A few years ago, I was reading about a contest open to everyone. The question was something like : if you were on a desert island, with unlimited natural resources, and access to a book with the most accurate values of properties for various natural things (i.e. gravitational acceleration on Earth, density of sea water), how would you measure the SI units of meter, kilogram and second. The prize was to the person(s) who could measure the units with the greatest accuracy. I wonder if anyone remembers this since I can't recall where I saw it. In either case, how would you go at it? For that matter, what about the other units (kelvin, ampere, candela)? I was thinking meter would be the easiest to guess at right away. Then using that, you could try to purify water and use a volume of water to define the kilogram, then use that and the meter to build a pendulum and define the second. But I'm sure my way could be improved upon. Yes, given enough time I suppose you could just build modern measuring device, but that's not really in the spirit of things. Maybe have a limit of 50 years or something.