National Geographic : 1914 Jun
THE SWEEP WITH WHICH THE COAST SURVEY FINDS SUNKEN ROCKS This is the most reliable apparatus yet devised for discovering the dangers to navigation that lurk beneath the surface ofthe sea. Itisknown as the wire sweep, and consists of a wire of any desired length, supported at intervals by buoys soarranged that the wire can belowered toany given depth. The sweep is then attached to two or three small power-boats, depending on its length, and istowed slowly along. The wire catches upon any obstacle that it encounters. This obstacle is then further investigated by means of lead soundings. These wire sweeps vary in size from Ioo feet to over a mile in length, and have been used to a depth of 45 feet (see page 660).