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 withstanding 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
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 chemical generation within the boat the oxygen which respiration consumes, and to absorb the water vapor and carbonic acid gas which respiration produces, those who have built the latest boats seem to have abandoned 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-
