Hal and Chester then examined a map of the country carefully and laid out a course.  It was agreed that Hal should follow the same course, for, as Chester said, there was little likelihood of anything going wrong, but coming along the same route the second craft would always have a chance of rendering aid should it be needed.  The lads agreed to meet at Saloniki the following day.

It was nearly dark when the machine carrying Chester, Colonel Anderson and Ivan soared in the air and headed south over Macedonia—­once the kingdom of Philip and Alexander the Great.  Stubbs, Nikol and Hal watched their friends disappear in the distance with some misgiving, which was given expression by Stubbs.

“I hope they get there safely,” he muttered, “but I have my doubts.”

“See here, Mr. Stubbs,” said Hal.  “You’ve gone through a lot, but you are still here, aren’t you?”

“I am,” said Stubbs calmly, “but I wish I were some place else.”

“Well, give me an hour or two to look over our machine and you will soon be some place else,” said Hal.

“And the chances are I’d rather be some place than where I am likely to be if I keep monkeying around in the air,” replied the little man.

Hal raised both hands in a gesture of hopelessness.

“There’s no use talking to you,” he said.  “I’ll leave you both here while I overhaul the plane.”

He took himself off.

Chester, Colonel Anderson and Ivan sailed swiftly through the air.  Darkness fell, but it was a bright night and Chester, at the wheel, could see without difficulty.  The passengers were quite comfortable in spite of the cold.

“Aren’t you getting a bit too low?” asked Colonel Anderson after a couple of hours flying in the darkness.

“Thousand feet,” said Chester after a glance at the indicator.

“Doesn’t seem like it to me,” said the colonel.  “Think I can see the ground below.”

“You shouldn’t at this altitude,” said Chester.

“I know it.  Guess I was mistaken.”

Half an hour later the colonel spoke again.  “Have you come down any, Chester?”

“No; why?”

“I’m sure I can see the ground below,” returned the colonel.

Chester glanced over the side of the plane.

“By Jove!  So can I,” he exclaimed.  He glanced at the indicator again.  It still read a trifle over a thousand feet.  “Something wrong some place,” he said to himself.

He tilted the elevating lever, but the plane did not answer by a sudden rush upward.  Chester gave a long whistle.

“What’s the matter?” demanded Colonel Anderson.

“I don’t know,” returned Chester.  “We’re going down gradually, I know that, but the indicator still reads a thousand feet and I can’t move the plane any higher.”

“And you don’t know what is wrong?”

“Haven’t the slightest idea.  I’m no airship expert.”