as a waste of grey, tumbling waves, unbroken by a sail of any
description.

The _Hoorn_ was ploughing her way at a modest ten knots. Short, beamy,
and deep-draughted, she was pitching heavily, sending a frothy bow wave
far to leeward each time she dipped her nose into the steep seas.

"I'd give a fiver for the sight of a good old White Ensign at the
present moment," remarked the Flight-Sub anxiously. "Good heavens,
what's that?"

Ten seconds later he laughed mirthlessly.

"Nerves going to blazes," he muttered. "A bit of wreckage gave me the
jumps. By Jove, don't we look a pair of comical objects?"

They had discarded their grotesque head-dress. Ross had a woollen
muffler wrapped round his head, while his companion had been given the
loan of a red stocking-cap, but they still retained the weird garb in
which they had made their journey down the ship canal.

Suddenly Ross gripped his companion's arm and pointed with his right
hand to a spar-like object projecting a few feet, close to the waves,
at less than a cable's length on the port quarter.

"A periscope!" ejaculated the Flight-Sub.

"Let's hope it's one of our own submarines," said Ross.

"We'll soon find out," added his companion. "It's forging ahead.
Whatever it is, they've got us under observation."

Jan, who was now on the bridge, had his attention called to the
disconcerting fact. He beckoned to his two passengers.

"You had better go below and stow yourselves away," he suggested. "We
will be boarded before long."

"Not I," replied the Flight-Sub. "They've marked us already. If they
do take us they won't have to dig us out of a coal-bunker."

The submarine was emerging. At a pace that more than held its own with
the _Hoorn_, she shook herself clear of the water, although green seas
were breaking across the flat deck as far aft as the conning-tower.

Then muffled forms clambered through the hatchway; a young,
yellow-bearded officer appeared on the navigation platform and hailed
the _Hoorn_ in Dutch to heave to instantly.

Even then the tough old Dutch skipper was not going to give in without
a protest.

"For what reason?" he shouted back. "This is a Netherlands ship."

"That I do not doubt," rejoined the officer of the submarine. "But you
have two Englishmen on board who have broken their parole----"

"You lie!" interrupted the skipper vehemently.

"Not a word more!" exclaimed the German fiercely. "Heave to, or we
sink you!"