The Clausius pistol is a prototype German pistol.
The Clausius was designed by German designer Claus Clausius some time around 1895. It was tested by the Swiss Army around that time as well, but was rejected and remained unadopted. Clausius later decided to make a second design with a floating chamber and an action akin to that of a short-recoil firearm which he himself decided to drop for unknown reasons. He made one final design in 1905 which abandoned the whole floating chamber concept altogether and was practically a fairly simple short recoil design.
The original 1895 Clausius used an unconventional long recoil system with a rotating bolt where the bolt would remain open until a lever located on the side of the pistol was pushed to release the bolt; however, as seen in the picture of the 1895 pistol above and from a picture from a book, no such lever is present on either side of the pistol. There were also an unusual set of controls which the energy from the compressed firing spring is used to push the bolt back open, which was supposed to allow the Clausius to be unloaded before firing.
The 1900 Clausius used a similar recoil system, but only have a portion of the barrel actually move, unlike the whole barrel. In his 1900 patent, the barrel would have been fixed which would have reduced the chances of fouling via dirt or grime. However, this system would have been quite impractical due to the likelihood of high pressure gas leaking from the chamber and sprayed into the shooter's face, making for an unpleasant experience.
The 1905 Clausius drops the whole idea of a floating chamber altogether, and has the barrel recoil just a short distance rearwards, with the inertia from said movement driving the barrel rearward.
The first variant of the pistol. Used a long recoil action. Only one is known to have been made.
The second variant of the pistol. Used an odd short recoil action with a floating chamber. It is unclear if any were even produced at all due to the impracticalities of its action; Clausius even ended up dropping the idea himself.
The final variant of the pistol, and unarguably the simplest of the three. Used a short recoil action with a moving barrel that only moves a short distance. One is known to exist.