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Galileo GALILEI
1592 - The thermometer is invented (Galileo, Italy)

"The first thermometer was constructed by Galileo. It consisted of a small glass globe fitted with a long tube. The open end of the tube was inserted in water, and the globe was heated until some of the air was expelled from it. When it was allowed to cool, the pressure of the external air was greater than that of the air in the globe, and so a column of water was forced up the tube, until equilibrium was established. The air thus shut off in the globe formed what we may call the thermometric body, and the top of the water column determined its volume. In using this instrument, as in using any thermometer, it was assumed on the basis of general experience, that when two bodies whose temperature are different are brought into each other's presence, their temperatures will finally become equal. This equalization of temperature is brought about by a lowering of the temperature of the hotter body and a raising of the temperature of the hotter body. It was also assumed that a change in the temperature of a body is accompanied by a a change in its volume, and in particular that the volume of air increases as its temperature rises.

With these fundamental principals the mode of operation of Galileo's thermometer is evident. When it is kept in a room at the ordinary temperature, the top of the water column will stand at a certain point in the tube, which may be marked. If it is then transferred to a hotter room, or if a hot body is placed around the globe, the equalization of temperature already described will occur, and the volume of air in the instrument will increase. The distance through which the top of the water column is forced down the tube is a measure of the change of temperature. For various reasons, the principal one of which is the irregularity introduced by changes in the external atmospheric pressure, Galileo's thermometer will not give consistent indications of temperature. It was very soon superseded by instruments constructed on a different plan."

Principles of Physics:Designed for Use as a Textbook of General Physics, William Francis Magie, 1911

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