According to Gareth Owen :
"Lloyd E. Sponenburgh" writes:
Nope, it has a quite definite etymology, but a rather exotic one.
The story I've heard is that it refers (in some way I could never
figure) to cannonballs and powder monkeys on Naval ships.
I was afraid that this was where this was leading. :-)
I don't believe this though ... it has a certain aura of
implausibility to it, and just *feels* like a back-formation.
I've certainly never seen convinving evidence of it.
If you want support, try this URL:
http://www.snopes.com/language/stories/brass.htm
Snopes makes a career out of investigating urban legends, determining
the truth or falsehood of each one, and has set up an excellent website
covering them. This is the sub-page dealing with this particular one.
One facet which he does not cover is the actual difference in
the thermal expansion rates for brass and cast iron.
First off -- bronze would be more likely at sea than brass, but
I'll list both:
18.50e-6/deg C Brass, ordinary yellow
21.16e-6/deg C Admiralty bronze
11.20e-6/deg C Cast Iron, gray
Now -- let's make some assumptions about the size of the
supposed "brass monkey", and the range of temperatures covered by a ship
from tropics to arctic sailing conditions:
1) Low temperature is perhaps on the order of -50F. Below that,
the sea would tend to freeze over, even short term.
2) High temperature is perhaps on the order of 120F.
For a range of 170F, or 94.44 C. (Let's call it 100 C for
convenience.
3) Let's say that the "brass monkey" was 24" maximum dimension
(and the stack of cannonballs would have to be somewhat smaller,
but let's call it identical for convenience.)
4) So -- over that range of temperatures:
the brass would expand 100 x 18.50e-6 x 24 = 0.037"
the bronze would expand 100 x 21.16e-6 x 24 = 0.051"
the CI would expand 100 x 11.20e-6 x 24 = 0.027"
or a total of 0.024" difference in size -- about 1/40th of an
inch, one turn of your micrometer thimble, which is hardly
likely to be enough to nudge any of the balls out of their
pockets.
Enjoy,
DoN.
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