I think I figured out the real differences there. It's not the "power of 
ten" stuff.  After having to decide between 0.2 liters of beer or 0.3 liters 
of beer, I realized it's something different.

Imperial measurements are generally based on humans, so it's pretty 
convenient for everyday usage.

Temperature: 0 is about as cold as you're likely to run into in most places, 
100 is uncomfortably hot and basically body temperature.  50-75 is 
comfortable for the most part.

Distance: As big as your finger (inch), your foot (foot), your arms (yard), 
how far you can conveniently walk (mile).

Volume: A sip (spoonful), a mouthful (ounce), a mug full (pint), a bottle 
full (quart), a bucket full (gallon), and about as much as you can 
conveniently move by yourself (a barrel).

Weight: Again, a little bit almost unnoticeable size of a coin (ounce), 
convenient to carry in one hand (pound), convenient for weighing people or 
animals (stone, which nobody seems to actually use any more), about as much 
as you can reasonably ship someplace (a ton).

All very convenient for human scales.

But metric wins for science.

The difference with metric is that it's easy to calculate big numbers.  In 
imperial measurements, the next thing up from a mile is an AU, or a 
light-year, or a parsec (assuming you want to call these "imperial" at all). 
There's really nothing smaller than an inch, except fractions of an inch, 
and nobody goes below a 64'th of an inch. There's nothing lighter than an 
ounce, or heavier than a ton - no way to express the weight of an atom or a 
planet.

Metric has the advantage that you don't get into weird problems with units 
when you do science.  I don't think it really matters that things are 
specifically multiples of 10 until you start getting into figuring out 
ranges of numbers where exponents come into play. I'm not sure how the 
interrelatedness of the numbers matters at all: is it really useful that 
grams are defined in terms of the weight of a certain substance? I can't 
imagine how that would be useful in science, especially when things like 
"moles" and "c" are defined in terms of natural units.




In practice (for those of you who are used to metric), in every-day usage, 
imperial weird units don't really cause the sort of problem that people seem 
to think it causes. I no more have to frequently convert inches to miles 
than you frequently need to figure out the number of minutes in a year. Once 
you're talking miles, the number of inches is as immaterial as asking how 
many miles are in an inch when you're measuring to construct a chair or 
something. The worst stuff is measuring your room to be 12 3/4 feet, and 
finding out carpet is sold by the inch or something.

-- 
Darren New, San Diego CA, USA (PST)
    Quoth the raven:
        Need S'Mores!

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