Tom, "The International Bureau of Weights and Measures 1875-1975," NBS Special Publication 420, discusses the 1791 decision to use the length of the earth's quadrant rather than the length of the seconds pendulum for the length of the metre because gravity (and thus the pendulum length) varied over the surface of the earth. See my History page <<http://www.ubr.com/clocks/hist/hist.html>http://www.ubr.com/clocks/hist/hi st.html> for articles (by Nelson and Olsson, and by Heyl and Cook) on using the pendulum for determining gravity.
Delambre and Me'chain measured the arc of the meridian between Dunkirk and Barcelona. It states that the Metre des Archives is about 0.2 mm shorter than 1 ten-millionth of the quadrant. From the Explanatory Supplement (p. 220) I see that the value for the earth's semimajor axis has changed from 6377563 meters in 1830 to 6378136 meters in 1989, about +0.009%, in those years alone. Gordon At 05:26 PM 10/16/1999 , Tom wrote: >I tried to verify your statement to the effect that: "We now know however that the >length of the quadrant is really 0.023% larger than originally surveyed, ..." and >got hopelessly lost in Chapter 4 of the Explanatory Supplement (© 1992) and in >Appendix X of Bowditch, 1977 Ed. Gordon Uber [EMAIL PROTECTED] Reynen & Uber Web Design http://www.ubr.com/rey&ubr/ Webmaster: Clocks and Time http://www.ubr.com/clocks/
