SUBMARINE NAVIGATION. 



163 



The method answers very well for the peaceful commercial work 

 of the 'Argonaut.' In war, however, this would usually he impossible. 

 The 'Holland' in action must he entirely concealed from the enemy 

 for considerable periods of time. The normal air capacity of her hull 

 is, therefore, supplemented by compressed air tanks capable of with- 

 standing pressures upwards of a ton to the inch, and of holding 4,000 

 feet of free air compressed into the volume of thirty cubic feet. These 

 tanks are recharged by her own engines when at the surface. 



Ever since the days of Drebbell's 'Quintessence of Air' a great deal 

 of thought has been given to the problem of purifying the air once 



Fig 3. The 'Argonaut' in Dry Dock. 



vitiated by respiration and thus rendering it tit for use again. While 

 it would seem to be a very simple task to restore from tanks or by chemi- 

 cal generation within the boat the oxygen which respiration consumes, 

 and to absorb the water vapor and carbonic acid gas which respira- 

 tion produces, those who have built the latest boats seem to have aban- 

 doned the attempt entirely. It is easy to imagine emergencies where 

 fresh air could not well be obtained, and where such means of restoring 

 air once breathed would be of prime value. 



Objects under water are subject to pressure, which varies with the 

 depth of submergence. At a depth of thirty-three feet this water pres- 



