A fathometer is used in ships to tell the distance it is from the ground. It uses Echo sounding, a type of SONAR, to bounce sound waves to the ocean floor and back to the fathometer. It measures the time the wave takes to reach back to the fathometer and uses that to find the distance between the ship and the floor. Fathometer was invented by Herbert Grove Dorsey. Dorsey invented the practical fathometer in 1923, which is also referred to as the Dorsey Fathometer by the director of the U. S. Coast and Geodetic Survey in 1934. It worked by sending a high pitched sound through the water. It traveled to the bottom at a known speed and returned an echo at the rate of four pings per second. This could then be measured and calculated as to the depth since sound travels at a precise speed in the water. The “Fathometer” was so named by Dorsey because it measures fathoms. Altimeter used by aeroplanes cannot be used for measuring the depth of sea as sound travels in water as speed of sound through air at 343 m/s at normal room at normal temperature of 20 °C, whereas, speed of sound in water range from 1450 to 1498 meters per second in distilled water and 1531 m/s in sea water at room temperatures (20 to 25 °C).