An early boxwood differential thermometer by Watkins & Hill of London, circa 1825.

Signed to the rear of the thermometer scales: WATKINS & HILL 5 CHARING CROSS LONDON

Both scales divided 50 – 0 – 50.

Height: 37 cm.

Width: 26 cm.

Depth: 14 cm.

The thermoscope, or differential thermometer, used to measure the transmission of heat was invented in the early nineteenth century by the physicist Benjamin Thompson, Count Rumford.

Watkins & Hill traded at 5 Charing Cross, London, England; partnership between Francis Watkins and William Hill, 1810-47. Optical, Mathematical & Philosophical Instrument Makers, To their Royal Highnesses The Dukes of York & Clarence, and the Hon. East India Company.