>Every sailing ship had to have cannons for protection.
>Cannons of the times required round iron cannonballs
>The master wanted to store the cannonballs such that
>they could be of instant use when needed, yet not roll
>around on the deck.
>The solution was to stack them up in a square-based
>pyramid next to the cannon. The top level of the stack
>had one ball, the next level down had four, the next
>had nine, the next had sixteen, and so on. Four levels
>would provide a stack of 30 cannonballs. The only real
>problem was how to keep the bottom level from sliding
>out from under the weight of the
>higher levels. To do this, they devised a small brass
>plate (Brass Monkey) with one rounded indentation for
>each cannonball in the bottom layer.
>Brass was used because the cannonballs wouldn't rust
>to the "Brass Monkey," but would rust to an iron one.
>When temperature falls, brass contracts in size faster
>than iron. As it got cold on the gun decks, the
>indentations in the brass monkey would get smaller
>than the iron cannonballs they were holding. If the
>temperature got cold enough, the
>bottom layer would pop out of the indentations
>spilling the entire pyramid over the deck. Thus is
>was, quite literally, "Cold enough to freeze the balls
>off a brass monkey."