Napier's bones vs Napier's rods

Napier's bones and Napier's rods both are nouns.

Word	Noun	Adjective	Verb	Adverb
Napier's bones	Yes	No	No	No
Napier's rods	Yes	No	No	No

As nouns, Napier's bones and Napier's rods are synonyms defined as:

Napier's bones and Napier's rods: a set of graduated rods formerly used to do multiplication and division by a method invented by John Napier

Napier's bones (noun)	Napier's rods (noun)
a set of graduated rods formerly used to do multiplication and division by a method invented by John Napier	a set of graduated rods formerly used to do multiplication and division by a method invented by John Napier