Andy. "After that?"

"There is enough stored in the small tanks of the diving suits to last
another two hours, perhaps. Seven hours in all."

"Then what?" asked Mark.

"We shall smother to death," said the professor in a low tone. "That
is," he went on, "unless before that time we can raise the _Porpoise_ to
the surface of the sea and get a fresh supply of air."

"Then we must work to raise the ship," put in Bill. "Let' get out and
see if we can't cut through the sea weed."

"It would be useless," said Mr. Henderson. "We can only depend on the
power of the ship herself. But do not be discouraged. We may escape.
Come, Washington, start the engine again. By keeping it going constantly
we can, perhaps, break loose from the grass. It is our only hope."

Steadily the machinery worked. It might as well have remained
stationary, however, as far as any noticeable effect was made on the
boat's progress. The grass of the Sargasso Sea held the _Porpoise_ in a
firm grasp.

Four hours passed. There was nothing to do but wait and see what would
happen. It all depended on the engines. Silently the navigators of the
realms under the ocean sat and hoped. Now and then the professor would
go to the engine room to adjust the machines.

The atmosphere in the cabin was growing noticeably heavier. The boys'
heads began to ring with strange noises, and there was a tightness
across their chests. The lack of fresh air was beginning to tell.

"We might as well use that in the diving suit reservoirs," remarked the
professor. "We will feel better, at least for a little while."

The helmets of the suits were brought in, and the vapor released from
the small tanks. A change was at once noticed. The old stale air in the
cabin was forced out of the exhaust pipes, and the fresh took its place.
Every one felt better.

Faster and faster revolved the big screw. The ship vibrated more and
more. Yet it did not move, nor did it rise. The crew were still
prisoners beneath the water.

For an hour or so conditions were fairly comfortable. Then the same
unpleasantness was experienced as was noticed before.

"If we could only open a window," sighed Mark, "and let in a lot of
fresh air, how nice it would be."

The air rapidly became more foul. Soon Washington was gasping for
breath. Tom and Bill showed signs of uneasiness.

"Lie down on the floor," counseled the professor. "You will find the air
a little fresher down there."

They all did as he