This is the sequel to The History of the Barometer (1965) and prequel to the more ambitious Invention of the Meteorological Instruments (1969). Middleton has a very particular area of expertise and knows it very well. He seems to have examined every thermometer that was produced before the year 1800 which still exists and to have read almost every text that references them, in the original language. Why was I interested in such a particular book? Middleton tells the reason in his Preface: unlike barometers, where almost all the progress since the 1600s has been technical, the history of thermometers is as much about what we think ‘temperature’ is as it is about the device itself. My interest is in how these philosophical questions about temperature were asked and answered.