In the heyday of sailing ships, all war ships and many freighters carried
iron cannons. Those cannons fired round iron cannon balls. It was necessary
to keep a good supply near the cannon. However, how to prevent them from
rolling about the deck? The best storage method devised was a square-based
pyramid with one ball on top, resting on four resting on nine, which rested
on sixteen Thus, a supply of 30 cannon balls could be stacked in a small
area right next to the cannon. There was only one problem...how to prevent
the bottom layer from sliding or rolling from under the others. The solution
was a metal plate called a "Monkey" with 16 round indentations.
However, if this plate were made of iron, the iron balls would quickly rust
to it. The solution to the rusting problem was to make "Brass Monkeys." Few
landlubbers realize that brass contracts much more and much faster than iron
when chilled. Consequently, when the temperature dropped too far, the brass
indentations would shrink so much that the iron cannonballs would come right
off the monkey. Thus, it was quite literally,
"Cold enough to freeze the balls off a brass monkey."