Experiments! I've been trying out mechanisms that use pneumatic power. 

In this mechanism the top block with the air hole is a valve, below that are two opposing pistons. 

The pistons move the valve, the valves move the pistons and so the whole thing goes back and forth - or that's the theory. Unfortunately is doesn't work. It is really close though. I reckon I need to make the air holes bigger, to let more air flow through. 

So, I'll make some modifications and try another iteration.

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