Ejs Open Source Cartesian Driver Model Java Applet by Fu-Kwun Hwang
this version is slightly edited by lookang
based on the original version http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=728.0 by Fu-Kwun Hwang
work in progress

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description by Fu-Kwun HWang
The following are copied from http://lectureonline.cl.msu.edu/~mmp/applist/f/f.htm

This is the idealization of a common toy, the Cartesian diver. It demonstrates Archimedes' Principle as well as the ideal gas law.

You have an external glass container filled with a liquid (blue). You can exert pressure on the liquid by pushing (via mouse-drag) on the plane on top of the spring.

In the liquid is an inner container, in which air is trapped (white). By pressing on the plate with the spring and increasing the pressure, the liquid is not compressed, but the trapped air is. Consequently the displaced volume of liquid changes, and the diver can be made to rise or sink.

Through variation of the spring force, you can achieve that the diver floats at a certain height. However, it is an un-stable equilibrium.  Have fun!