The second volume, entitled "Air Service Boys Over the Enemy's Lines;
or The German Spy's Secret," takes the two young men through further
adventures. They had become acquainted on the steamer with a girl named
Bessie Gleason and her mother. Carl Potzfeldt, a German sailing under
false colors, claimed to be a friend of Bessie and her mother, but Jack,
who was more than casually interested in the girl, was suspicious of
this man. And his suspicions proved correct, for Potzfeldt had planned a
daring trick.

After some strenuous happenings, in which the Air Service Boys assisted,
Bessie and her mother were rescued from the clutches of Potzfeldt,
and went to Paris, Mrs. Gleason engaging in Red Cross work, and Bessie
helping her as best she could.

Immediately preceding this present volume is the third, called "Air
Service Boys Over the Rhine; or Fighting Above the Clouds."

By this time the United States had entered the great war on the side of
humanity and democracy.

Then the world was startled by the news that a great German cannon was
firing on Paris seventy miles away, and consternation reigned for a
time. Tom and Jack had a hand in silencing the great gun, for it was
they who discovered where it was hidden. Also in the third volume is
related how Tom's father, who had disappeared, was found again.

The boys passed through many startling experiences with their usual
bravery, so that, when the present story opens, they were taking a
much needed and well-earned rest. Mr. Raymond, having accomplished his
mission, had returned to the United States.

Then, as we have seen, came the news of the arrival of the first of
Pershing's forces, and with it came the sad message that Harry Leroy,
the chum of Torn and Jack, had fallen behind the German lines. And
whether he was alive now, though wounded, or was another victim of the
Hun machine guns, could not be told.

"Harry's sister couldn't have come at a worse time," remarked Tom, as he
rejoined Jack, having carried the unconscious girl to the same hospital
where Du Boise lay wounded.

"I should say not!" agreed Jack. "Do you really suppose she's Harry's
sister?"

"I don't see Any reason to doubt it. She said so, didn't she?"

"Oh, yes, of course. I was just wondering. Say, it's going to be tough
when she wakes up and realizes what's happened."

"You bet it is! This has been a tough day all around, and if it wasn't
for the good news that our boys are in France I'