Museo Galileo
Virtual Museum
Letter by Castelli on the invention of the thermoscope

In a letter to Ferdinando Cesarini (1604-1646) of September 20, 1638, Benedetto Castelli (1577/8-1643) described the use of the thermoscope: "I recall an experiment shown to me more than thirty-five years ago by our master Galileo; who, taking a glass flask the size of a small hen's egg with a neck around two palms in length and as thin as a barley-stalk, heated the flask with the palms of his hands and then, turning the mouth of it over into a vessel placed under it, in which there was a little water, when he left the vessel free of the heat of his hands, suddenly the water began to climb up the neck and rose to above the level of the water in the vase by more than a palm. Galileo made use of this effect to fabricate an instrument to estimate the degrees of heat and cold."

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