story of the Petrograd adventure from the youths he addressed.

“We thought the walk would do us good.”

Henri had some difficulty in keeping a serious face when Billy offered this plea as an excuse for the performance that had almost brought nervous prostration to Salisky and Marovitch, the dispatch bearers.

In a quiet corner later on, Henri had no desire to even smile when Billy gravely reviewed the possibility of the vengeful Cossack tracing them to Warsaw.

“You know,” said the boy from Bangor, “those fellows hang on like grim death when they have a grudge against anybody, and this wild and woolly scout is evidently anxious to stick his claws into us.”

“Maybe after all,” suggested Henri, “it is just because he thinks we are spies, having seen us working with or, rather, for the other side.”

“Why, then, didn’t he make his spring when we were within easy reach?”

“You forget, Billy,” replied Henri, “that by the time he had patched up his memory we were in Malinkoff palace, and even the tiger of the plains would hesitate before attempting to rough it with a Russian duke.”

“And there was a good reason why he did not have it out with us when we left the palace,” added Billy.

“A backway reason,” concluded Henri.

The Russian secret service, reputed to be a wonderfully efficient system, had now advices of the activities of that eminent arch-schemer, Roque, or whatever other name by which he was known, in this section of the war zone.

The blowing up of the war depot in Warsaw was less a mystery since the authorities had learned of the presence of this dreaded operator even so close as the width of a river.

If the wily Cossack could connect our boys with the previous movements of the aforesaid Roque, then, as Billy would say, “good night.”

In Colonel Malinkoff would be vested their only hope.

That the boys were not crazy about making another journey at present to Petrograd, goes without saying. They would be insane if they did, of their own accord.

But, luckily, their next flying assignment was the piloting of scouts sent out daily to observe the maneuvers of the great army in gray, then working on a new tack to break into the coveted city of Warsaw.

The aviators operated near a battle front nearly forty miles wide, and above a veritable hurricane of gunpowder, but in this experience Billy and Henri had grown old.

Once away from the city, and up in the air, their chief worry was behind them—their Cossack Nemesis could go hang!

From Salisky, now acting as observer in one of the biplanes, the boys learned of the fall of the great underground fortress of Przemysl, in and out of which they had served as aerial messengers, and where they had, not so long ago, bidden farewell to that gallant soldier-aviator, Stanislaws.

“I hope that ‘Stanny’ will be given a soft berth as a prisoner,” said Billy to his chum.

In the presence of the other airmen, however, the boys kept discreetly silent as to their acquaintance with the Austrian fort and town now overrun by the Russian forces.

Now and again there were days when Billy and Henri were relieved of the strain of constant aeroplane driving, and which was given to wandering about the streets of busy Warsaw.

One afternoon their steps inclined to the well remembered square with the tall column and heroic statue of bronze. In the door of a shop bearing the symbol of a silversmith, the proprietor happened to be standing when the boys strolled by.

This tradesman, at the time without trade, suddenly changed from sleepy attitude to one of alert anticipation after second view of the strollers.