SUBMARINE NAVIGATION 271 



extrusion or withdrawal of cylindrical plungers for the purpose of vary- 

 ing the displacement) were the only means of controlling vertical move- 

 ment, it would be exceedingly difficult to reach or to maintain any 

 desired depth. This difficulty was anticipated on theoretical grounds, 

 and has been verified on service — in some cases, with considerable risks 

 to the experimentalists — the submarines having reached the bottom 

 before the vertical motion could be checked. It has consequently 

 become the rule for all submarines to be left with a small reserve of 

 buoyancy when brought into the diving condition. Submergence is 

 then effected by the action of horizontal rudders controlled by operators 

 within the vessels. Under these conditions, submergence only con- 

 tinues as long as onward motion is maintained, since there is no effective 

 pressure on the rudders when the vessel is at rest. The smallest reserve 

 of buoyancy should always bring a submarine to the surface if her 

 onward motion ceases, and, as a matter of fact, in the diving condi- 

 tion that reserve is extremely small, amounting to only 300 lbs. (equiva- 

 lent to 30 gallons of water) in vessels of 120 tons total weight. This 

 is, obviously, a narrow margin of safety, and necessitates careful and 

 skilled management on the part of those in charge of submarines. A 

 small change in the density of the water, such as occurs in an estuary 

 or in the lower reaches of a great river, would speedily obliterate the 

 reserve of buoyancy and cause the vessel to sink if water was not 

 expelled from the tanks. Moreover, variations in weight of the sub- 

 marine (due to the consumption of fuel, the discharge of torpedoes or 

 other causes) must sensibly affect the reserve of buoyancy, and arrange- 

 ments must be made to condensate for these variations by admitting 

 equal weights of water in positions that will maintain the ' trim ' of 

 the vessel. Additional safeguards against foundering have been pro- 

 vided in some submarines by fitting detachable ballast. The more 

 common plan is to make arrangements for rapidly expelling water from 

 the tanks either by means of pumps or by the use of compressed air. 

 In modern submarines, with locomotive torpedoes, compressed air is, of 

 course, a necessity, and can be readily applied in the manner described 

 if it is desired to increase their buoyancy. 



The conditions of stability of submarines when diving, are also 

 special. At the surface, owing to their singular form, the longitudinal 

 stability is usually much less than that of ordinary ships. When sub- 

 merged, their stability is the same in all directions, and it is essential 

 that the center of gravity shall be kept below the center of buoyancy. 

 This involves no difficulty, because water-ballast tanks can be readily 

 built in the lower portions of the vessel. Small stability in the longi- 

 tudinal sense, however, necessitates great care in the maintenance of 

 trim, and in the avoidance of serious movements of weights within the 

 vessels. Moreover, when a vessel is diving under the action of her 

 longitudinal rudders, she is extremely sensitive to changes of trim, and 



