Navigators of surface vessels are interested principally in knowing the amount of water they have beneath their keel and the nature of the bottom, so that they may judge of their location by soundings, especially in time of fog. It is not essential, therefore, to know every foot of the bottom, but it is essential to know that no obstructions exist extending nearer to the surface than their keel. It is also essential for submarine commanders to know that there are no obstructions nearer the surface than their depth of submergence, if they are running submerged at speed. It is possible that collisions with submerged obstructions may have been responsible for some of the mysterious submarine fatalities. This method of bottom investigation permits of very accurate contour lines being run as close together as may be desired for harbor work. On the coast, in depths exceeding fifty to seventy-five feet, if contours were run one-half mile apart, it would probably be satisfactory if a guaranty could be given that there were no obstructions over five or six feet in height which lay between such contours. Two vessels as herein described are capable of automatically recording parallel contours at the rate of two or three miles per hour and to guarantee that there are no dangerous obstructions lying between them. Referring to page 267, a surface vessel is shown with a well which extends from under the pilot-house and out under her stem. An access tube extends from this well forward to a small submarine vessel. The upper end of this access tube is pivoted to strong bearings secured in the sides of the well, and is further secured by tension rods extending from part way down the tube to bearings secured to the outer skin of the ship in line with the bearings in the well. Large bearings with stuffing boxes in the submarine boat end of the access tube permit of access through a door to an air-lock compartment, and a second door leads from the air-lock into the diving compartment, a sliding door in the bottom of the diving compartment permitting the door to be opened for inspection of the bottom. By donning a diving suit members of the crew may also leave or enter the vessel when on the bottom. The water is kept from entering the diving compartment by air compressed to the same pressure as the surrounding water pressure, corresponding to the depth of submergence, the same as is done in my military submarine boats. A motor, drawing its power from a dynamo on the surface vessel, drives through suitable gearing the tractor wheel arranged near the bow of the submarine. This tractor wheel may be turned by its vertical steering post so as to propel the vessel in any desired direction.