How’s the Weather?
This device is called a Galilean Thermometer. When the temperature changes, the density of the liquid in the tube changes. Any bulbs denser than the liquid sink, and any less dense float. Only a bulb with the same density as the liquid remains in place, which is known as neutral buoyancy. This is what happens when you see a bulb sitting in the middle of the tube. The bulbs are designed to be neutrally buoyant when the temperature is the same as their label. There aren’t bulbs for every temperature, so sometimes you don’t see a bulb in the middle. In this case, you average the labels of the bulbs nearest to the middle to estimate the temperature.