Robert Bushnell wrote in USMA 21975: > 2002 Aug 29 >The Fall 2002 issue of American Heritage of Invention & Technology has an >article about the survey from Dunkirk to Barcelona to size the metre. The >article, by one Ken Adler, reports that Mechain made a mistake in finding the >latitude at the south end of his survey. He could not repeat the value of a >year before. He kept this secret. After the death of Mechain, Delambre >turned in only the results of the survey, not the details. > >Adler makes unneeded remarks. In large type the article says "The meter, it >turns out, is a mistake." Adler speaks of Mechain's "duplicity". The >"mistake" is about 2 parts in 10 000, not enough to call for bad words. > >I quote from the end of the article, "Ken Adler is associate professor of >history at Northwestern University. His book on the origins of the metric >system, The Measure of All Things: The Seven-Year Odyssey and the Hidden >Error that Transformed the World, is being published by the Free Press in >October 2002." > >The error did not transform the world. I hope the book gets Prof. Adler his >promotion but I can not call him a good historian. > > ------------------------------------- >The article has a good picture of the "cercle repetiteur", which also appears >on page 41 of L'Aventure du Metre, but I can not figure out how it works.� >Somebody, tell me how it works; what is repeated? > > Robert Bushnell
The international conference of 1799 determined the length of the prototype metre on the basis of the survey of M�chain and Delambre, combined with the results of previous surveys in Peru, France, and Lapland. It now appears that the provisional metre of 1795 was a better approximation to 1/10 000 000 of the distance from the north pole to the equator. However it is not a critical matter because the length of a degree of latitude increases as one goes from the equator to the pole. The metre can only be accurate at some intermediate latitude. The international prototype metre was embodied in a platinum bar that was made in 1799. Since then that bar has represented the metre, and there has been no further reference to the distance from the pole to the equator. Malicious rumor has it that Borda promoted the survey of M�chain and Delambre to demonstrate the "cercle r�p�titeur" theodolite that he had invented. This had its angle divisions on a scale about 20 cm in diameter which could be rotated. An ordinary theodolite has a fixed scale. With Borda's theodolite after an angle observation was made, the scale could be rotated so that the measurement was repeated, using a different part of the scale. Making several measurements of an angle the errors in the scale would be averaged out. Borda hoped the accuracy would be 1 second of arc. -- Joseph B. Reid 17 Glebe Road West Toronto M5P 1C8 Tel. 416 486-6071
