(_a_) A fathometer is installed on the navigating bridge to determine the depth of water under the ship. This instrument works on the echo depth-finding process. Briefly, it consists of a submarine oscillator in the bottom of the ship that produces a sound of sufficient intensity to travel to the ocean floor and reflect back to a sensitive receiver also located in the ship’s bottom. The difference between the time of sending the sound and receiving the echo is measured by the fathometer, translated into depth, and flashed on the dial as a red light opposite a numeral corresponding to the depth in fathoms. (_b_) Near the fathometer is a fathometer recorder that records on a paper chart the depths indicated by the fathometer. This gives a continuous depth profile of the ocean bottom along the ship’s course. (5) _Chartroom equipment._ In the chartroom are sextants, protractors, dividers, parallel rules, chronometer, hydrographic charts, and a chart table. =c. Communications.= (1) _Radio._ The radio equipment consists of a main receiver and transmitter with emergency sets and a short-wave receiver and transmitter. The radio sets presently used are shown in figures 9 and 11. The manuals describing this equipment are listed in ASF Catalog Sig 10-1. (2) _Radio telephone._ Radio telephone communication is handled by the short-wave transmitter and receiver. (See fig. 11.) They operate on frequencies between 2 and 3 million cycles per second with separate channels designated for specific purposes. For ship-to-shore communication, frequencies range between 2,100 and 2,200 kilocycles. Direct ship-to-ship communication is on 2,738 kilocycles. The sets thus send and receive on different frequencies. (3) _Visual signal equipment._ (_a_) _Flashing light signals._ Flashing light signals are made with a dot-and-dash blinker light. The light is on top of the pilot house for greater visibility and is mounted so it can be shone in any direction. (_b_) _Flag signaling._ The ship carries complete sets of International flags and pennants and hand flags for signaling by semaphore. =d. Main engine.= (1) The main engine is an 8-cylinder, supercharged 4-cycle Diesel capable of developing 1,300 hp. (See fig. 12.) (2) The engine has an air starting system; pressure lubrication for all running parts except the pistons, which are lubricated by oil thrown into the cylinder walls by the cranks; and an indirect cooling system in which clean soft water is circulated in a closed circuit and cooled by sea water in the heat exchangers. The fuel system uses a manifold in which fuel is maintained at constant pressure but in which there is no pressure on the injection valves except during the actual time of injection. =e. Auxiliary engines.= Diesel and gasoline auxiliary engines furnish power for the ship’s generators, compressors, and pumps. Typical of the auxiliary engine installations are the 120-hp Diesels used to drive the fire pumps. (See fig. 13.)