The Fathom and Cable System
How a sailor divided the sea, from arm span to horizon.
Origin
Ancient in origin and standardised by the Royal Navy through the eighteenth and nineteenth centuries.
The catch
The whole system starts with a man’s outstretched arms. A fathom is the reach from fingertip to fingertip, which is how a sounding line was hauled in: arm span by arm span, counted as it came.
The word fathom comes from the Old English for outstretched arms, and the unit survives in speech long after it left the charts: to fathom something is to sound its depth. The cable had two definitions that never quite agreed. Counted in fathoms it was a hundred of them, 600 feet; the Admiralty instead made it a tenth of a nautical mile, 608 feet, so that ten cables made a mile exactly. British Admiralty charts were marked in fathoms until metrication began in the 1970s, and a great many older charts are still in use.