In 1751 a certain Mary Midnight wrote to the gentlemen of the Royal Society regarding her research and accomplishment on the cat organ: ". . . viz. A plain harpsichord, which instead of having strings and jacks, consists of cats of different sizes, included in boxes, whose voices express every note in the gamut, which is extorted from the imprison’d animals, by placing their tails in grooves, which are properly squeezed by the impression of the organist’s fingers on the keys." She goes on to make some points about the musicality of cats, the delicacy of a castrated male cat's voice, and how her cats are aligned in two rows for gentle and hard squeezing depending on meow.
While there actually were cat organs (here's one with not-actual cats, but you get the idea), Mary Midnight didn't have one, she wasn't really Mary Midnight, and she wasn't even really writing the Royal Society. The writer was actually Christopher "Kit" Smart, best known for the poem Jubilate Agno (and its section "For I will consider my cat Jeoffry"). The letter was a spoof published in his magazine The Midwife.
A long excerpt of the letter is found here at History Extra. The essay just afterward points out that this is the kind of wacky English humor that directly led to, say, Monty Python - and then namechecks Terry Jones' whack-a-mouse squeak organ.