Repeating circle

The Bordakreis, also Repetitionskreis, from the Paris mathematicians Jean Charles de Borda (1733-1799) was developed and is an instrument for measuring angular degrees. Simultaneously with two mutually displaceable telescopes, two goals targeted and the angle between them can be read as an arc minutes.

Were made ​​the instruments of brass instrument maker Etienne Lenoir ( 1744-1832 ). The measuring circuit was used for the triangulation of the meridian arc from Pierre Méchain and Jean -Baptiste Joseph Delambre ( 1792-1799 ). The measurement accuracy is approximately one minute of arc.

The telescopes of the Bordakreises are stacked firmly to the level of the pitch circle. One of the telescopes is firmly connected to the pitch circle, the other moves in the measurement with the coaxially with the rotating circle reading sites. The measurement of vertical angles is possible without problems. The measurement of horizontal angles are not directly possible because the scopes for targeting of points in various angles of elevation are not tilted. For triangulations with target distances of over 20 km of this is usually still possible.

An original copy is in Paris at the Museum possession.

Pictures

At

• J. Loridan: Voyages of Astronomes français à la recherche de la figure de la terre et de ses dimensions. Publisher Desclée, France, 1890, 330 pp. (Reprint 2010 Publisher Kessinger Publishing Company. ISBN 978-1-160-93874-7 ) there are two drawings which illustrate such a device and its use in the field.
• A photograph su triangulation in ingenieurgeograph.de