Observations

Historic Parramatta Observatory instrument discovered through “citizen science”

Dip circle magnetic instrument from Parramatta Observatory, image Chris Brothers, courtesy Powerhouse Museum

Dip circle magnetic instrument from Parramatta Observatory, image Chris Brothers, courtesy Powerhouse Museum

At its Treasures day on Sunday 19 April 2009 Sydney Observatory proudly displayed a newly identified instrument from the historic Parramatta Observatory that operated in Parramatta Park from 1821 to 1847. The story of its identification was briefly told in the Sun-Herald newspaper, but here it is in more detail.

A few weeks ago Ms Sharon Rutledge, a member of the public but who has studied the history of Parramatta Observatory, was trawling through the Powerhouse Museum’s publically accessible database and found listed in the Museum’s collection an HP Gambey magnetic dip circle. She knew that one of these had been at Parramatta Observatory as it is listed in the 1825 and 1847 list of Parramatta instruments as well as being mentioned in research papers on magnetism by observatory staff. Could the Powerhouse Museum instrument be from Parramatta?

Sharon contacted myself as the relevant Museum curator. And yes, it was almost certainly the one from Parramatta. It seems that the Gambey dip circle came with other Parramatta Observatory instruments to Sydney Observatory in 1858. A few decades ago Sydney Observatory disposed of the old and unused instrument to the NSW Government Stores that in turned transferred it to the Powerhouse Museum where no provenance or history was recorded. As this took place before the Observatory itself came under the auspices of the Museum, the association of the dip circle with the historic Parramatta Observatory was forgotten.

Portrait of Polar explorer Captain James Clark Ross who explored Antarctica between 1839 and 1843, discovering the south magnetic pole with the pictured dip circle.

Polar explorer Captain James Clark Ross who explored Antarctica between 1839 and 1843 discovering the south magnetic pole with the pictured dip circle. Ironically, he was indirectly responsible for the closure of Parramatta Observatory. Picture courtesy Wikipedia

What does the dip circle do? It measures the inclination of the Earth’s magnetic field compared with the horizontal. We all know that a magnetic compass points horizontally, but the magnetic field is only horizontal at the equator. The higher the latitude the more magnetic field points downwards. Dip circles were used in surveying, mining and, as at Parramatta, studying the Earth’s magnetic field. For accurate readings the instrument had to be carefully constructed. Henri Prudence Gambey was the best known French maker of geomagnetic instruments. Accordingly, this instrument is carefully made with attention to the reduction of friction in the pivoting of the magnetic needle. On depressing a lever, the needle pivots on a pair of jewels on either side to reduce friction.

When was the instrument built? Sir Thomas Brisbane who founded Parramatta Observatory spent 1815 to 1818 in Paris with Wellington’s victorious army after the Battle of Waterloo. During that time we know he purchased the Breguet and Sons clock, another important item that he eventually brought to Parramatta. It is likely that Brisbane purchased the Gambey dip circle at that time as well.

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