The surface vessels, by means of their athwartship propellers, are always kept headed due west, therefore the course must also be due west. Each operator in the submarine keeps watch on his indicator, which records the amount of line paid out, and also enables him to be kept advised, by frequent inquiry through the telephone, of the amount of line his companion vessel has out. The operators also keep each other advised of the distance their respective vessels have travelled and the direction of lead of sweep line. Thus they can always keep each other on lines due north and south. If now an obstruction is struck, such as a rock, a sunken ship, etc., the strain on the sweep line becomes greater than normal, and the line commences to run off its drum. After running a short distance the sweep line will begin to lead aft instead of at right angles to the course. The two operators then stop and advise each other of the lead of the line. The one whose line leads the greater number of degrees off from right angles to the course is nearest the obstruction. He now turns his tractor wheel in the direction of the lead and wheels over to the obstruction, taking in his sweep lines as fast as he goes. The characteristics of the obstruction are noted, and its position accurately located by the triangulation method and recorded on the chart. In practice this sweep line extends a few feet above the bottom so as not to pick up small boulders, stones, etc., and would be caught only on the larger submerged objects. In taking off the readings from the contour sheets, when plotting the depths on the charts, the assurance can be had that no obstructions exist between the surface and the depth of the sweep line, as the depth and contour recording gauge is located at the height of the sweep line. The actual contour depth would be the distance between the sweep line and the water-bed, which could be added if desired. As the submarine may be used for purposes of making navigation more safe, so also may it be used for the recovery of ships' cargoes and for salvaging ships which have had the misfortune to be sunk.