Information

A Galileo Thermometer Galilean thermometer, or thermoscope is a thermometer made of a sealed glass or plastic cylinder containing a clear liquid solution.

Suspended in the liquid are weights, the number of which depend on the size of the thermometer in question. Commonly those weights are themselves sealed glass globes with coloured liquid within for an attractive effect.

As the liquid changes temperature it changes its density and the suspended weights rise and fall to stay at the position where their density is equal to that of the surrounding liquid. If the weights differ in density by a very small amount and are ordered such that the least dense is at the top and most dense at the bottom, they can form a sliding temperature scale.
The temperature is typically read from an engraved metal tag, usually brass, on each weight. The lowest weight of the top set of weights indicates the correct room temperature. To achieve this requires manufacturing the weights to a tolerance of less than 1/1000 of a gram or 1 milligram.