Before revision: Previously, depth was determined by taking soundings with a lead line, a time-consuming and difficult method.<ref>* Jeffrey L. Kieve, The Electric Telegraph: A Social and Economic History, p. 238, David and Charles, 1973 {{ ocl They are used for measuring ocean water dept
After revision: Previously, depth was determined by taking soundings with a lead line, a time-consuming and difficult method.<ref>* Jeffrey L. Kieve, The Electric Telegraph: A Social and Economic History, p. 238, David and Charles, 1973 {{ ocl oc