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Wendy,  

I have seen similar motions controlled by using a visitor-operated lever
damped by a double-acting air (or oil) piston/cylinder.   The damping is
controlled by adjusting a valve in the tube linking the two ends of the
cylinder.  The lever end that the visitor operates protrudes only a little,
so that it is operated by the fingers, not the large muscles of the arm
......

It might be easier to raise the glycerol container - and the visitor's view
direction to the glass samples would remain the same, so that they are less
likely to think that the different position of the glass samples may have
caused the effect.  (People can invent some surprising ideas ...)

Good luck,  Peter Anderson

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