Minimum spirit thermometer on porcelain mount. Original mounting broken and now on wooden board. Graduated from 100 to -40 degrees Fahrenheit. Made by Horne and Thornthwaite, serial number 22243.

This thermometer was one of a pair of instruments used to measure the highest (instrument 22242) and lowest (instrument 22243) water temperatures of the River Thames during the 1870s. The instruments were suspended alongside the Police ship ‘Royalist’ in a wooden trunk with perforated sides to allow the water to flow through. The first readings were taken on 5 May 1874 and were continued daily at 9am by the Resident Inspector until October 1879 when the ‘Royalist’ was taken out of service.

For more details see ‘Results of the Magnetic and Meteorological Observations Made at the Royal Observatory in the Year 1876’, Section 19, ‘Thermometers immersed in the Waters of the Thames’, pp.lvi-lvii.