rbia. Since the early days of the struggle, heavy battles had
been fought upon the Austro-Serbian frontier, with success first to one
side and then to the other.

Belgrade, the Serbian capital, had been bombarded time after time by the
great Austrian guns and once the city had been occupied by the foe.
Later, however, the Serbians had driven out the invader and reoccupied
the capital. And now, the Austrian army, reinforced by a hundred thousand
Germans, bringing the total number of troops to half a million, was again
knocking at the gates of Belgrade; and the Serbians, realizing the utter
hopelessness of their cause unless aid arrived from the Anglo-French
troops at Saloniki, were preparing to flee.

This was the situation when the aeroplane bearing Hal, Chester and their
friends descended just outside the city.

Hardly had they alighted when they were taken in charge by a squad of
Serbian troops. Colonel Anderson, acting as spokesman for the party,
explained their presence in a few well-chosen words and asked to be taken
to the commanding officer. There was considerable red tape to go through
before the friends finally were ushered into the presence of the Serbian
commander, and that worthy immediately informed them he had but a few
moments to give them.

Colonel Anderson, therefore, came to the point at once. He told him of
the conversation he and Chester had overheard a short time before.

"And you say one of the men wore a Serbian uniform?" asked the general.

"Yes, sir."

"You don't know who he is--you didn't hear his name mentioned?"

"No, sir; but I would know him again if I saw him."

"Good. You shall have the chance. Now, how far from the city do you say
this conversation took place?"

"Must have been all of ten miles, sir."

"Then the men have hardly returned to the city yet. And you say you did
not hear the name of the member of the general staff, the first traitor,
or spy mentioned as having divulged information?"

"No, sir."

"Very well. Now I will leave all of you here for an hour or so. I have
some matters to attend to. When I come back we'll see if you can identify
the man you speak of."

The general bowed to them and took his departure, leaving them alone in
his quarters.

From without a heavy cannonading could be heard.

"I guess the last advance has begun," said Chester slowly.

"You probably are right," agreed Hal. "And I feel sorry for these
Serbians. If the British and French c