Built around 1852, this precision timepiece on the wall at OS HQ has intrigued many of us here. According to the accompanying plaque, this Dent Master Chronometer was the standard on which all other mobile chronometers owned by OS were checked against before being used in the field astronomical observations by surveyors. An accurate knowledge of time is necessary in the calculation of latitude and longitude.
More explicitly, a chronometer is an instrument for keeping highly accurate time and was especially used in navigation to figure out a distance between two locations. Before the invention of radio time signals and later Global Positioning Systems (GPS) for example, the chronometer was the only device that kept accurate enough time for a ship at sea to find out its longitude.
Since manufacture, this chronometer has also been modified at some stage to include a switch, activated by way of two electrical contacts. Expert opinion is that these alterations were made around 1900. As the writing on the clock face suggests, this was made by a ‘Clock Maker to the Queen’. Now what do Big Ben, the Royal Exchange and Ordnance Survey have in common? They all have clocks made by Edward John Dent, clockmaker to Queen Victoria!
Edward John Dent (1790–1853) was a famous English watchmaker renowned for his highly accurate clocks and chronometers. Having established his own shop in 1843, Sir George Airy, Astronomer Royal, recommended Dent to make the clock for the new Royal Exchange. This was installed in 1844 and in 1852 Dent won the commission to make the clock for the Houses of Parliament that we now recognise as Big Ben. Although unfortunately Dent died before its completion, his stepson, Frederick Rippon Dent, finished Big Ben in 1854.
How we survey today
Means of measuring location such as this are now mostly redundant for national mapping. For example, the modern equivalent to the network of trig pillars is the OS Net network of 110 Global Navigation Satellite System (GNSS) receivers. Our surveyors use OS Net and GNSS technology daily to instantly position new map detail to within a few centimetres.