Cambridge Observatory eight-foot mural circle

Made:
1832 in London
maker:
Troughton and Simms Limited

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Science Museum Group Collection
© The Board of Trustees of the Science Museum

Science Museum Group Collection
© The Board of Trustees of the Science Museum

Science Museum Group Collection
© The Board of Trustees of the Science Museum

Science Museum Group Collection
© The Board of Trustees of the Science Museum

Science Museum Group Collection
© The Board of Trustees of the Science Museum

Large 8-foot mural Circle, by Troughton & Simms, London, 1832, one of the original meridian instruments commissioned for the newly established University Observatory, Cambridge, England.

The Troughton and Simms 8 ft Mural Circle (1832) is one of the original meridian instruments commissioned for the new Cambridge University Observatory. It was based on the 6ft mural circle Edward Troughton had made for the Greenwich Observatory (1810).

A mural circle is a telescope mounted on a circular frame which is then mounted on a wall aligned north / south. The telescope is pivoted at the centre of the circle to measure the angular height of the star from 0 to 360 degrees.

The physical structure of this mural circle consists of a limb (outer circle), connected to the centre by 16 hollow spokes. The spokes are also connected in the middle by a second ring. This second ring is half the diameter of the limb. A 4 ½ ft hollow cone mounts the circle on a ‘pier’ of Portland Stone, weighing about 30 tones. A ‘pier’ being a structural mount for a telescope, which looks akin to a pillar. On the external edge of the limb are the divisions; the scale from 0 to 360 degrees. The hollow cone allows the telescope to be pivoted in a circle.

Microscopes are also attached to the stone pier by brass supports. The eyepieces of these are then attached to micrometres, used in the readings of the limb’s divisions. The telescope is clamped to the circle using a steel rod through the conical axis. It is then pivoted in the circle to measure the angular height of the stars. The circle can be fixed in place by five clamps with tangent screws which are spaced around / connected directly to the limb.

At the Cambridge Observatory, a wooden screen was used to protect the mural circle from the sun’s radiation.

The mural circle is significant because of the meticulous and difficult manner in which it was made. Firstly, Troughton divided the mural circle by hand when it was mounted on the pier. His method was so remarkable that the Royal Society of London in 1809 awarded him its Copley Medal when he gifted them the description of this. Secondly the outer circle of the mural, ‘the limb’, was cast in several pieces and then joined together by the ‘burning together’ process, which again when performed meticulously, as in this example, makes the limb look as if it were cast as one piece. It is one of the largest examples of a mural circle to be built and it is made rarer by being one of few to survive.

The mural circle was used at Cambridge from 1835 to 1870. Although no longer used, it was kept at the Observatory until 1937 owing to it being considered an esteemed object by those who worked there. In the announcement of the mural circle’s gifting to the Science Museum, it was noted ‘Thus the march of progress which has recently swept a number of ancient instruments out of the observatories in Oxford into the Museums reaches the sister University…’.

Troughton and Simms Ltd began as an instrument-making business in 1764, set up by John Troughton (1716-1788). The business was passed down to his nephews John Troughton (1739-1807) and then Edward Troughton (1756-1835). In 1826 William Simms (1793-1860) was brought on as a partner by Edward Troughton.

Edward Troughton was praised for making ‘considerable improvements in many types of instruments used in astronomy, navigation and geodetic survey’. He was world renowned and sold much of his astronomical equipment internationally. He subcontracted this work to other firms, for example, Dollond or Tulley, demonstrating an element of essential collaboration that was common in astronomical work. For example, Troughton was colour blind, therefore did not undertake the optical work in his astronomical equipment.

Details

Category:
Astronomy
Object Number:
1937-599
Materials:
brass (copper, zinc alloy), complete, glass and steel (metal)
Measurements:
overall (assembled on stand): 3000 mm x 3000 mm x 1700 mm, 1500kg
type:
mural circle
taxonomy:
  • furnishing and equipment
  • measuring device - instrument
credit:
Cambridge Observatory