I always thought it was forty verses, as in the Calabar:

Come all ye dry-land sailors and listen to my song,
It has only forty verses, so I won't detain you long.
It's all about the history of this here British tar
Who sailed as a man before the mast on board of the Calabar.

Wasn't there a Hamish Imlach song that started something like that as well?

John