William Thomson's machine provided more accurate measurement of ocean depths than could be done with the hand-thrown lead weight and measured line then in everyday use. The machine's brake and drum allowed the rapid release of great lengths of fine piano wire weighted by a lead sinker. A dial registered the amount of wire let out, from which the water's depth could be determined if a calculation were done to compensate for the forward motion of the ship. For greater accuracy, Thomson specified the use of a pressure gauge to be mounted alongside the lead sinker. The gauge comprised a brass case protecting an open-bottomed glass tube coated with chemicals that would change color when exposed to seawater. The deeper the tube sank into the sea, the greater the amount of the tube's interior that would change color. Using a scale on board for comparison, the depth the gauge had reached could be determined.
"In sounding from a steamer running at fourteen knots in water of seventy fathoms depth," Thomson wrote, "I have found the sinker to draw about one hundred and ninety-five fathoms of wire off the wheel, and to take about forty seconds of time to reach the bottom. The hauling in by two men turning the wheel easily by handles occupied about seven minutes."
William Thomson (1824-1907), a resident of Glasgow, Scotland, was a leading mathematician, physical scientist, and engineer. In a long and active career, he made significant contributions to the study of thermodynamics, electricity, and magnetism. In the 1850s and 1860s, he was centrally involved in efforts to create the transatlantic telegraph cable. This engineering work brought him wide public note, and his patents for electrical devices brought him considerable wealth. Knighted in 1866 for his work on the transatlantic cable, he was honored further with a peerage and the title Baron Kelvin of Largs in 1892. The Kelvin scale of absolute temperature, developed from his initial ideas, is named for him.
William Thomson, Deep-Sea Sounding Apparatus, U.S. patent no. 210,067, November 19, 1878.
Alan Gurney, Compass: A Story of Exploration and Innovation (New York, 2004), 235 ff.
"Sounding machine," The Oxford Companion to Ships and the Sea, Peter Kemp, ed. (London, 1976), 817-18.