That's correct Torctgyd,

Iron balls on iron monkey in a pyramid... Same rate of contraction, balls stay put.

Iron balls in a pyramid on a brass monkey... Brass contracts more than iron and squeezes the bottom layer, till a certain point is reached when the balls are explosively ejected, flying all over the gundeck.

Got that from the guide when I visited HMS Victory, and he assured me it is historically correct.

Victory, of course, carrying Nelsons flag, used brass.

Don T.