It’s cold enough to freeze the balls off of a brass monkey. The first time that I heard this phrase, many years ago, it had a vulgar connotation. More recently, I have heard an explanation that allegedly dates back to the days of cannon balls on sailing ships.
The story goes that, in order to hold cannon balls in a neat pyramid stack; a metal base was used to hold the iron cannon balls. To keep the iron balls from rusting to an iron base, the base was made of brass. This brass base was called a brass monkey. When it was very cold aboard the ship, the brass monkey tended to shrink up more than an iron monkey would, and thus, lost the balls all over the deck.
It is my strong opinion that this story is a hoax made up by somebody who didn’t know squat about naval vessels. Further, that person likely had never been closer to wooden sailing ship than a painting, in which, an artist may have taken license to show cannon balls stacked in pyramids. It just couldn't be so.
First, consider the need for a brass monkey, instead of an iron monkey, to keep the balls from rusting to the monkey. To be sure, salt air causes considerable rust and corrosion to many metals. However, what is to keep the iron cannon balls from rusting to each other where they touch in the pyramid stack?
During the 16th through the 19th centuries the British Man O’ War, of 200 hundred feet in the length, was the sizable warship of the era. Anyone who has served on a 20th century or newer destroyer or frigate, of twice that length, would have to doubt that any monkey, brass or otherwise, could hold cannon balls in rough seas by virtue of gravity alone. When pitch and roll momentum, of the ship underway was added, the balls are going to tumble at any temperature. On a modern frigate, in rough seas, no sailor is going to set down his coffee cup in anything but a secure cup holder lest he’ll see it shattered on the deck.
Brass, does in fact, shrink when chilled. Using liquid nitrogen to chill brass, it will contract to 99.6% of the original size. Considering the lack of precision in manufacturing cannon balls in the 16th through the 19th century, less than one-half of one percent shrinkage in the base is not going to release the cannon balls to roll about the deck. Cannon balls were not machined to precision as we might do ball bearings in modern day.
The shrinking brass monkey story simply cannot be true.