The principal upon which this calorimeter is founded is, that a contraction of a definite amount takes place on the conversion of ice at 0° C. into water at 0° C., and that a definite amount of heat is required for this conversion. Early in the year 1866 I sent a description and drawing of the instrument to Messrs Kemp & Co., instrument-makers here, with an order to have it constructed. Some mechanical difficulties occurred which prevented its completion at the time.