Mark V Rae aeronautical bubble-sextant, 1926-1939.
© Science Museum / Science & Society Picture Library
Sextants are optical instruments for measuring angular distances, determining latitude. In 1919, sextant design advanced with the invention of a bubble artificial horizon by Lionel Burton Booth and William Sydney Smith, of the Royal aeronautical Establishment (Rae) at Farnborough. A bubble floats in liquid contained in a dome-topped chamber. When held vertically, the bubble floats to the dome's centre, forming a vertical line of reference. Patented in 1923 (no 193,638), the modified Mark V was introduced in 1925.