Many thermometers operate on the basis that some internal fluid expands when heated.
Today’s invention turns that on its head by making a thermometer consisting of a fluid with a very low coefficient of thermal expansion (eg coloured water) in a glass enclosure which expands greatly on heating (eg soda glass).
When this device comes into thermal equilibrium with something hot, the glass will expand, whilst the liquid stays at almost constant volume. The temperature will thus be measured by the fall in the liquid level.
Similarly, a temperature fall will be registered as a rise on the coolness scale.