"According to the Naval Historical Center of the U.S. Department of the Navy, there's no evidence that there was ever a device called a brass monkey:
The first recorded use of the term "brass monkey" appears to dates to 1857 when it was used in an apparently vulgar context by C.A. Abbey in his book Before the Mast, where on page 108 it says "It would freeze the tail off a brass monkey."
It has often been claimed that the "brass monkey" was a holder or storage rack in which cannon balls (or shot) were stacked on a ship. Supposedly when the "monkey" with its stack of cannon ball became cold, the contraction of iron cannon balls led to the balls falling through or off of the "monkey."
This explanation appears to be a legend of the sea without historical justification. In actuality, ready service shot was kept on the gun or spar decks in shot racks (also known as shot garlands in the Royal Navy) which consisted of longitudinal wooden planks with holes bored into them, into which round shot (cannon balls) were inserted for ready use by the gun crew.
In very cold weather it is claimed the brass contracted more than the iron balls, consequently toppled the pile and thereby giving rise to the aforementioned phrase. If you determined veracity only by the predominance of opinion (i.e. the vocal majority), that explanation is undeniable. The only problem is that it is not true and none of these thousand "authorities" bothered to verify that fact, which would have been very easy to do.
The differential linear coefficient of linear expansion between iron and brass is 0.000008 per degree centigrade. It means the "brass monkey" shrinks about 0.01 inch in the worst of weather - hardly enough to topple the balls."
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