Much has been said of the work of spies--said and written. Here is a
woman in Paris sending forbidden messages on a marked coin. Men are
tapped on the shoulder by a civil gentleman in a sack suit, and walk
away with him, never to be seen again.

But of one sort of spy nothing has been written and but little is known.
Yet by him are battles won or lost. On the intelligence he brings
attacks are prepared for and counter-attacks launched. It is not always
the airman, in these days of camouflage, who brings word of ammunition
trains or of new batteries.

In the early days of the war the work of the secret service at the Front
was of the gravest importance. There were fewer air machines, and
observation from the air was a new science. Also trench systems were
incomplete. Between them, known to a few, were breaks of solid land,
guarded from behind. To one who knew, it was possible, though dangerous
beyond words, to cross the inundated country that lay between the Belgian
Front and the German lines, and even with good luck to go farther.

Henri, for instance, on that night before had left the advanced trench
at the railway line, had crawled through the Belgian barbed wire, and
had advanced, standing motionless as each star shell burst overhead, and
then moving on quickly. The inundation was his greatest difficulty.
Shallow in most places, it was full of hidden wire and crisscrossed with
irrigation ditches. Once he stumbled into one, but he got out by
swimming. Had he been laden with a rifle and equipment it might have
been difficult.