In Galileo's extant writings there is only one reference to the thermometer and this corresponds in time with the letters of his friend Sagredo. In this fragment Galileo tries to explain the principle of the thermometer; he says that "when the air in the bulb contracts through cold, the wine in the stem rises to take the place of the void thus formed, and when the air is warmed it is rarefied and takes up more space so that it drives out and presses down the wine; "from this," says Galileo, "it follows that cold is nothing but absence of heat."
This correspondence and this fragment establish several things: 1st. The thermometer was invented by Galileo Galilei, between the years 1592 and 1597. 2nd. The instrument was an inverted air thermoscope containing either water or wine, and provided with a scale of degrees. 3rd. By its use Galileo determined relative temperatures of different places and of the same place at different seasons. 4th. Galileo made thermometric observations of freezing mixtures.
Galileo's method of graduating the stem cannot be ascertained, and was undoubtedly arbitrary, but the fact that he cites "degrees"
