[jjtheblunt]

apparatus is a past participle from apparare, so as an adjective plural would follow second declension, not fourth, i thought.

used as a noun, it's an (implicit thing) prepared (which preparatus would describe, but i guess we don't have any other word in English other than apparatus.

i don't have a Latin dictionary though

[jacobmartin]

apparatus is first/second as a past participle. I suppose it could be used as a substantive, but there is also a fourth declension noun with the same lemma meaning implements, tools, etc.: https://logeion.uchicago.edu/apparatus