The atmosphere was bad up here, as all the foul air drifted upwards, and Seagrave soon screwed back the post and returned to his book in the fore compartment.
Every ten minutes came the monotonous order, ‘Eighteen feet,’ the periscope was hoisted and lowered, and the boat descended to the thirty-foot level.
Nearly every one was asleep, the heavy atmosphere making them drowsy, and it was very quiet and peaceful, when the second coxswain, who was at the hydroplanes, called out sharply,—
‘Getting ’eavy, sir! I can’t ’ol ’er up!’
Now, no submarine can descend to an unlimited depth, because the external pressure becomes too great, and below two hundred feet or so they are liable to crush and flatten in like pancakes. Also if she begins to leak owing to excessive depth, water may get to the batteries, whereupon chlorine gas will form and suffocate all hands. Therefore a decent haste is necessary if at any time, owing to an increase of water in the bilges, the boat becomes heavy and has what is known as ‘negative buoyancy.’
Raymond and Seagrave hurried into the control room, the captain giving his orders as he came.
‘Start the pump on the auxiliary. Speed up to 500 on the motors.’
The purr of the motors increased, and the adjusting pump added its clack to the subdued noises.
Raymond hoped that the extra speed by giving her more steerage-weigh, and with the hydroplanes ‘hard-a-rise,’ would bring her up without the tedious necessity of blowing main ballast.
She was going down fast, however. Forty, fifty, sixty, seventy feet the depth-gauge recorded. At seventy-five feet she stopped.