Galileo thermometer
 Question: A Galileo Thermometer is a device made of a sealed glass cylinder, containing a clear liquid (water,  for example), and several floating glass bubbles, each filled with a different color liquid, and a small metal tag hanging from it.  Each bubble represents a certain temperature (which is indicated on the metal tag).All the bubbles are produced, and filled with liquid, so to have the same density (mass-to-volume ratio).The metal tags, being of very similar volume to each other, are slightly different by weight, so that the bubbles+tags, are different very slightly from each other in their "average density", yet, being very close to that of the surrounding water.   As the temperature changes, some of the bubbles are floating at the top part of the cylinder, while others are sinking at the lower part of it. The lower bubble of the floating ones (the one closest to the middle section of the tube) indicates the room temperature   Why ???
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 The glass bubbles are expanding and contracting, as a result of the changes in the surrounding water temperature. The change in their volume results in floating or sinking of them, in accordance to the temperature.
 When the surrounding air temperature changes,the water temperature inside the cylinder changes as well, causing it to expand or to contract, thereof changing its density.At any given density, some of the bubbles will float,while others will sink.The bubble floats at the most lower part of the floating bubbles, has the most similar density to that of the water, at that certain temperature.
 When the surrounding air temperature changes, the volume of the air inside the bubbles changes simultaneously. When the air volume decreases, the bubble will sink, and when it increases, it will float.
 When the surrounding air temperature changes, the water temperature inside the cylinder changes as well, changing the specific weight of the liquid inside the bubbles. As the specific weight decreases, the bubble sinks, while in case it increases, the bubble floats.