_Control in the Vertical Plane._ That beautiful machine, the Whitehead torpedo, is maintained at a set depth below the surface by means of a pendulum and a hydrostatic valve which regulate the horizontal rudders, and also in its true course by the gyroscope. In the case of the submarine it is necessary that it should not pass a certain limit when on its downward course, and that it keep so far as is possible the same level throughout its run under water. The control of the submarine in the vertical plane may be accomplished by the manipulation of the rudders, either automatically, by means of some such arrangement as the hydrostatic valve or pendulum, or by hand, and she can be kept on an even keel and prevented from rising to the surface or sinking to the bottom, when running beneath the waves, by the pumping of water from a reservoir situated aft to one situated forward, or vice versâ, by the admission of water into trimming tanks, by shifting weights, &c. These operations can be carried out either automatically or by hand-operated mechanism. It will be readily understood that while it is a comparatively simple matter to force a vessel beneath the surface to a depth previously determined, it is not so easy to ensure its keeping at this depth during the whole time it is submerged and maintaining throughout the run a perfectly even keel. One of the greatest difficulties the inventor of submarine boats has to overcome is their lack of longitudinal stability. Submerged vessels are of two classes, those which are equal in weight to the water they displace, and those which are lighter. Both classes are subject to various disturbances which tend to upset their longitudinal stability and send them up to the surface and down towards the bottom. In Chapter XV. mention is made of the difficulties experienced by those who had to navigate the Nordenfelt. The principal causes of disturbance have been summed up by Captain Hovgaard in a paper entitled, “The Motion of Submarine Boats in the Vertical Plane,” read before the Institution of Naval Architects at the Annual Meeting in 1901. 1. Faulty use of horizontal rudder. 2. Admission of water through leakages. 3. Expulsion of foul air and products of combustion. 3a. Firing of torpedoes and projectiles. 4. Movements of crew. 5. Existence of free surfaces of liquid. 6. Movements of loose weights, such as fuel. 7. Variations of buoyancy caused by varying density of sea water. 8. Grounding and collision. 9. Variations in speed. Some of the most important of these disturbances may be briefly discussed. 1. Most modern submarines are provided with more than one pair of horizontal rudders, but if all the rudders should refuse to act and the boat is running down an inclined plane, the only thing to be done is to pump the water out of the tanks and thus bring the boat to the surface.