How does a Galileo thermometer work?
A Galileo thermometer consists of a sealed glass tube filled with a clear liquid that contains small glass bulbs of varying densities. As the outside temperature changes, the liquid’s density changes.
How did Galileo measure the degrees of heat and cold?
When he took away the heat of his hands from the flask, the water at once began to rise in the neck, and mounted to more than a span above the level of the water in the vessel. The same Sig. Galileo had then made use of this effect in order to construct an instrument for examining the degrees of heat and cold. 
What did Castelli write about Galileo’s device?
Benedetto Castelli wrote in 1638 about a device he had seen in Galileo’s hands around 1603:
When was the first temperature scale invented?
In the early eighteenth century, universal temperature scales based on several fiduciary points (e.g. a mixture of ice and brine, a mixture of ice and water, body temperature, the boiling point of water) were developed by Daniel Gabriel Fahrenheit (1686-1736), Anders Celsius (1701-1744), and René-Antoine Ferchault de Réaumur (1683-1757).
Who invented the first air thermometer?
Over the next several years this thermoscope was developed by Santorio Santorio and Galileo’s friend Gianfrancesco Sagredo (both in Venice), Galileo, and others to include a numerical scale. It had thus become a full-fledged air thermometer.
Do all glass globes in a thermometer have the same density?
Each glass globe in the thermometer has a different density. Some bubbles are the same density of the water they are submerged in at a specific atmospheric temperature. The other globes all have different densities (read mass), varying from slightly to substantially lighter or heavier than the density of the water.