and they pursued him, believing him to be a German
spy.

Their surmise proved correct, for the man, who was hurt when his machine
got beyond control, was none other than Adolph Tuessig, the German who
had vainly tried to buy Mr. Raymond's stabilizer from him, and who had,
later, stolen the paper.

In our second volume, entitled, "Air Service Boys Over the Enemy's
Lines; or The German Spy's Secret," Tom and Jack found further
adventures. On their way to England, whence they had gone to France,
they had met on the steamer a girl named Bessie Gleason. She was in the
company of Carl Potzfeldt. The girl seemed much afraid of him, though he
was her guardian, said to have been so named by Mrs. Gleason, a distant
relative of his. Mrs. Gleason had been on the ill-fated _Lusitania_, and
it was related by Potzfeldt, for purposes of his own, that Bessie's
mother had been drowned. Moreover, he declared that before she died she
had given him charge of Bessie.

Tom and Jack, the latter especially, grew very fond of Bessie, but there
seemed to be a mystery about her and something strange in her fear of
her guardian.

When the two young men reached England, they lost sight, for a time, of
their fellow passengers, but they were destined to meet them again under
strange circumstances.

During one of their flights they landed near a lonely house behind the
German lines. They were traveling in a Caudron, which contained them
both, and on investigating the building after dark they found, to their
surprise, that Bessie and her mother were kept there, prisoners of Carl
Potzfeldt, who was a German spy.

Bessie and her mother were rescued and then departed for Paris, the
latter to engage in Red Cross work, and the boys, remaining with their
fellow aviators, longed for the time when they might see their friends
once more.

But they had enlisted to help make the world safe for democracy, and
they intended to stay until the task was finished. Over a year had
elapsed since the sensational rescue of Bessie and her mother. The
United States had entered the war and the Air Service boys were thinking
that soon they might be able to join an American aviation service in
France.

"What is it? What has happened?" Tom demanded of one of the aviators on
the outskirts of the throng about the messenger. "Have we won a victory
over the Germans?"

"No, but we're going to," was the answer. "Oh, boy! It's great! We're in
it now sure! Hurray!"

"In i