of a room to the open door, or on approaching them to a person or a lamp. He remarks at the same time that instruments made of thick and of thin glass do not change with equal rapidity, the thinnest moving the quickest; he also surmises that the unequal viscosity of water and of wine makes a difference. This interesting letter concludes with the remark: "Signor Gageo is in my room and disturbs me, and I do not want him to see what I am writing, so my letter will be disconnected for my mind is occupied in several ways."
The instrument Galileo used is described in a letter written by Father Castelli to Monsignor Cesarini, dated 20th September, 1638, in which he says it was used in public lectures thirty-five years before. Recounting what he remembers seeing, he writes: "Galileo took a glass vessel about the size of a hen's egg, fitted to a tube the width of a straw and about two spans long; he heated the glass bulb in his hands and turned the glass upside down so that the tube dipped in water held in another vessel; as soon as the ball cooled down the water rose in the tube to the height of a span above the level in the vessel; this instrument he used to investigate degrees of heat and cold." This lecture experiment dates from 1603.
