ied them along the bank of the
placid Ourthe, flowing peacefully, calmly along toward its confluence
with the more important stream of the Meuse at Liege. Behind them one
strange thing proved that all was not quite normal. From Fort
Boncelles a searchlight began to play. They had seen that light
before, but only when it was being tested or when there were manoeuvres
in progress. Now it seemed to have a sinister meaning.

"I think that means that there is war," said Paul. "They are keeping
the searchlight going so that they may be sure to escape a surprise."

"I think it's the Germans who will get the surprise," said Arthur,
confidently.

But most of the time they walked on in silence. Both were thinking a
good deal; thinking of what war might mean, and wondering what part
they themselves might play if it came. Of one thing they were sure.
All Belgium would rise to repel the invader, no matter what the pretext
for the invasion might be.

"Here we are," said Arthur, suddenly. "That's the house, Paul."

"It looks quite dark, Arthur. But let's go along toward it. Not by
the road--we'll cut through this field here."

This they proceeded to do. But suddenly, as they neared the house, the
ground seemed to give way beneath Paul. He suppressed a cry, and the
next moment he was vigorously turning back the treacherous ground with
his foot. Arthur turned on his light. And there, beneath the soft
loam, they saw a plate of shining steel.

CHAPTER III

THE HOUSE OF MYSTERY

Utterly bewildered, they stared down at the steel.

"Put out your light!" said Paul, suddenly. His voice was tense. "Keep
still a moment! See if you can hear anyone moving around near us."

They were absolutely still for a full minute, but there were only the
familiar sounds of the night.

"All right," said Paul. "Now you watch and listen while I dig down
here and see what this is about."

"Why can't I dig, too?"

"Because it's better for you to watch. Besides, I want to dig so that
I can put the earth back in a hurry, and fix this place so that it
won't look as if it had been disturbed."

Then he fell to, working silently and quickly, like a mole, digging
with his hands until his nails were torn and his fingers were raw and
bleeding. But Paul did not mind that. He had already made a guess,
and a shrewd one, as to the meaning of this strange discovery that they
had made. It was not long before he found that the steel p