Under ideal conditions of manipulation, the withdrawal of heat from the calorimeter should be at just such a rate as to exactly compensate for the heat developed by the resistance-coil. Under these conditions, then, there would be no heat abstracted from nor stored by the calorimeter and its temperature should remain constant throughout the whole experiment. Practically this is very difficult to accomplish and there are minor fluctuations in temperature above and below the initial temperature during a long experiment and, indeed, during a short experimental period. If a certain amount of heat has been stored up in the calorimeter chamber or has been abstracted from it, there should be corrections made for the variations in the temperature of the chamber. Such corrections are impossible unless a proper determination of the hydrothermal equivalent has been made. A number of experiments to determine this hydrothermal equivalent have been made and the results are recorded beyond, together with a discussion of the nature of the experiments. As a result of these experiments it has been possible to make correction for the slight temperature changes in the calorimeter. It is interesting to note that these fluctuations are small and there may therefore be a considerable error in the determination of the hydrothermal equivalent without particularly affecting the corrections applied in the ordinary electrical check-test. The greatest difficulty experienced with the calorimeter as a means of measuring heat has been to secure the average temperature of the ingoing water. The temperature difference between the mass of water flowing through the pipes and the outer wall of the pipe is at best considerable. The use of the vacuum-jacketed glass tubes has minimized the loss of heat through this tube considerably, but it is advisable that the bulb of the thermometer be placed exactly in the center of the water-tube, as otherwise too high a temperature-reading will be secured. When the proper precautions are taken to secure the correct temperature-reading, the results are most satisfactory. In testing both calorimeters a large number of electrical check experiments have led to the conclusion that discrepancies in results were invariably due, not to the loss of heat through the walls of the calorimeter, but to erroneous measurement of the temperature of the water-current. DETERMINATION OF THE HYDROTHERMAL EQUIVALENT OF THE CALORIMETER. While the temperature control of the calorimeter is such that in general the average temperature varies but a few hundredths of a degree between the beginning and the end of an experimental period, in extremely accurate work it is necessary to know the amount of heat which is absorbed with any increase in temperature. In other words, the determination of the hydrothermal equivalent is essential.