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.
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