rang upon him.

Chester had entered the room upon Hal's heels; and, as his friend jumped
for the first intruder, Chester rushed at the man in the next room. The
latter heard him advance, and, stepping back, picked up a chair, which he
brandished over his head. Taking a rapid stride forward, he swung his
improvised weapon at Chester's head.

Chester avoided the blow with a quick, backward leap, and the chair was
smashed to fragments against the door. Then Chester jumped forward and
closed with his opponent.

With a rapid movement he placed his knee behind the other's leg and
pushed suddenly. The man went over backward, with Chester on top of him.
As the intruder fell, his head came into contact with the sharp
projection of the bureau, and when he struck the floor he lay still.
Chester rose to his feet.

As Hal's opponent sprang toward him, the lad stepped in close and
delivered a stinging short-arm blow over the other's heart. He staggered
back, and, as Hal took another step forward, Chester, having disposed of
his adversary, threw his arms about the man from behind, and bore him to
the floor, where both boys piled on top of him.

While the three were struggling on the floor, a voice from the doorway
exclaimed:

"What is going on here?" and Fritz rushed into the room.

He took in the situation at a glance, and, rushing forward, lent a hand
in subduing the boys' opponent.

The struggle was over quickly, and, seizing a strong rope, which hung
from the wall, Fritz soon had the two men safely bound. Then he turned to
his mother, who still sat huddled on the chair, where she had been when
the boys entered the room. The excitement had been too much for her, and
she had fainted.

She was soon revived, however, and, when she was strong enough to sit up,
jumped to her feet, and, throwing her arms around Hal, kissed him loudly.
Then she turned her attention to Chester, and repeated the operation.

"My preservers!" she cried, laughing and crying at the same time. "Fritz,
but for these two boys your old mother would now be dead."

Rapidly and somewhat incoherently she related what had occurred, and
Fritz was no less warm in his praise for the actions of the two boys.

"Those men are undoubtedly spies," he declared. "They most certainly had
designs upon my biplane, which they evidently knew had been completed. I
shall turn them over to the military authorities."

He left the house, and in a few moments returned with a