Hourglass
An hourglass (also: Hour glass ) is a simple, known about since the beginning of the 14th century timepiece. His earliest presentation can be found on the 1338 created by Ambrogio Lorenzetti fresco "Allegory of Good Government " in the Palazzo Pubblico (Siena).
Is mentioned explicitly the hourglass in a 1379 inventory of Charles V of France. However, an earlier sources of the 14th century refer to it as a "glass watches ", which were in use at the time of measurement. Although their exact origin remains unclear, it is ensured that the hourglass was about the same time the mechanical clock distribution.
Construction of an hourglass
The oldest hourglasses consisted of two individual glass flasks connected together at her throat. The sand flowed through a pinhole made of metal, glass, mica or wood from a flask in the other. These pinhole used through the use of the clock slowly and were expanded by abrasion. Thus, the sand trickled faster through the aperture and the run time of the hourglass decreased. After 1750, hourglasses were made in one piece, in which a tough narrowing between the glass bulb took the place of the pinhole. The sand was filled using a small opening in the glass bottom, which was then sealed, for example with wax, or with a cap. From about 1800, the hourglasses were completely sealed by subsequent fusion of the filler hole.
Sand is not suitable as a filling for usually. It takes a fine grain, homogeneous as possible grain size, spherical particle shape possible and abrasion resistance. The individual particles must not stick and must be insensitive to temperature and humidity fluctuations. In the late Middle Ages, besides marble dust and tin or lead sand and finely ground egg shells were used. From the 13th century "recipes" for the special preparation of the " sand " are known. He was finely sieved, washed and graded and scoured by the book. Today we used to fill very fine glass beads.
The glass body of the hourglass is very sensitive to impact. To protect and so you can set up the hourglass, it is mounted in a holder. The hourglass on a ship is suspended so that the ship motion and tilt it - and thus the accuracy of the measurement - affected as little as possible.
Operation
The principle of operation is similar to the hourglass known from ancient Egypt water clocks. From the upper piston of the sand trickles through an open and close junction of the two glass plunger slowly in the lower piston. On the basis of the passed amount of sand can be measured up to several hours depending on the size of the hourglass periods between a few seconds.
Sand trickles only depressurized. Above the constriction the sand loosens under the influence of gravity, the lowest grains begin to fall freely down loosely follow other after. It is formed from a cavity that extends to the glass surface of the funnel-shaped inlet for the narrow side. This funnel angle must significantly steeper than the angle of repose for the particles to be so that they can not remain lying there. The stream of scattered glass beads runs against the initially stationary gas phase, which is, however, urged with increasing progress trickle through the bulk material from bottom to top and begins to flow so slightly upward. Rate-determining step is just this trickling case of small round particles through the viscous gas (dry air ), for a given grain size arises from the balance of gravity and drag on the particle a specific case speed. In addition, given a suitable shaping of the constrictions, a certain throughput as a dynamic state of equilibrium, because too many particles at the same time make it difficult to fall through. The constant flow of " Sands " also brings a certain dynamic pressure increase in the lower gas volume. When the flow of grain last ends suddenly, a few extra small granules are sometimes visible from the bottom then currently relaxing gas blown a few millimeters upwards, before she finally disappear down.
Common Areas
Seafaring
In addition to the chronometer the hourglass was an important tool for measuring time in the maritime field. A classic measure are the 30 minutes, the ship's bell clock of seafarers. Four hours ( eight bells ) was the usual duration of a service station. With the change of half an hour the ship's bell was struck once in the hour twice ( double whammy ). Four double strokes meant the changing of the guard.
The hourglass was additionally used in the determination of the speed of ship, relative to water. As for timing an additional log is thrown into the water, this particular hourglass is also called Logglas.
Since hourglasses wear by friction effects between sand and glass over time, they change usually their original duration by a few minutes. For this reason, small hourglasses were called on ships also runners.
Medicine and Health
Physicians employed earlier hourglasses with a term of 15 seconds for measuring the pulse. For children there is sand clocks, specify the time to brush your teeth. In saunas, there are sauna hourglasses (15 minutes with 5 -minute schedule ).
Cost control
In the 1980s, telephone hourglasses came on the market in order to give people a feel for the length of the inserted in the local rate minutes clock.
The largest hourglasses
For the duration of an hourglass not necessarily their size is critical. If their maturity, however, several days or weeks, they 'd have to be pretty big. Two such giants are the wheel of time in Budapest and the hourglass in the sand Museum of the Japanese city of Nima. With a height of eight and six meters and a term of one year, they are among the greatest timepieces. Another giant was in July 2008 as part of a marketing events on the Red Square in Moscow. With a height of 11.90 m and a weight of 40 tons, there were believed to be the largest hourglass in the world. In contrast, the smallest hourglass of the world only measures a height of 2.4 cm. It was made in Hamburg 1992, and require slightly less than 5 seconds for a run.
Paradoxical hourglasses
There is also absurd or paradoxical hourglasses, in which, as in the picture, after the fall of the double piston a specifically lighter fluid, because optically dark and opaque colored, but subjectively " harder " acts slightly appearing in a water-clear transparent, but in with respect to the gravity denser liquid to flow upwards. The light liquid can be based on paraffin oil, which does not mix with the water phase, the beam can be turbulent schwänzelnd, depending on the design and viscosity ratios also trickling of equal counterflow of the transparent, heavy water downward, however, is hardly noticeable.
Such paradoxical hourglass can in principle be filled with air-filled glass balls as floats. In general greater transparency are the balls that can be filled with colored water, in turn, partially visible through its wall reflections. Here it is meant that solid body should rather fall and is looking forward to the floating piece by piece through the constriction.
Similar optical effects, which are also caused by buoyancy, but are caused by thermal expansion, are the lava lamp with thermally driven convection and the Galileo thermometer with a series of floating bodies.
The hourglass in the visual arts and literature
In art, the hourglass appears first in 1338 in a fresco painting by Ambrogio Lorenzetti in the town hall to Siena. In the 16th century it was often portrayed as a vanitas symbol, as a symbol of transience and death ( death symbolism ). An art-historically significant example are the Dance of Death woodcuts (1538 ) by Holbein the Younger.
Ernst Jünger wrote under the title The Sanduhrbuch (1954), a monograph on the art historical and philosophical significance of the hourglass as a symbol.
The hourglass as a symbol of transience
In artistic representations, and as a decorative and functional object in living rooms the hourglass reminds them that life is short and that man should use his as the sand verrinnende time wisely. It is in the theory of archetypes within the meaning of Carl Gustav Jung, an archetypal symbol of the transience of all earthly things, called in Western tradition also Vanitas.