SUBMARINE NAVIGATION. 241 



The more common plan is to make arrangements for rapidly expel- 

 ling Avater from the tanks either b}^ means of pumps or by the use of 

 compressed air. In modern submarines Avith 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 longitu- 

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

 Allien submerged their stability is the same m all directions^ and it 

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

 of buo^yancy. 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 Avithin the vessels. MoreoA'er, Avhen a A'essel is di^nng 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 underAvater speed is in- 

 creased the pressure on the rudders for a giA^en angle increases as the 

 square of the velocity, and sensitiA^eness to change of trim becomes 

 greater. This fact makes the adoption of higher iniderAvater speed 

 a matter requiring very serious consideration. Some authorities 

 AA-ho liaA'e giA^en great attention to the construction of submarines 

 have been opposed to the adoption of high speeds under water, be- 

 cause of the danger that A^essels Avhen diving quickly may reach much 

 greater depths than are desirable. Causes of disturbance Avhich 

 might be of small imjjortance when the underAvater speed is moder- 

 ate may haA-e a greatly exaggerated effect Avhen higher speeds are 

 reached. Cases are on record where modern submarines in the hands 

 of skilled crcAvs haA'e accidentally reached the bottom in great depths 

 of Avater and haA'e had no easy task to regain the surface. For these 

 reasons it is probable that Avhile speeds at the surface Avill be in- 

 creased, underwater speeds Avill not groAv correspondingly. Indeed, 

 the tactics of submarines hardly appear to require high speed under 

 Avater, seeing that it is an im]:>ortant element in successful attaciv to 

 make the final dive at a moderate distance from the enemy. It is 

 authoritatiA'ely stated that in our submarines complete control of ver- 

 tical moA'ements has been secured by means of skilled operators, and 

 that a constant but moderate depth beloAV the surface can be main- 

 tained. Proposals haA^e been made and successfully applied to small 

 submarines for automatically regulating the depth of submergence 

 by apparatus similar to that used in locomotive torpedoes. "For the 

 larger submarines noAV used such automatic apparatus does not find 

 faA^or, and better results are' obtained Avith trained men. 



