Napier's rods vs Napier's bones

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

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

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

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

Napier's rods (noun)	Napier's bones (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