Derevaux, a Frenchman, had taken the boys with them on their flight and
the four had later encountered each other in many strange and
unexpected places.

Stubbs they also had met while on one of their many missions and had
earned the little man's undying gratitude; but he had repaid whatever
they had done for him, with interest, more than once.

The boys, in their latest exploit, had been with the Italian army in
the Alps. Two of the four friends having fallen into the hands of the
enemy, the others had entered the enemy's lines in an effort to effect
their escape.

It was a daring adventure, but after a fight and chase, the four had
managed to seize the airship in which we now find them and had at last
fought their way clear. They had then held a council of war and decided
that it was best to head for the Balkans, rather than to run the gauntlet
of the Austrian flying craft which kept constant vigil in the direction
of the Italian lines.

Hal and Chester, typical American lads, were large and strong for their
ages, which were within a year of each other, seventeen and eighteen now.
In the rough lumber camps of the north, the two had had considerable
experience in the use of firearms and the art of self-defense--fists.
Also, during the school term each had practiced the use of the sword
until, though by no means experts, they could give a fair account of
themselves with this weapon--as each had done more than once.

Fortunately, both lads had made a study of languages and spoke French and
German fluently. They never had trouble on that score.