Bygrave slide rule
Encyclopedia
The Bygrave slide rule is a slide rule
Slide rule
The slide rule, also known colloquially as a slipstick, is a mechanical analog computer. The slide rule is used primarily for multiplication and division, and also for functions such as roots, logarithms and trigonometry, but is not normally used for addition or subtraction.Slide rules come in a...

 named for its inventor, Captain L. G. Bygrave of the RAF. It was used in celestial navigation
Celestial navigation
Celestial navigation, also known as astronavigation, is a position fixing technique that has evolved over several thousand years to help sailors cross oceans without having to rely on estimated calculations, or dead reckoning, to know their position...

, primarily in aviation. Officially, it was called the A. M. L. Position Line Slide Rule (A.M.L. for Air Ministry Laboratories).

It was developed in 1920 at the Air Ministry Laboratories at Kensington in London and was produced by Henry Hughes & Son Ltd of London until the mid-1930s. It solved the so-called celestial triangle accurately to about one minute of arc and quickly enough for aerial navigation. The solution of the celestial triangle used the John Napier
John Napier
John Napier of Merchiston – also signed as Neper, Nepair – named Marvellous Merchiston, was a Scottish mathematician, physicist, astronomer & astrologer, and also the 8th Laird of Merchistoun. He was the son of Sir Archibald Napier of Merchiston. John Napier is most renowned as the discoverer...

 rules for solution of square-angled spherical triangles. The slide rule was constructed as two concentric tubes with spiral scales, like the Fuller slide rules, with yet another tube on the outside carrying the cursors.

During the Second World War, a closely related version was produced in Germany by Dennert & Pape as the HR1 , MHR1 and HR2.

Famous users

Sir Francis Chichester was a renowned aviator and yachtsman. He used a Bygrave Slide Rule as an aid to navigation during flights in the 1930s, one of which was the first solo flight from New Zealand to Australia in a Gipsy Moth biplane. He later completed a round the world cruise in his yacht Gipsy Moth IV
Gipsy Moth IV
Gipsy Moth IV is a yawl that Sir Francis Chichester commissioned specifically to sail single-handed around the globe, racing against the times set by the clipper ships of the 19th century.-Background and design:...

. This was the first solo circumnavigation. Sir Francis Chichester wrote about these exploits in his autobiography, entitled The Lonely Sea and the Sky.
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