Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A sounding line or lead line is a length of thin rope with a plummet, generally of lead, at its end. Regardless of the actual composition of the plummet, it is still called a "lead." Sounding lines were widely used in navigation until the development of echo sounding. Ultrasonic depth sounders provide an accurate graphical profile of the depth of the seabed. The word derives from the Old English sund, meaning swimming, water, sea. It is easy to measure a length of line or rope as a rough number of fathoms by repeatedly stretching the rope between the two outstretched arms. Water depths have traditionally been measured this way using a weighted sounding line. The word fathom can be used as a verb to describe this process.

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