In the heyday of the sailing ship, every ship had to have cannons for protection. Cannons of the times required round iron cannon balls. The master wanted to store the cannon balls such that they could be of instant use when needed, yet not roll around the gun deck.
The solution was to stack them 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 16, and so on. Four levels would provide a stack of 30 cannon balls.
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 plate ("monkey") with one rounded indentation for each cannon ball in the bottom layer.
When iron was used to make this plate ("monkey"), the cannon balls would rust to the plate. As a result, these plates were made of brass to prevent this problem-- thus the name "brass monkey".
When temperatures fall, brass contracts in size faster then iron. As it got cold on the gun decks, the indentations in the brass monkey would get smaller than the iron cannon balls 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, |
