There are seven vitreous in the cylinder. The measuring range is between 18 and 24 ° C. The balls share to the measuring range in intervals of 1 ° C. If there are, for example, the top three balls (24 ° C (top sphere), 23 ° C and 22 ° C (lowest ball) above, it is 22 ° C warm.
The thermometer reacts quite slow, because the temperature of the liquid slowly follows the changes in the air temperature.
It is certainly not the fastest and most precise measurement tool to fast times read the temperature but it is a piece of applied physics, which also still looks chic.