At the time of writing NASA scientists are eagerly awaiting the results of soil sampling from their latest Martian probe Phoenix. Crucial to that experiment is confirmation of the presence of water. That precious substance essential to all life both here on Earth and maybe elsewhere. It also plays a big part in shaping the world geologically and meteorologically both here and possibly on Mars. What more natural a substance to choose for defining a unit of mass as was the case originally with the metric system.
The gram came into being as the mass of a cubic centimetre of liquid water (under normal terrestrial conditions of course) and for a time was adequate to meet the technological and scientific as well as ordinary commercial needs of the day. But even with the much more stable and precise reference used nowadays in the form of a metal prototype the difference between a litre of pure liquid water and the kilogram is immeasurable by most ordinary standards.
Much the same can be said of the origin of the metre. That 10 000 km quadrant of the Earth is still so close that it remains useful in visualising or estimating international distances.
In contrast, imperial measures have an obscure origin and a poor relationship to anything that perisists in the natural world. The pound weight owes its name to the fact that it equated to a pounds worth of coinage of a type that no longer exists. Linear measures, although they ostensibly relate to parts of the body, hardly constituted a meaningful reference even at the time of their invention, given the wide ranges of human stature.
By comparison with metric, imperial measures are abstract, ephemeral and archaic. Whatever the future may hold for systems of measurement one can at least say the Earth and water will remain meaningful to us for generations to come.
Metric Views 
It is a myth that the pound weight is related to a pound in coinage. Precious metals and medicines used to be measured in Troy units, but other commodities in Avoirdupois units. One Troy pound (12 Troy ounces) was defined to be 5760 grains while a pound Avoirdupois (16 ounces Avoirdupois) was 7000 grains. A pennyweight (Troy units) was 24 grains which is 1/240 of a Troy pound and was the weight of a silver Anglo-Saxon penny.
The pennyweight and Troy pound ceased to have a legal status in the UK in 1878, but the Troy ounce may still be used legally for the sale of precious metals.
For the record
1 troy pound = 373.24g
1 troy ounce = 31.103 g
1 pennyweight = 1.555 g
1 pound avoirdupois = 453.59 g
1 ounce avoirdupois = 28.35 g
1 grain = 64.80 mg
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Thank you Martin for the correction. I take it that your contribution was intended to show that the origin of the pound weight is even more obscure than I was suggesting.
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Elsewhere, you’ve suggested USA is not metric…… Well, try this reference:
http://lamar.colostate.edu/~hillger/laws/metric-conv.html
and Wikipedia too:
http://en.wikipedia.org/wiki/Metrication_in_the_United_States
And I’ve seen metric speed signs in Hawaii.
So they are even more confused than UK.
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