the Humber.

Then, after a time, he rose from where he sat, and seemed to be
measuring a distance by taking paces, his companion walking at his side
over the level expanse of sand.

Suddenly he halted, pointing to the ground.

Dubois examined the shore at that point with apparent curiosity. With
what object I could not imagine.

They remained there for fully an hour, and the sun had already set when
they returned to Patrington, and took the train back to Hull.

That old Busch was a spy I had proved long ago, but what part Dubois and
Gessner were playing was not yet at all clear.

On the following evening, about ten o'clock, I saw Dubois near the Dock
office, and on watching him, followed him to the factory, which he
entered with his key. Beyond the gate was the small paved courtyard in
which rose the high chimney. Within the factory he lit the gas, for I
could see its reflection, though from the street I could not get sight
of the lower windows.

The night was bright and moonlit, and as I waited I heard within the
grinding of a windlass, and saw to my surprise, a thin light iron rod
about six feet long and placed vertically rising slowly up the side of
the chimney stack, evidently being drawn up to a pulley at its summit.

Dubois was hoisting it to the top, where at last it remained stationary,
its ends just protruding beneath the coping and hardly visible.

Scarcely had this been done when Busch came along, and I had to exercise
a quick movement to avoid detection. He was admitted by Dubois, and the
door was closed and locked as usual.

I stood beneath the wall, trying to overhear their words. But I could
understand nothing.

Suddenly a dull, crackling noise broke the silence of the night, as
though the sound was dulled by a padded room.

Again I listened. Then at last the truth dawned upon me.

The spies had put in a secret installation of wireless telegraphy!

Those intermittent sounds were that of the Morse code. They were
exchanging signals with some other persons.

Gessner was absent. No doubt the corresponding station was at that house
high upon Sydenham Hill to the south of London, two hundred miles
distant!

I waited for a quarter of an hour, listening to those secret signals.
Then I hurried to the telephone, and fortunately found Raymond at home.
I told him what I had discovered, and urged him to take a taxi at once
down to Sydenham and ascertain whether they were receiving signals
there.