Lord Kelvin's sounding machines, which enable a ship when travelling through the water to find the depth, proved almost as great a boon to navigation as his perfecting of the mariner's compass.
Before that the ship had to stop before sounding, and the depth of the water was ascertained by means of a rope attached to a lump of lead thrown overboard. This was a clumsy and dangerous method and could not be used when the ship was in motion. Rope was unsuitable; owing to its thickness it was tossed about by the waves, and satisfactory results could not be obtained.
Kelvin used pianoforte wire instead, and found he could take soundings, without stopping with a 30lb. leas-sinker at a depth of 2,700 fathoms. Regarding this invention there is a story that the great scientist, when busy in his workshop, was one day visited by Joule, distinguished as the discoverer of the mechanical equivalent of heat.
Joule inquired what all the wire was for and was told it was for sounding. "Sounding what note?" asked Joule, surveying with perplexity the coils of pianoforte wire. "The deep C," replied Kelvin, with a merry twinkle in his eye.
The Irish Times, March 6th, 1931.