• George Biddel Airy, Esq., A.M, F.R.S., Astronomer Royal
• Originally Published 1830
• Encyclopædia Metropolitana, Volume V, Article: Figure of the Earth
• Pages 185-260
• Specific numbers on Page 240
• a = 20,923,700 feet = 6,377,543.76 meters
• b = 20,853,810 feet = 6,356,241.288 meters
• Alexander Ross Clarke
• Published 1833
• Account of the Observations and Calculations, of the Principal Triangulation; and of the Figure, Dimensions and Mean Specific Gravity of the Earth as Derived Therefrom
• Survey of Great Britain
• a = 20,927,005 feet = 6,378,551.12 meters
• c = 280.4 feet = 85.47 meters
• Friedrich Wilhelm Bessel
• Published 1841
• Astronomische Nachrichten 333 and 438
• a = 3,272,077.14 toises = 20,922,829.22 feet = 6,377,278.35 meters
• b = 3,261,139.33 toises = 20,852,888.96 feet = 6,355,960.55 meters
• Astronomische Nachrichten 333:
• Astronomische Nachrichten 438
• Sir George Everest
• “Transcontinental Triangulation and the American Arc of the Parallel”
• “The Figure of the Earth and Isostasy from Measurements in the United States”

Spherical Excess

For a triangle on a plane, the sum of the interior angels is 180°.  For a triangle on a sphere, the sum of the interior angles must be more than 180°.  The amount over 180° is called “spherical excess”.

Overview of several different measurements circa 1906:

History of the Determination of the Figure of the Earth from Arc Measurements