roup of detectives,
with eyes ceaselessly watchful, passed comments in a low voice. They
were not hopeful of finding their quarry yet. The search was merely one
of precaution. Now and again one of the boats stopped and a man
clambered aboard a barge, dropping back in a few minutes with a shake of
the head. Foyle and Green left all this to the river men. They knew the
work.

But, swift as they were, they made slow progress. Foyle glanced uneasily
at his watch. It was already growing dusk, and the lights on the bridges
were reflected in fantastic shapes from the dark waters. The
superintendent spoke in a low voice to Wrington, who jerked his head in
sharp assent.

"You're right, sir. If we take the likely one now we can leave the
others till we've finished. We'll get on. Let her out, boys."

The two boats leapt forward, unobtrusively stealing a course in the
shadow of the barges. It was delicate work in the gathering darkness,
for many times a lighter swinging at its moorings threatened to crush
them; but always they avoided the danger, though to the untrained
faculties of Foyle it seemed that the margin of safety was no more than
the breadth of a knife blade.

At London Bridge they crossed to the northern side, and here the real
hunt began. Wrington signalled for the lights to be put out, and they
stole forward, two black blotches on the dark water. Once they narrowly
escaped running down a Customs' patrol boat, and voices cursed them
with vigour out of the gloom. Again, as they were about to pass under a
mooring rope, some one yelled to Foyle to duck. The warning came too
late, and he would have been swept into the water but that a ready knife
severed the rope. Then there was a halt for a little, while the barge
was secured again.

"There's a new caretaker on a tier of barges just above Tower Bridge,"
whispered Wrington tensely. "We'll try there first. Keep your voice low
if you want to speak, sir. Sound travels a long way on the water. Ah,
there it is."

Foyle had got good eyesight, but he could make out nothing but a smudge
where Wrington pointed--a smudge emphasised by a tiny point of twinkling
light. The two motor-boats slowed down and approached, as it were, on
tiptoe one on either side of the vessel. As they came nearer a barge
took shape at the head of a long string.

"Stop her," ordered Wrington. "Now, sir, will you board her with me? Get
ready."

As they lurched against the sides of the craft the two