This echo sounding unit uses a paper readout, visible behind the glass panel, above which is a fathom scale marked from 0 to 90.

It has always been important to be able to measure the depth of water below a ship, but it was not unitl the 20th century that entirely automated systems for doing this were developed. Alexander Behm (1880-1952), a German physicist, first demonstrated the principal of measuring water depth by timing the echo from a sounding source such as an explosion in 1911, and working echo sounders for use on ships were devloped in the early 1920s. In Britain, the Royal Navy began developing equipment in collaboration with Hughes & Son Ltd, one of the leading navigational instrument makers.