Temporarily Assigned to Planet Earth

From Sailing Terms Explained For 'Lubbers by Vance Broad:
Between the guns, pyramids of cannon balls stood upon lipped edged trays called monkeys. In some ships these monkeys were made of brass (for ceremonial reasons). In cold weather, the different coefficient of expansion meant that the brass trays would contract faster than the iron cannon balls. Sometimes it was cold enough to freeze the balls off a brass monkey.

From Nautical Expressions in the Vernacular by Gibbons Burke:
It is not what you think. On ships, cannon balls were sometimes stacked in what was called a monkey, usually made from brass. When it got really cold the monkey would contract forcing some of the cannon balls to fall off. [Steve Rose (rose@rtl.ENET.dec.com)]

Brrrr...