The boats of the "Octopus" type are strictly speaking "diving boats," while the Lake boats are of the "even-keel" type. These terms refer to the method of submergence, the diving boats changing their horizontal trim when submerging, while the even-keel boats retain their horizontal trim, or nearly so. The Lake boats have some features not usually embodied in other submarines, since some of the boats are designed for purposes other than warfare. Thus, they are equipped with wheels, or buffers, on which they can roll along the bottom of the ocean or bay. In the bow is an air-tight compartment with an opening in the bottom through which a diver can emerge and work on wreckage, or laying and disconnecting mines. These boats have also a safety device in the form of a detachable keel weighing several tons. In case of accident, when it might otherwise be impossible to rise to the surface, this keel can be detached simply by pulling a lever, thus giving the boat sufficient buoyancy to rise to the surface. This particular feature of the detachable keel is not peculiar to the Lake boats alone, some of the foreign submarines using a similar arrangement as a safeguard. [Illustration: THE AMERICAN SUB-MARINE BOAT "CUTTLEFISH" IN DRY DOCK AT THE BROOKLYN NAVY YARD.] Technically speaking the name "submarine" is now used only as applying to those boats that are operated solely by electric power, have little buoyancy, and do very little running at the surface. The term "submersible" is applied to a submarine boat, actuated by electricity while submerged, but using gasoline motors for motive power while running at the surface. These gasoline engines are used at the same time for charging the storage batteries; so that the submersible is a much more practical boat than the submarine, and at the same time is quite as good a diver. Indeed, although many naval writers are very careful to make a distinction in the use of these terms, there seems little need of doing so, since only one type of boat--the submersible--is now considered practical. But as the word submarine is the older and more popular, it is used here to cover both classes except in specific cases. For several years there were two classes of submarines under observation--those possessing no floatability when submerged, and those having some reserve buoyancy. The advantage claimed for the no-floatability class of boats is that, having no buoyancy, they are kept more easily at a certain depth below the surface of the water instead of tending to come to the surface constantly as in the case of boats of the other type.