BATITOM'ETEH, an instrument invented by C. William Siemens, for indicating the depth of the sea beneath a passing vessel. The density of sca-water is about that of solid earth or rock has an average of shout 2.75, Hence, the attraction emanat ing from the water whicit lies beneath the ship, is less than that which would he exerted by earth or rock occupying the same relative position, and the greater the depth of the water, the greater the loss of attraction. Hence, further, the weight of a given mass of matter on board the ship will be greater when the ship is ashore than when afloat, by an amount which may be made appreciable by an instrument of sufficient delicacy, and this diminution of weight may become a recognizable function of the greater depth of water. Dr. Siemens fills with mercury a vertical steel tube of small bore, fitted below with a cup-shaped expansion closed with a corrugated steel-plate diaphragm. The pres
sure of the mercury upon the upper surface of the diaphragm is antagonized by a plate adjusted to bear upon the center of its under surface, and this plate is supported by steel spiral springs that are attached to the top of the column. In the construction of the instrument, care has been taken to compensate for the variations caused in the density of the mercury, and in the elasticity of the steel springs, by change of temperature. As the tension of the springs varies with their extension, while the pressure of the mercury on the diaphragm varies with the attraction from deep or shallow water, the two forces adjust themselves by a movement of parts, and the motion is so magnified by a microm eter screw, having an electric tell-tale, that the apparatus indicates a change of a fathom in depth of water for each division on the scale of the micrometer.