At the beginning of last year I proposed that a cable might be laid down in the sea, and, by changing the electric state of the cable, that vessels passing near or over it might be able by means of a detector on board to discover that they were in its vicinity, and hence to locate their position. Some experiments showed that the method was feasible, and that water offered no insurmountable difficulty; and having been assured that no known instrument could detect the currents that could, with our present machinery, be passed through a submarine cable, a series of exhaustive experiments were made, with the result that I have constructed two such instruments that will act through over 30 fathoms of water, and I have been unable to discover their like.