A Galileo thermometer, or Galilean thermometer, is a thermometer made of a sealed glass cylinder containing a clear liquid and filled with several glass vessels of varying density. The glass vessels are marked with different temperatures and they will rise and fall within the cylinder in proportion to their respective density and the density of the surrounding liquid as the temperature changes.  

They're read by either reading the value of the vessel floating alone in the middle between the upper and lower group of vessels or by taking the average of the value of the lowest vessel of the upper group and the value of the highest vessel of the lower group.

h4. External links

* "Wikipedia: Galileo thermometer":[https://en.wikipedia.org/wiki/Galileo_thermometer]
galileo thermometer