160 Sir WiUiam H. White [June 9, 



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 

 ^11 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 continues as long 

 as onward motion is maintained, since there is no eJBFective 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 :-J00 lbs. 

 (equivalent 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 submarine (due to the consumption of fuel, the dis- 

 charge of torpedoes or other causes) must sensibly affect the reserve 

 of buoyancy, and arrangements must be made to compensate for these 

 variations by admitting equal weights of water in positions that will 

 maintain the "trim" of the vessel. x^Ldditional safeguards against 

 foundering have been provided in some submarines by fitting 

 detachable ballast. The more common plan is to make arrangements 

 for rapidly expelling Avater from the tanks either by means of pumps 

 or by the use of compressed air. In modern submarines, with loco- 

 motive 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 longitu- 

 dinal stability is usually much less than that of ordinary ships. 

 When submerged, their stability is the same in all directions, and it 

 is essential that the centre of gravity shall be kept below the centre 

 of buoyancy. This involves no difficulty, because water-ballast tanks 

 can be readily built in the lower portions of the vessels. Small sta- 

 bility in the longitudinal 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 great skill is required on the part of operators 

 in charge of working the rudders. As the under-water speed is in- 

 creased, the pressure on the rudders for a given angle increases as the 

 square of the velocity, and sensitiveness to change of trim becomes 



