word or gesture. That will be a punishable offence of the
second degree. Anything more?"

"My friend has had a nasty knock on the head," said Trefusis. "Have
you a doctor on board?"

Again Kapitan Schwalbe smiled broadly.

"No," he replied. "There is no need. Cases of illness must wait till
we return to port. The only injuries we are likely to sustain would
put us beyond all medical aid. But several of the men are fairly
skilled in rough surgery, so I will----"

"Vessel on the port bow, sir; she's showing no lights," announced a
voice.

"All hands to stations!" ordered the skipper.

"Down below with you!" hissed the petty officer, who during the
interview had stood rigidly at attention at two paces to the rear of
his charges.

Already the hitherto recumbent men were alert. Quickly, yet in order,
they disappeared down the fore hatchway, and amongst them were Ross and
Vernon.

The officers had taken their places inside the shelter of the
conning-tower. Everything was battened down from within, and with a
gentle purr the electric motors were set in motion, while at the same
time water ballast was admitted into the trimming-tanks.

Swift and stealthy had been their preparations, but the presence of the
submarine was betrayed by the phosphorescent swirl of the water caused
by the churning of the twin propellers as she slipped beneath the
surface.

Twenty seconds later a swift vessel that looked suspiciously like a
trawler, although her speed belied her, tore over the place where U75
had disappeared. Bare inches only separated the top of the latter's
conning-tower from the massive keel plates of the craft that had all
but accomplished its mission.

The watch-dogs of the British Navy were at work.

CHAPTER VI

The Tramp

Like a startled hare the unterseeboot fled for shelter. Not until she
reached a depth of fifteen fathoms did she check her diagonally
downward course. At intervals a dull booming, audible above the rattle
of the motors, proclaimed the unpleasant fact that her antagonist was
circling around the spot marked by the phosphorescent swirl and the
iridescence of escaped oil, and was firing explosive grapnels in the
hope of ripping open the U-boat's hull.

Kapitan Schwalbe, looking very grey in the artificial light, was
standing behind the quartermaster. His hands were clenched in
momentary apprehension. Beads of perspiration stood out upon his
forehead. He was experiencing