The entire shell of the submarine was under an enormous pressure.
Matt gave the order to start the motor, and the popping of the engine soon settled into a low hum of perfectly working cylinders. A forward motion was felt by those in the submarine.
"Not many people have ever had the novel experience of navigating the ocean seventy yards below the surface," remarked the captain, with a slow smile.
"It's a wonderful thing!" exclaimed Matt. "The Grampus seems equal to any task you set for her, captain."
The air of the periscope room was being exhausted by the breathing of Matt, Nemo, Jr., and Cassidy. Matt ordered the bulkhead doors opened, in order that fresh oxygen might be admitted from the reservoirs. Just before the doors were opened, Captain Nemo, Jr.'s face had suddenly paled, and he had swayed on his seat, throwing a hand to his chest.
"You can't stand this, captain!" exclaimed Matt, jumping to the captain's side. "Hadn't we better ascend?"
The captain collected himself quickly and waved the youth away.
"Never mind me, my lad," he answered. "I feel better, now that a little fresh oxygen is coming in to us. Go on with your maneuvring."
All was silent in the submarine, save for the croon of the engine, running as sweetly as any Matt had ever heard. Aside from a faint oppression in the chest and a low ringing in the ears, the Grampus might have been cruising on the surface, so far as her passengers could know.
Cassidy was at the wheel, steering, his passive eyes on the compass.