vented the thermometer, a glass containing air and water which served to indicate changes and differences in temperature, an instrument afterwards perfected by Ferdinand II, of Tuscany. This assertion is confirmed in letters addressed to Galileo by his friend Francesco Sagredo, of Venice, and made public by Nelli in his biography of Galileo. The first of these letters is dated 9 May, 1613. Sagredo writes: "The instrument for measuring heat, which you invented, I have made in several convenient styles, so that the difference in temperature between one place and another can be determined up to 100 degrees." And he then gives examples of phenomena that he has examined by the aid of the instrument.
Two years later, 7 February, 1615, Sagredo wrote to Galileo more fully: "The use of the instrument for measuring heat and cold has been improved by me, and I think there is opportunity for many observations, but without your cooperation I had hardly succeeded. With this instrument I see clearly that the water of our fountain is colder in winter than in summer, and I imagine that the same is true of springs and subterranean places, although our feelings seem to indicate the contrary. …
