As the plane rose from the ground, Hal grabbed the side of the seat and hung on for dear life.  Looking down and seeing the ground dropping rapidly away, he experienced a choking sensation in his throat.

As the machine stopped rising, however, and stretched itself out for a straight flight, Hal’s composure came back to him, and he looked around with interest.

Then Fritz explained the mechanism of the machine to him.  He showed him how to stop, how to increase the speed of the plane; how to rise and how to glide to earth.  He also showed him how to work the steering wheel.

While they were sailing about in the air he told Hal that, if necessary, his craft could make a speed of one hundred miles an hour for hours.  He declared it could attain an altitude of a mile.  Practically the only danger, he said, came from conflicting air currents.

After sailing around for nearly half an hour, Fritz again brought the machine to the ground a few feet from where Chester stood.

“Great!” exclaimed Hal, as he alighted and helped Fritz roll the machine back into the shop.  “No more automobiling for me.  When I get home I am going to get an airship.”

“Wouldn’t you like to go up with me to-morrow, Chester?” asked Fritz, as he locked the door to the shop.

“I believe I would,” was the reply.  “I guess I can stand it if Hal can.”

“Then you shall,” said Fritz, and the three turned toward the house, where Frau Schmidt stood in the doorway, calling to them that supper was ready.

CHAPTER X.

In the air.

The boys were busily engaged in disposing of a hearty supper when there came a knock at the door.  Frau Schmidt answered the knock, and, returning a few moments later, placed before Hal an important-looking letter, bearing the official seal of the German government.

Hal opened the document and read.

“Great Scott!” he exploded, after a hasty perusal.

“What’s the matter?” demanded Chester anxiously.

“Why, here is an order, commanding us to report to the commanding officer the first thing in the morning, so that we may be transported back to Berlin!”

“Berlin!  What in the world do we want to go back to Berlin for?”

“We don’t; but it looks as though there were no help for it.  The letter says that, after an investigation of our case, it has been decided that we shall be sent back to Berlin and that, if we are to be allowed to leave the country, such arrangements must be made by the United States ambassador.”

“Well, what do you think of that!”

“It’s too bad,” declared Fritz; “but an order is an order.  I am afraid you must go!”

“You poor boys!” exclaimed Frau Schmidt “I can’t see why they won’t let you stay here.”

“No more do I,” declared Hal.  “But I guess this letter means business.”