=Holding Depth When Not Under Way.=--If it is desired to bring the boat to rest while submerged, but when no motive force is being used, other methods must be used than that just described. One method is to have an anchor or anchors to hold the vessel at the desired depth. If it is desired to lie at rest off the entrance of the enemy's harbor to wait for her ships to come out, the submarine proceeds to her station submerged with a small amount of buoyancy,--which is the usual method of navigating submerged. When she arrives at the desired station the speed is reduced and an additional amount of water is gradually admitted to give her a small amount of negative buoyancy. At the same time her anchoring weights are paid out until they touch bottom. As soon as they do so water is forced out of the ballast tanks by compressed air until positive buoyancy is restored and the vessel stops sinking and remains at rest anchored between the surface and the bottom, like an anchored buoyant mine. By winding in on the anchor cables a submarine may then be hauled down nearer the bottom, and by paying out the cables she may rise nearer the surface. On picket duty off harbor entrances she remains sufficiently near the surface to project her telescoping periscope occasionally above the crest of the waves to keep watch and see that an enemy ship does not enter or clear. In this condition there is no necessity to have any machinery running on board the submarine, therefore she can remain for weeks at a time on station without exhausting her fuel supply. It is only necessary for her to renew the air supply now and then, which can be done at night. Another method for holding a vessel at rest is by taking in and forcing out alternately small quantities of water so as to keep her in equilibrium between positive and negative buoyancy. Another method is to use vertical propellers operating in wells extended from the sides, and by running these it is possible to exert an upward or downward pressure and so hold her at a depth. Neither of these methods is as satisfactory, however, as the anchor weights, because the vessel will not hold a definite position on station, but will drift off with the current. They also make a drain on the storage battery and require constant attention on the part of the members of the crew. By the anchor weights scheme the vessel may stay on station as long as the food and fuel supply holds out. [Illustration: SHOWING VARIOUS CONDITIONS IN WHICH A SUBMARINE OF THE LEVEL KEEL TYPE FITTED WITH BOTTOM WHEELS, MAY NAVIGATE 1, running light on surface; 2, awash, ready for submergence; 3, submerged, depth controlled by hydroplanes; 4, running on bottom.] The above facts set forth simply the outstanding mechanical principles upon which the operation of the submarine is based. The submarine of to-day, however, has many auxiliaries, to describe which in detail would require several volumes of technical description.