"Upon one end of a narrow plate of metal about a foot long, _a_, are fixed three scales of temperature, _h_, which ascend from 25 deg. to 100 deg. F., and which are shown more clearly in the drawing detached from the instrument. Upon one of these scales the present temperature is shown by the pointer, _e_, which turns upon a pivot in its centre. The register index, _g_, to the maximum temperature, and the index, _f_, to the minimum temperature, are moved along the other scales by the pin upon the moving pointer, at _e_, where they are retained by stiff friction. At equal distances from the centre of the pointer are two connecting pieces, _d d_, by which it is attached to the free ends of two compound bars, _b b_, and its movements correspond with the movements of the compound bars under variations of temperature. The other ends of the bars are fastened by the plate, _c_, to the plate, _a_, on which the scales of temperature are fixed. The connection of the bars with both sides of the centre of the pointer prevents disturbance of indication by lateral concussion. The case of the instrument has been improved at the suggestion of Admiral FitzRoy, and now presents to the water a smooth cylindrical surface, with rounded ends, and without projection of fastenings. "In surveying expeditions, this instrument would be found useful in giving notice of variation of depth of water, and of the necessity for taking soundings. A diminution of the temperature of water has been observed by scientific voyagers to accompany diminution of depth, as on nearing land, or approaching hidden rocks or shoals. Attention would also thus be attracted to the vicinity of icebergs." This thermometer might easily be modified to serve for several other important purposes, such as the determination of the temperature of intermittent hot springs, and mud volcanoes. [Illustration: Fig. 71.]