On a former occasion, I had the honour of describing and exhibiting to this Society a new Registering Thermometer, by means of which the atmospheric temperature may be ascertained at any given instant during absence. In the construction of the instrument then described, a sliding index within the tube is indispensable; and, whenever such an index is employed, the diameter of the tube, and consequently that of its bulb, must be such as to render the instrument defective, when great accuracy and attention to minute fractions is requisite, such as in barometrical measurements, and various delicate experiments. Besides, though an instrument made with such an index may be so constructed as to perform with great accuracy, still, inasmuch as it is a complication, it is defective, and is more or less liable to error, as its construction may have been moire or less perfect.