About half past three o'clock
`` Here we go ! '' cried Harry , increasing the speed with a touch on the levers . `` Let 's keep a sharp lookout , though ! '' About half past three o'clock the boys decided to rise to the surface for the necessary airing of the vessel and storing of another supply of fresh air in the tanks provided for that purpose . In furtherance of this plan , the rudders were shifted while Harry slowed the engines . Directly the craft ascended . The gauges indicated a depth of about eight fathoms when Jimmie , who was at the helm , requested that the rudders be again deflected .