In particular, his katzenklavier, or "cat piano."
From Wikipedia (because I couldn't say it better myself):
A cat piano or Katzenklavier is a hypothetical musical instrument consisting of a line of cats fixed in place with their tails stretched out underneath a keyboard. Nails would be placed under the keys, causing the cats to cry out in pain when a key was pressed. The cats would be arranged according to the natural tone of their voices.The Kircher Society quotes Thomas Hankins and Robert Silverman on the subject:
In order to raise the spirits of an Italian prince burdened by the cares of his position, a musician created for him a cat piano. The musician selected cats whose natural voices were at different pitches and arranged them in cages side by side, so that when a key on the piano was depressed, a mechanism drove a sharp spike into the appropriate cat’s tail. The result was a melody of meows that became more vigorous as the cats became more desperate. Who could not help but laugh at such music? Thus was the prince raised from his melancholy.This was all in 1650; an eighteenth-century quack named Johann Christian Reil brought some more innovative thinking to bear on the project. He suggested the Katzenklavier as a treatment for inattentive patients, who would be forced to focus on the howling kitties.
Said Wikipedia article quotes Jean-Baptiste Weckerlin's book Musiciana, extraits d’ouvrages rare ou bizarre:
When the King of Spain, Felipe II was in Brussels in 1549 visiting his brother the Emperor Charles V, each saw the other rejoicing at the sight of a completely singular procession. ... The most curious [thing in the procession] was on a chariot that carried the most singular music that can be imagined. It held a bear that played the organ; instead of pipes, there were sixteen cat heads each with its body confined; the tails were sticking out and were held to be played as the strings on a piano, if a key was pressed on the keyboard, the corresponding tail would be pulled hard, and it would produce each time a lamentable meow. The historian Juan Christoval Calvete noted the cats were arranged properly to produce a succession of notes from the octave ... This abominable orchestra arranged itself inside a theater where monkeys, wolves, deer and other animals danced to the sounds of this infernal music.