Devil in the bottle

These devils in a bottle have been invented by the Frenchman Renatus Cartesuis over 300 years ago.
They are handmade form glass and hollow inside.
The bottle is full of water and closed with a rubber cap. Inside the devil is some water and some air. If you press onto the rubber cap, you give pressure onto the water. That causes that water gets into the devil and compresses the air in the devil. The devil gets heavier and sinks in the bottle. If you let the cap go, the water gets out of the devil again and, caused ot the form of the tail , he starts to turn around.

We bought this little fellow in November at the Matinsmarkt . He is handmade from a glassblower at the middle age market.
And I love playing with him :-D (the devil, not the glassblower :-P  )