Today, a relatively inexpensive G.P.S. receiver can indicate the position of a ship within a few meters, and moving charts on a video screen facilitate navigation. Until relatively recently, however, the compass was not particularly accurate, and navigators were dependant on charts of widely varying accuracy. Since the depth of water beneath a hull is always a primary concern, the sounding-weight—known in the Mediterranean by at least the sixth century B.C.—remained a critical navigational aid on seas and rivers well into the twentieth century.
A sounding-weight is a roughly bell-shaped mass—usually made of lead—averaging about five kilogrammes in weight, with a sturdy attachment lug at its apex and a tallow cup in its spreading base. Ancient mariners used sounding-weights, the oldest known marine navigational instrument, not only to determine the depth of water, but also to bring up samples of the bottom, comparing the result with their knowledge of coastal geography and river behavior.
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