240 SUBMARINE NAVIGATION. 



reaching and maintaining any desired depth below the surface 

 simply by the admission of the amount of water required to secure 

 a perfect balance between the weight of the vessel and all she con- 

 tains and the weight of water which would fill the cavity occupied by 

 the submarine when submerged. For all practical purposes and 

 within the depths reached by submarines on service water may be 

 regarded as incompressible. The submarine should, therefore, rest 

 in equilibrium at any depth if her total weight is exactly balanced 

 by the weight of water displaced. If the weight of the vessel ex- 

 ceeds by ever so small an amount the weight of water displaced, that 

 excess constitutes an accelerating force tending to sink the vessel 

 deeper. On the contrary, if the weight of water displaced exceeds 

 by ever so small an amount the total weight of the vessel, a vertical 

 force is produced tending to restore her to the surface. Under these 

 circumstances it is obvious that if the admission or expulsion of water 

 from internal tanks (or the extrusion or withdrawal of cylindrical 

 plungers for the purpose of varpng the displacement) were the only 

 means of controlling vertical movement, 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 serv- 

 ice — in some cases with considerable risks to the experimentalists — 

 the submarines having reached the bottom befo\|e the vertical motion 

 could be checked. It has consequently become the rule for all sub- 

 marines 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 on- 

 ward motion is maintained, since there is no effective pressure on 

 the rudders when the vessel is at I'est The smallest reserve of buoy- 

 ancy should always bring a submarine to the surface if her onward 

 motion ceases, and, as a matter of fact, in the diving condition that 

 reserve is extremely small, amounting to only 300 pounds (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 

 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. Additional safeguards against foundering 

 have been provided in some submarines by fitting detachable ballast. 



