The weight of the submarine upon the bottom is regulated by water ballast. A depth-recording device operates in connection with a distance-recording apparatus, so that an exact contour of the bottom is reproduced on a roll of paper, the record being made by the revolution of the tractor wheels. Corrections of errors are made by taking observations from the surface vessels from known points on shore by the usual triangulation method. A drum is mounted on the submarine on which is wound a double wire. The upper wire is an insulated wire and is used to telephone between the two submarine vessels. The lower wire is a bare wire and is used to locate obstructions. The two wires are secured together as shown. Suitable recording devices in the interior of each vessel give the amount of wire unwound from its drum. A tension regulator holds a certain desired strain or pull upon the sweep lines, and another indicator gives the direction of lead of the wires during the "sweeping" operations. The surface vessel has a propeller in her skeg operating athwartship in addition to the usual stem propeller. The method of operation is as follows: Two vessels are required, which proceed to the location to be charted. In surface navigation the submarine, carried at the forward end of the access tube, is emptied of her water ballast and floats on the surface in front of the surface vessel, being pushed ahead of the latter vessel by the access tube, the pivoted bearings at each end of the tube giving sufficient flexibility to prevent any damage to the tube because of strains set up by the waves. One of the vessels takes her station at the point of beginning the day's survey and anchors; the other vessel then comes sufficiently near to secure the end of the sweep line from the anchored ship and then moves over to her starting point, which might be only a few yards away or as much as a mile. I have found, in wreck-sweeping operations, that it is practical to go as much as a mile apart, depending upon how close together the contours are desired. These sweep lines of the two vessels are then joined together and the submarines sink to the bottom, on which they are allowed to rest with sufficient weight to prevent their being drifted out of their course. We will assume that their starting points are one-half mile apart, and that they are to run contour lines due west from their respective starting points. The boats should therefore lie due north and south from each other, and the sweep lines should lead at right angles from each toward its companion boat. The dynamo is now started in the surface vessel to supply the motors in the submarines with power. The two submarines now start ahead.