What is the difference between hydrolics and pneumatic?

Hydraulic cylinders will be linked to the bath at both ends, as the oil needs to remain pressurised to work effectively.

Pneumatic cylinders have springs if they're single way cylinders, as generally they can't rely on gravity to revert back to the first stage - dual cylinders on the other hand have 2 springs so they can switch sides.

That aside, the hissing from Pneumatics should just about give it away.