In this Study Unit we are going to look at some of the practical difficulties involved in the measurement of quantities of heat. We shall restrict ourselves mostly to the field of calorimetry. The calorimetric unit of heat-although it is very useful for certain kinds of experiment, especially in connection with chemistry-obscures the link between thermal physics and what we ordinarily know as mechanics. At the end of the Study Unit, in the Viewpoints, we raise a number of questions concerning the assumptions which are made in calorimetric and other experimental methods.