BATHOMETER, Gk. pctOos, bathos, depth + tarpov, met ion, measure). An instrument in vented by C. Williams Siemens, for indicating the depth of the sea beneath a passing vessel. The density of sea-water is about 1.026, while that of solid earth or rock has an average of about 2.75. Hence, the attraction of the water which lies beneath the ship is less than that which would be exerted by earth or rock occupy ing the same relative position, and the greater the depth of water, the less the attraction. The weight of a given mass on board the ship will be greater, therefore, when the ship is in a harbor or in shallow water than when on the deep sea by all amount which may he determined by an instrument of sufficient delicacy, and this change in weight becomes a recognizable function of the depth of water. Dr. Siemens filled with mercury a vertical steel tube of small bore, fitted below with a enp-shaped expansion closed with a corrugated steel-plate diaphragm. The pressure
of the mercury upon the upper surface of the diaphragm is opposed by a plate adjusted to bear upon the centre of its upper surface, and this plate is supported by steel spiral springs, which are attached to the top of the column. In the construction of the instrument, care was 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. The pressure of the mercury in the diaphragm causes an extension of the steel springs by an amount depending on the force of attraction act ing on the mass of the mercury. This extension, of the spring is so magnified by a micrometer screw that the apparatus indicates a change of a fathom in depth of water for each division on the scale of the micrometer.