Horizontal rudder set down aft inclines the vessel down by the bow, in which condition, with only a small reserve of buoyancy, she will "dive." When she reaches the desired depth a lesser inclination of the diving rudder is supposed to reduce her angle of inclination sufficiently so that the pressure on the top of her hull will offset the tendency to rise due to her positive buoyancy. To be successful there must be no movable ballast, and variable stream line effect requires expert manipulation of the diving rudder.] =Depth Control.=--Practically all modern submarines use hydroplanes with a horizontal rudder for the control of depth when under way. Hydroplanes might be said to correspond to the side fins of a fish. They are substantially flat vanes that extend from either side of the vessel. They are set on shafts that may be partially rotated by mechanism in control of a man within the vessel. They readily control the depth of the vessel with a certain amount of either positive or negative buoyancy. For instance, submarines are usually submerged with a small amount of positive buoyancy. If a vessel has positive buoyancy she will float. We have seen that in a surface condition the five-hundred-ton submarine has about one hundred and twenty-five tons of positive buoyancy. [Illustration: METHOD OF CONTROLLING HYDROPLANE BOATS Showing a proper arrangement of hydroplanes and horizontal rudders. C B represents the centre of buoyancy of the vessel when submerged. G represents centre of gravity, which lies directly beneath centre of buoyancy. Now if hydroplanes are located at equal distances fore and aft their up or down pull is always balanced and does not cause the vessel to dive or broach, but holds her to a level keel. If stream line pull tends to upset this level keel, horizontal rudders may be used to correct it.]