a very heavy false keel of iron, which may almost instantly be detached by the throwing of a lever or the turning of a screw within the boat, The effect is precisely the same as that produced by throwing out a large quantity of ballast from the car of a balloon.
To sink a boat, take on sufficient ballast; to rise, discharge ballast, as, in a balloon. But the ballast that will sink a boat beneath the surface aft all will sink her to the bottom, and on the other hand if ballast be-discharged until the rise begins, the rise will continue till the boat is, again at the surface. To regulate the depth of submergence, therefore, something more is needed than mere adjustment of ballast. Practically there are but two ways of securing this regulation. One, represented
in the Nordenfeldt boats and in some others, depends on the action of propellers arranged to act vertically instead of horizontally as do the ordinary. Although this method has the advantage of being applicable whether the boat is progressively in motion or not, it is now entirely abandoned. No sane person would advocate lateral propellers for moving a boat to right or left, and the disadvantages of vertical propellers for vertical motion are of the same order. The 'Holland' dives, rises or runs at a constant depth by the use of a rudder at the stern set at right angles to that for steering to right and left. By means of this rudder in the hands of a skilled steersman the 'Holland' can be held for a mile or over to within less than a foot of any depth desired.
