Pat Naughtin wrote on 2002-07-13 23:03 UTC: > Thomas Jefferson independently developed a system of measures very similar > to the French decimal metric system. He differed from the French in that he > wanted the metre to be the length of a pendulum that beats seconds because > other countries could readily reconstitute that at any time.
Considering, how much gravitational/centrifugal acceleration varies on the surface of the earth with latitude (more than 1%?), I doubt that this definition of the length unit would have found widespread sympathy, even at the time. It's a bit of a shame that a lasting system for units of measurement is likely to be developed in any society *before* significant advances in experimental physics happen, and not afterwards. Otherwise, it would have been possible to choose round values for fundamental physical constants such as the speed of light, the charge of an electron, Plank's constant, etc., as opposed to round values for geophysical constants such as the circumference or rotational period of the planet and the density of its dominant liquid, as was done for the initial French metric system. Or are geophysical constants more important to the majority of unit users than fundamental physical ones? In any case, the selection that was made by the French let by accident (or careful search?) to additional nicely round values for important geophysical reference values, such as 100 kPa for sea-level athmospheric pressure and 10 m/s� for surface gravitational accelleration on Earth, which I guess is rather convenient. I'd expect this choice of base units to last at least until we start communicating with inhabitants of other planets, at which point SI would become too Earth-centric and therefore politically incorrect and probabaly to be replaced with a universal system of units like the ones used in ETI communication attempts such as the messages on NASA's interplanetary probes. Markus -- Markus G. Kuhn, Computer Laboratory, University of Cambridge, UK Email: mkuhn at acm.org, WWW: <http://www.cl.cam.ac.uk/~mgk25/>
