Second only to the compass in its value to the sailor is Thomson's sounding apparatus, whereby soundings can be taken in 100 fathoms by a ship steaming at 16 knots; and by the employment of piano-wire of a breaking strength of 140 tons per square inch and an iron sinker weighing only 34 lb, with a selfregistering pressure gauge, soundings can be rapidly taken in deep ocean.
This length has been standardized to a measure of 6 ft., and as such is used mainly in soundings as a unit for measuring the depth of the sea.
Here the tidal phenomenon called the bore, or Pororoca, occurs, where the soundings are not over 4 fathoms. It commences with a roar, constantly increasing, and advances at the rate of from 10 to 5 m.
Large portions of the bed still remain unexplored, but a fair knowledge of its general form has been gained from the soundings of H.M.S.
Jour., March 1907 and July 1908.) Its greatest ascertained depth is some 270 ft., which compares with soundings of 2000 ft.