SUBMARINE NA VIGA TION. 



167 



a very heavy false keel of iron, which may almost instantly be de- 

 tached 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. Practi- 

 cally there are but two ways of securing this regulation. One, repre- 



Fig. 7. Cross Section of the 'Holland' Amidships. 



sented 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 rud- 

 der 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. „ _, ^ 



