e man lying upon the mattress.
He had raised himself upon one elbow.

"Oh, Robert!" he cried. "Not that!"

"Tom!" gasped Mackinder. Soon the two men were shaking hands at a great
rate, tears in their eyes.

"Boys," Mackinder announced at length, "I must introduce my brother Tom."

"We have had the pleasure of meeting the gentleman," stated Ned. "In
fact, we owe our lives to his kindness."

"But, see here," demanded Jimmie, stepping forward, "this needs an
explanation. Which one of you fellows was at the little cabin on the
Holland border?"

Tom Mackinder smiled, in spite of the pain of his crushed leg. He turned
his glance toward his brother, whose hand he held.

"We both were there, Jimmie," he said. "I took the package from the
window. You see," he continued, "it contained plans of my submarine, with
which you are familiar. I tried to sell the plans to Germany, but found
they had beaten me. So upon my return trip I slipped the package into
your baggage, thinking to escape search and detention at the border. I
have it here now."

As he ceased speaking he drew from his pocket the same flat package the
boys had seen before.

"Hurrah!" cried Jimmie. "Now we can explain how your brother came to be
captured by the Germans, and how under his direction we stole the other
'U-13' and escaped from Helgoland."

"If what you say is true, young man," put in the officer, "the
anticipated court martial may never convene."

"We can prove it!" protested Jimmie vigorously.

"Then we have solved the Mystery of the 'U-13'!" declared the officer,
with evident relief.

"And now we'll head for the little old U. S. A. and peaceful neutrality!"
was Jimmie's joyful comment.

"But you'll first arrange to care for your share of the cargo aboard my
boat," interposed the injured Mackinder.

He would not entertain any of the objections raised by the boys, but
insisted that they share in the treasure which had been recovered from
the ocean's grasp.

A few days later as the boys watched the chalk cliffs of Dover slip away
into the eastern horizon Jimmie turned from the rail of the steamer upon
which they had taken passage.

"Good by, England, and good by the Mackinders," he said. "I'm glad we are
out of the war zone at last and that we solved the Mystery of the
'U-13'."

THE END

* * * * *

The Aeroplane Series
By John Luther Langworthy

1. The Aeroplane Boys; or, The Young Pilots First Air V