gerhard Dohrn-van Rossum : The story of the hour. Clocks and modern time systems. Anaconda, Cologne 2007, ISBN 978-3-86647-139-9 , p. 157.
- ↑ Charles K. aked: Brief history of the hourglass. In: Old clocks. Timepieces, scientific instruments and automatons. 3, 1980, ISSN 0343-7140 , pp. 22-37.
- ↑ Victor Pröstler: Callwey's manual of the clock types. From the wristwatch to the zappler. Callwey, Munich 1994, ISBN 3-7667-1098-2 .
- ^ Jürgen Abeler : Ullstein watch book. A cultural history of time measurement. Ullstein, Frankfurt am Main a. a. 1994, ISBN 3-550-06849-2 , p. 21.
- ↑ Time and its keepers (English).
- ↑ Sand Museum in the Japanese city of Nima ( Memento of the original from February 18, 2009 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (English).
- ↑ http://openlibrary.org/works/OL10343418W/Das_Sanduhrbuch
