In the earlier years of the United States Navy
, every Naval sailing ship
was required to have a cannon for protection
. Cannons of the times required round iron cannonballs
. The ship's master
wanted to store the cannonballs such that they could be of instant use when needed, yet not roll around the gun deck
. The solution was to stack them up in a square-based pyramid next to the cannon. The top level had one, the next had 4, the third level down had nine, the lowest had sixteen. Four levels would then provide a stack of thirty cannonballs. The only real problem was how to keep the bottom level from sliding out under the weight of the other fourteen cannonballs on top. To do this, they devised a small brass
plate (a "brass monkey
") with one rounded indentation for each cannonball in the bottom layer. Brass was used because the iron cannonballs would not rust to the brass monkey, but would have rusted to an iron one.
When temperature falls, brass contracts in size faster than does iron. As it got cold on the gun decks of the vessels, 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, it was (quite literally) cold enough to freeze the balls off a brass monkey.