§ 1. The thermometer described in the present communication is a constant volume one. Its advantages proceed mainly from a simple arrangement whereby the total pressure of the enclosed air, and hence its temperature, since these are proportional, is measured directly by the height of one column of mercury. The external atmospheric pressure is eliminated by the adjustment of an auxiliary reservoir of mercury.

§ 2. In fig. 1, A is the air-bulb, B is the pressure-gauge, which is an ordinary barometer tube with a vacuum above the mercury connected at its lower end to the bent stem of the air-bulb and also by a flexible tube to the reservoir C. D is the constant volume mark on the stem of the air-bulb. The part of the instrument which is filled with mercury when the air is at 0° C. is shaded in the diagram.

§ 3. To make an observation of temperature the mercury is adjusted to the mark D by raising or lowering the reservoir. The height of the column in B above the level of the mark D now gives the total pressure of the enclosed air. The air-bulb and pressure-gauge may be cut off entirely from external pressure by closing the stop-cock between the pressure-gauge and the reservoir. If the absolute temperature corresponding to any given value of the pressure be known, we can calculate that corresponding to any other pressure by simple proportion.

Thus, in order to graduate the thermometer in degrees of temperature, it is only necessary to find the total pressure for any one temperature; that of melting ice, which we may take as 273° on the absolute scale of temperature, is the most convenient.