I'm a chemical engineer and I've worked in some old plants. Before modern electronic control, everything was controlled by pneumatics. A control loop would literally be something like a pressure gauge connected to an air valve, so that when pressure increased the air flow would change and proportionately close off another valve upstream, and when it would decrease, the air would open the valve up. There were also complex logic circuits set up this way entirely with air pressure. Really amazing stuff when you can look at the control panels because operators were opening and closing air valves to control everything and it works just like an analogue circuit, but it's all air pressure instead of voltage and air lines instead of wires.