The fathom - read all about it...
http://en.wikipedia.org/wiki/Fathom
(Contains mining references.)
I must admit, I just thought it was one of those old measurements that were
used at the time (like leagues, for instance) but which had become
associated specifically with depth. And as someone has already pointed out,
before mines were sunk, bodies of water were pretty much the only things
around where depth was a consideration. (Interestingly, Jules Verne's
"Twenty Thousand Leagues Under the Sea" would appear to buck that trend, but
the title relates to the distance travelled under the sea rather than the
depth achieved - a depth of 60,000 miles would seem a little excessive...)
A modern instance of a unit being used in a rather specific way (based on
the gathering of much evidence) would seem to be the centilitre, which
appears to be almost exclusively used in the measurement of wine...!
Just my two groats worth...
Howard.
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